Succession and toxicity of Microcystis and Anabaena (Dolichospermum) blooms are controlled by nutrient-dependent allelopathic interactions

Succession of particle-attached and free-living bacterial communities in response to microalgal dynamics induced by the biological cyanocide paucibactin A

Microalgae, including cyanobacteria and eukaryotic algae, are hotspots of primary production and play a critical role in global carbon cycling. However, these species often form blooms that poses a threat to aquatic ecosystems. Although the use of bacteria-derived cyanocides is regarded as an environmentally friendly method for controlling cyanobacterial blooms, only a few studies have examined their potential impact on ecosystems. This study is the first to explore the response of particle-attached (PA) and free-living (FL) bacteria to the dynamics of microalgal communities induced by the biological cyanocide paucibactin A. The microalgal community dynamics were divided into two distinct phases [phase I (days 0-2) and phase II (days 3-7)]. In phase I, paucibactin A caused a sudden decrease in the cyanobacterial concentration. Phase II was characterized by increased growth of eukaryotic microalgae (Scenedesmus, Pediastrum, Selenastrum, and Coelastrum). The stability of the bacterial community and the contribution of stochastic processes to community assembly were more pronounced in phase II than in phase I. The microalgal dynamics triggered by paucibactin A coincided with the succession of the PA and FL bacterial communities. The lysis of cyanobacteria in phase I favored the growth of microbial organic matter degraders in both the PA (e.g., Aeromonas and Rheinheimera) and FL (e.g., Vogesella) bacterial communities. In phase II, Lacibacter, Phycisphaeraceae, and Hydrogenophaga in the PA bacterial community and Lacibacter, Peredibacter, and Prosthecobacter in the FL bacterial community showed increased relative abundances. Overall, the FL bacterial community exhibited greater sensitivity to the two sequential processes compared with the PA bacterial community. These results highlight the need for studies evaluating the impact of biological cyanocides on aquatic ecosystems when used to control natural cyanobacterial blooms.

Intensification of harmful cyanobacterial blooms in a eutrophic, temperate lake caused by nitrogen, temperature, and CO2

Warmer temperatures can significantly increase the intensity of cyanobacterial harmful algal blooms (CHABs) in eutrophic freshwater ecosystems. However, few studies have examined the effects of CO2 enrichment in tandem with elevated temperature and/or nutrients on cyanobacterial taxa in freshwater ecosystems. Here, we observed changes in the biomass of cyanobacteria, nutrients, pH, and carbonate chemistry over a two-year period in a shallow, eutrophic freshwater lake and performed experiments to examine the effects and co-effects of CO2, temperature, and nutrient enrichment on cyanobacterial and N2-fixing (diazotrophic) communities assessed via high throughput sequencing of the 16S rRNA and nifH genes, respectively. During both years, there were significant CHABs (50-500 μg cyanobacterial chlorophyll-a L-1) and lake CO2 levels were undersaturated (≤300 μatm pCO2). NH4+ significantly increased the net growth rates of cyanobacteria as well as the biomass of the diazotrophic cyanobacterial order Nostocales under elevated and ambient CO2 conditions. In a fall experiment, the N2 fixation rates of Nostocales were significantly higher when populations were enriched with CO2 and P, relative to CO2-enriched populations that were not amended with P. During a summer experiment, N2 fixation rates increased significantly under N and CO2 - enriched conditions relative to N-enriched and ambient CO2 conditions. Nostocales dominated the diazotrophic communities of both experiments, achieving the highest relative abundance under CO2-enriched conditions when N was added in the first experiment and when CO2 and temperature were elevated in the second experiment, when N2 fixation rates also increased significantly. Collectively, this study indicates that N promotes cyanobacterial blooms including those formed by Dolichospermum and that the biomass and N2 fixation rates of diazotrophic cyanobacterial taxa may benefit from enhanced CO2 levels in eutrophic lakes.

Grazing preferences of three species of amoebae on cyanobacteria and green algae

Twenty species/isolates of cyanobacteria and green algae were isolated from cyanobacterial bloom samples in lakes associated with the upper Qu'Appelle River drainage system in southern Saskatchewan, Canada. Three amoebae species (Cochliopodium sp., Vannella sp. and Vermamoeba vermiformis) were also isolated from one of these samples, and were subjected to grazing assays to determine which species of cyanobacteria or algae could potentially serve as a food source. Amoeba grazing rates were quantified based on the diameter of the plaque after 12 days on agar plate assays, and by estimation of the amoeba population growth rate from the rate of increase of plaque area. The common cyanobacterial bloom-formers Dolichospermum sp. and Aphanizomenon flos-aquae supported high growth rates for all three amoebae, while green algae, with the exception of one green alga/amoeba combination, did not support growth of the tested amoebae. Many of the cyanobacterial and algal isolates that did not support amoebae growth were ingested, suggesting that ingestion did not determine grazing success. Overall, while the cyanobacteria Dolichospermum sp. and Aphanizomenon flos-aquae were suitable food sources for the amoebae, the other cyanobacteria were grazed in an unpredictable manner, with some species/strains grazed by some amoebae and some species not grazed at all.

Role of serotonin in the lack of sensitization caused by prolonged food deprivation in Aplysia

Food deprivation may cause neurological dysfunctions including memory impairment. The mollusk Aplysia is a suitable animal model to study prolonged food deprivation-induced memory deficits because it can sustain up to 14 days of food deprivation (14DFD). Sensitization of defensive withdrawal reflexes has been used to illustrate the detrimental effects of 14DFD on memory formation. Under normal feeding conditions (i.e., two days food deprivation, 2DFD), aversive stimuli lead to serotonin (5-HT) release into the hemolymph and neuropil, which mediates sensitization and its cellular correlates including increased excitability of tail sensory neurons (TSNs). Recent studies found that 14DFD prevents both short-term and long-term sensitization, as well as short-term increased excitability of TSNs induced by in vitro aversive training. This study investigated the role of 5-HT in the absence of sensitization and TSN increased excitability under 14DFD. Because 5-HT is synthesized from tryptophan obtained through diet, and its exogeneous application alone induces sensitization and increases TSN excitability, we hypothesized that 1) 5-HT level may be reduced by 14DFD and 2) 5-HT may still induce sensitization and TSN increased excitability in 14DFD animals. Results revealed that 14DFD significantly decreased hemolymph 5-HT level, which may contribute to the lack of sensitization and its cellular correlates, while ganglia 5-HT level was not changed. 5-HT exogenous application induced sensitization in 14DFD Aplysia, albeit smaller than that in 2DFD animals, suggesting that this treatment can only induce partial sensitization in food deprived animals. Under 14DFD, 5-HT increased TSN excitability indistinguishable from that observed under 2DFD. Taken together, these findings characterize 5-HT metabolic deficiency under 14DFD, which may be compensated, at least in part, by 5-HT exogenous application.

Water quality seasonal variation assessment of the Gongji and Yaksa streams, Chuncheon, South Korea

Gongji Stream flows into Lake Uiam, a potable water source for the capital region of Chuncheon, South Korea. Algal blooms often occur downstream of the Gongji stream in combination with drastic flow rate variations. Downstream water quality may also be affected by Yaksa stream. Yaksa stream joins Gongji stream before it reaches Uiam Lake, which is a drinking water source for the city. Limited data exists on the Yaksa stream water quality. Therefore, water quality parameters (pH, electrical conductivity (EC), biological oxygen demand (BOD), total nitrogen (T-N), total phosphorous (T-P), chlorophyll-a (Chl-a), total coliforms, and Escherichia coli (E. coli) concentration) were sampled from Gongji (at sites GJ1 and GJ2) and Yaksa (at sites YS1 and YS2) streams from May to September, 2022. The results revealed the overall water quality of both streams was good (BOD = 0.27-3.66 mg/L; TP = 0.003-0.074 mg/L), except on August 3. On August 3, the concentrations of BOD, TP, total coliforms, and E. coli were elevated, with the highest concentrations in samples from GJ2. The recent heavy rainfall potentially caused sewage inflows near GJ2. The correlation analysis revealed positive linear relationships in the 1-day cumulative precipitation with BOD (r = 0.503), total coliforms (r = 0.547), and TP (r = 0.814). The Yaksa stream may be an Anabaena sp. source, which contaminated samples from YS1, YS2, and GJ2, but not at GJ1 (upstream of the tributary).

Nitrogen liberated via allelopathy can promote harmful algal blooms

Allelopathy is a biological mechanism that can promote harmful algal blooms (HAB) via the inhibition of sympatric phytoplankton. While nutrient loading can also promote HABs, the ability of allelopathy to stimulate HABs via the regeneration of nutrients has yet to be explored. To examine the impacts of allelopathically liberated N on HAB species, a series of experiments were performed using multiple allelopathic HAB species including the dinoflagellates Alexandrium catenella and Margalefidinium polykrikoides, and the pelagophyte, Aureoumbra lagunensis. These HAB species were paired with the cosmopolitan dinoflagellate, Akashiwo sanguinea, that was labeled with 15NO3- or 15NH4+, allowing the release and transfer of N to be traced as a time course during allelopathic interactions. During all experiments, the allelopathic inhibition of Akashiwo was accompanied by increases in cell densities, growth rates, and the δ15N content of the HAB species, evidencing the transfer of N liberated from Akashiwo. The cellular transfer of 15N and release of dissolved N was higher when Akashiwo was grown with 15NO3- compared to 15NH4+ suggesting a differential subcellular-compartmentalization of N sources. Regardless of the type of N, HAB species obtained 60 - 100% of their cellular N from lysed Akashiwo cells and there was an enrichment of the δ15N content of the dissolved NH4+ pool post-lysis of Akashiwo. Collectively, the results demonstrate that beyond facilitating species succession, allelopathy can supply HABs with N and, therefore, is likely important for promoting and sustaining HABs. Given that allelopathy is known to be a dose-dependent process, allelopathy may induce a positive feedback loop, whereby competitors are lysed, N is liberated, HABs are intensified and, in turn, become more strongly allelopathic.

Synchrotron-based X-ray fluorescence microscopy mapping the ionome of a toxic freshwater cyanobacterium

Harmful algal blooms (HABs) pose a major environmental concern across the globe. In abundance, cyanobacteria, or so-called green-blue algae can produce extremely dangerous cyanotoxins that harm humans and animals. This study focused on the mapping and distribution of intracellular macro-and micronutrients of the wide-spread freshwater cyanobacteria Microcystis aeruginosa (M. aeruginosa). Towards a better understanding of trace metal uptake and homeostasis throughout the cell cycle, we quantitatively mapped the spatial distribution of the elements P, K, Fe, Ca, Zn, Mn, and Cu across the ultrastructure of frozen-hydrated single cells using state-of-the-art X-ray nanofluorescence imaging at the Advanced Photon Source (APS) at Argonne National Laboratory. Bulk cellular nutrient and trace metal content correlated well with the total intracellular elemental content in individual cells obtained by quantitative synchrotron X-ray fluorescence measurements. Multi-dimensional mappings showed P and K atoms colocalized as discrete semicircular hotspots that were analyzed with respect to their stoichiometry. Elevated Cu and Ca concentrations were detected along division plane of cells. P and K were found to have similar spatial elemental distribution with about 65% and 69% of the total cellular P and K, respectively, located at the hotspots. The P and K colocalization were refined further using nanotomography, showing a K envelope surrounding the P core. Inorganic P and organic P compounds were specified using solution-state 31P nuclear magnetic resonance (NMR) spectroscopy from M. aeruginosa. Of the total extracted P determined by 31P NMR spectroscopy, 47% were found to be nucleotides while only 11% were polyphosphates. Multimodal X-ray imaging provides a better understanding of intracellular biochemical processes in cyanobacteria, helping us monitor and combat an emerging environmental threat.

Effects of dissolved organic matter from sediment and soil samples on the growth and physiology of four bloom-forming algal species

Algal blooms negatively impact the water quality of reservoirs; however, the role of dissolved organic matter (DOM) in bloom formation in reservoirs has not been investigated. Therefore, we assessed the compositions of sediment- and soil-derived DOM and their effects on the growth, physiology, and photosynthetic activity of Microcystis aeruginosa, Anabaena sp., Chlamydomonas sp., and Peridiniopsis sp. (bloom-forming species). Sediment DOM promoted the growth of all algal species, whereas soil DOM significantly promoted the growth of Chlamydomonas sp. and Peridiniopsis sp.; this effect was due to enhanced stress tolerance and photosynthetic efficiency exhibited by these algae under DOM treatment. However, soil DOM slightly inhibited the growth of Anabaena sp. by increasing reactive oxygen species levels and inactivating some photosystem II reaction centers. The tyrosine-like substance, humic acid-like substances, and unsaturated aliphatic compounds were the main DOM components that affected algal growth. The findings of this study will provide a theoretical foundation for the development of bloom-prevention strategies for river-type reservoirs.

Spatiotemporal diversity and community structure of cyanobacteria and associated bacteria in the large shallow subtropical Lake Okeechobee (Florida, United States)

Lake Okeechobee is a large eutrophic, shallow, subtropical lake in south Florida, United States. Due to decades of nutrient loading and phosphorus rich sediments, the lake is eutrophic and frequently experiences cyanobacterial harmful algal blooms (cyanoHABs). In the past, surveys of the phytoplankton community structure in the lake have been conducted by morphological studies, whereas molecular based studies have been seldom employed. With increased frequency of cyanoHABs in Lake Okeechobee (e.g., 2016 and 2018 Microcystis-dominated blooms), it is imperative to determine the diversity of cyanobacterial taxa that exist within the lake and the limnological parameters that drive bloom-forming genera. A spatiotemporal study of the lake was conducted over the course of 1 year to characterize the (cyano)bacterial community structure, using 16S rRNA metabarcoding, with coincident collection of limnological parameters (e.g., nutrients, water temperature, major ions), and cyanotoxins. The objectives of this study were to elucidate spatiotemporal trends of community structure, identify drivers of community structure, and examine cyanobacteria-bacterial relationships within the lake. Results indicated that cyanobacterial communities within the lake were significantly different between the wet and dry season, but not between periods of nitrogen limitation and co-nutrient limitation. Throughout the year, the lake was primarily dominated by the picocyanobacterium Cyanobium. The bloom-forming genera Cuspidothrix, Dolichospermum, Microcystis, and Raphidiopsis were highly abundant throughout the lake and had disparate nutrient requirements and niches within the lake. Anatoxin-a, microcystins, and nodularins were detected throughout the lake across both seasons. There were no correlated (cyano)bacteria shared between the common bloom-forming cyanobacteria Dolichospermum, Microcystis, and Raphidiopsis. This study is the first of its kind to use molecular based methods to assess the cyanobacterial community structure within the lake. These data greatly improve our understanding of the cyanobacterial community structure within the lake and the physiochemical parameters which may drive the bloom-forming taxa within Lake Okeechobee.

Chemodiversity of Cyanobacterial Toxins Driven by Future Scenarios of Climate Warming and Eutrophication

Climate change and eutrophication are two environmental threats that can alter the structure of freshwater ecosystems and their service functions, but we know little about how ecosystem structure and function will evolve in future scenarios of climate warming. Therefore, we created different experimental climate scenarios, including present-day conditions, a 3.0 °C increase in mean temperature, and a "heatwaves" scenario (i.e., an increase in temperature variability) to assess the effects of climate change on phytoplankton communities under simultaneous stress from eutrophication and herbicides. We show that the effects of climate warming, particularly heatwaves, are associated with elevated cyanobacterial abundances and toxin production, driven by a change from mainly nontoxic to toxic Microcystis spp. The reason for higher cyanobacterial toxin concentrations is likely an increase in abundances because under the dual pressures of climate warming and eutrophication individual Microcystis toxin-producing ability decreased. Eutrophication and higher temperatures significantly increased the biomass of Microcystis, leading to an increase in the cyanobacterial toxin concentrations. In contrast, warming alone did not produce higher cyanobacterial abundances or cyanobacterial toxin concentrations likely due to the depletion of the available nutrient pool. Similarly, the herbicide glyphosate alone did not affect abundances of any phytoplankton taxa. In the case of nutrient enrichment, cyanobacterial toxin concentrations were much higher than under warming alone due to a strong boost in biomass of potential cyanobacterial toxin producers. From a broader perspective our study shows that in a future warmer climate, nutrient loading has to be reduced if toxic cyanobacterial dominance is to be controlled.

Changes in the morphology and cell ultrastructure of a microalgal community exposed to a commercial glyphosate formulation and a toxigenic cyanobacterium

Human activities significantly influence the health of aquatic ecosystems because many noxious chemical wastes are discharged into freshwater bodies. Intensive agriculture contributes to the deterioration by providing indirectly fertilizers, pesticides, and other agrochemicals that affect the aquatic biota. Glyphosate is one of the most used herbicides worldwide, and microalgae are particularly sensitive to its formulation, inducing displacement of some green microalgae from the phytoplankton that leads to alterations in the floristic composition, which fosters the abundance of cyanobacteria, some of which can be toxigenic. The combination of chemical stressors such as glyphosate and biological ones, like cyanotoxins and other secondary metabolites of cyanobacteria, could induce a combined effect potentially more noxious to microalgae, affecting not only their growth but also their physiology and morphology. In this study, we evaluated the combined effect of glyphosate (Faena^®) and a toxigenic cyanobacterium on the morphology and ultrastructure of microalgae in an experimental phytoplankton community. For this purpose, Microcystis aeruginosa (a cosmopolitan cyanobacterium that forms harmful blooms) and the microalgae Ankistrodesmus falcatus, Chlorella vulgaris, Pseudokirchneriella subcapitata, and Scenedesmus incrassatulus were cultivated, individually and jointly, exposing them to sub-inhibitory concentrations of glyphosate (IC₁₀, IC_20, and IC₄₀). Effects were evaluated through scanning electron (SEM) and transmission electron (TEM) microscopy. Exposure to Faena^® produced alterations in the external morphology and ultrastructure of microalgae both individually and in combined cultures. SEM evidenced the loss of the typical shape and integrity of the cell wall and an increase in the biovolume. TEM revealed reduction and disorganization of the chloroplast, variation in starch and polyphosphate granules, formation of vesicles and vacuoles, cytoplasm degradation, and cell wall continuity loss. The presence of M. aeruginosa was, for microalgae, an additional stress factor adding to the chemical stress produced by Faena^®, increasing the damage in their morphology and ultrastructure. These results alert to the effects that can be caused by glyphosate and the presence of toxigenic bacteria on the algal phytoplankton in contaminated and anthropic and eutrophic freshwater ecosystems.

Microcystis spp. and phosphorus in aquatic environments: A comprehensive review on their physiological and ecological interactions

Cyanobacterial blooms caused by eutrophication have become a major environmental problem in aquatic ecosystems worldwide over the last few decades. Phosphorus is a limiting nutrient that affects the growth of cyanobacteria and plays a role in dynamic changes in algal density and the formation of cyanobacterial blooms. Therefore, identifying the association between phosphorus sources and Microcystis, which is the most representative and harmful cyanobacteria, is essential for building an understanding of the ecological risks of cyanobacterial blooms. However, systematic reviews summarizing the relationships between Microcystis and phosphorus in aquatic environments are rare. Thus, this study provides a comprehensive overview of the physiological and ecological interactions between phosphorus sources and Microcystis in aquatic environments from the following perspectives: (i) the effects of phosphorus source and concentration on Microcystis growth, (ii) the impacts of phosphorus on the environmental behaviors of Microcystis, (iii) mechanisms of phosphorus-related metabolism in Microcystis, and (iv) role of Microcystis in the distribution of phosphorus sources within aquatic environments. In addition, relevant unsolved issues and essential future investigations (e.g., secondary ecological risks) have been highlighted and discussed. This review provides deeper insights into the relationship between phosphorus sources and Microcystis and can serve as a reference for the evaluation, monitoring, and effective control of cyanobacterial blooms.

Expression of Microcystis Biosynthetic Gene Clusters in Natural Populations Suggests Temporally Dynamic Synthesis of Novel and Known Secondary Metabolites in Western Lake Erie

Microcystis spp. produce diverse secondary metabolites within freshwater cyanobacterial harmful algal blooms (cyanoHABs) around the world. In addition to the biosynthetic gene clusters (BGCs) encoding known compounds, Microcystis genomes harbor numerous BGCs of unknown function, indicating a poorly understood chemical repertoire. While recent studies show that Microcystis produces several metabolites in the lab and field, little work has focused on analyzing the abundance and expression of its broader suite of BGCs during cyanoHAB events. Here, we use metagenomic and metatranscriptomic approaches to track the relative abundance of Microcystis BGCs and their transcripts throughout the 2014 western Lake Erie cyanoHAB. The results indicate the presence of several transcriptionally active BGCs that are predicted to synthesize both known and novel secondary metabolites. The abundance and expression of these BGCs shifted throughout the bloom, with transcript abundance levels correlating with temperature, nitrate, and phosphorus concentrations and the abundance of co-occurring predatory and competitive eukaryotic microorganisms, suggesting the importance of both abiotic and biotic controls in regulating expression. This work highlights the need for understanding the chemical ecology and potential risks to human and environmental health posed by secondary metabolites that are produced but often unmonitored. It also indicates the prospects for identifying pharmaceutical-like molecules from cyanoHAB-derived BGCs. IMPORTANCE Microcystis spp. dominate cyanobacterial harmful algal blooms (cyanoHABs) worldwide and pose significant threats to water quality through the production of secondary metabolites, many of which are toxic. While the toxicity and biochemistry of microcystins and several other compounds have been studied, the broader suite of secondary metabolites produced by Microcystis remains poorly understood, leaving gaps in our understanding of their impacts on human and ecosystem health. We used community DNA and RNA sequences to track the diversity of genes encoding synthesis of secondary metabolites in natural Microcystis populations and assess patterns of transcription in western Lake Erie cyanoHABs. Our results reveal the presence of both known gene clusters that encode toxic secondary metabolites as well as novel ones that may encode cryptic compounds. This research highlights the need for targeted studies of the secondary metabolite diversity in western Lake Erie, a vital freshwater source to the United States and Canada.

The Potential of Allelochemicals from Microalgae for Biopesticides

Improvements in agricultural productivity are required to meet the demand of a growing world population. Phytopathogens, weeds, and insects are challenges to agricultural production. The toxicity and widespread application of persistent synthetic pesticides poses a major threat to human and ecosystem health. Therefore, sustainable strategies to control pests are essential for agricultural systems to enhance productivity within a green paradigm. Allelochemicals are a less persistent, safer, and friendly alternative to efficient pest management, as they tend to be less toxic to non-target organisms and more easily degradable. Microalgae produce a great variety of allelopathic substances whose biocontrol potential against weeds, insects, and phytopathogenic fungi and bacteria has received much attention. This review provides up-to-date information and a critical perspective on allelochemicals from microalgae and their potential as biopesticides.

Harmful cyanobacteria-diatom/dinoflagellate blooms and their cyanotoxins in freshwaters: A nonnegligible chronic health and ecological hazard

Human and ecological health depends on the vitality of freshwater systems, but these are increasingly threatened by cyanotoxins released from harmful algal blooms (HABs). Periodic cyanotoxin production, although undesirable, may be tolerable when there is enough time for cyanotoxins to degrade and dissipate in the environment, but the year-round presence of these toxins will be a chronic health for humans and ecosystems. The purpose of this critical review is to document the seasonal shifts of algal species and their ecophysiological acclimatation to dynamic environmental conditions. We discuss how these conditions will create successive occurrences of algal blooms and the release of cyanotoxins into freshwater. We first review the most common cyanotoxins, and evaluate the multiple ecological roles and physiological functions of these toxins for algae. Then, the annual recurring patterns HABs are considered in the context of global change, which demonstrates the capacity for algal blooms to shift from seasonal to year-round growth regimes that are driven by abiotic and biotic factors, leading to chronic loading of freshwaters with cyanotoxins. At last, we illustrate the impacts of HABs on the environment by compiling four health issues and four ecology issues emanating from their presence in the that covers atmosphere, aquatic ecosystems and terrestrial ecosystems. Our study highlights the annual patterns of algal blooms, and proposes that a "perfect storm" of events is lurking that will cause the 'seasonal toxicity' to become a full-blown, 'chronic toxicity' in the context of the deterioration of HABs, highlighting a non-negligible chronic health and ecological hazard.

Nitrogen forms and concentration influence the impact of titanium dioxide nanoparticles on the biomass and antioxidant enzyme activities of Microcystis aeruginosa

Nanoparticles (NPs) are becoming more widely produced, used, and released into the aquatic environment. In aquatic ecosystems, these NPs affect different populations of photosynthesizing organisms, such as cyanobacteria. This study aimed to evaluate the effects of titanium dioxide (TiO₂) NPs (48 mg l^-1) combined with low (0.04 mM) and high (9 mM) concentrations of urea and nitrate on Microcystis aeruginosa. Microcystins (MCs) production and release were monitored in the cyanobacterium. The results showed that high urea concentration (9 mM) combined with TiO₂ NPs inhibited growth, pigment, and malondialdehyde (MDA) content by 82%, 63%, and 47%, respectively. The treatment also increased the reactive oxygen species (ROS) and glutathione S-transferase (GST) activity by 40.7% and 67.7%, respectively. Similarly, low nitrate (0.04 mM) combined with TiO₂ NPs inhibited growth by 40.3% and GST activity by 36.3% but stimulated pigment production and ROS concentration in M. aeruginosa. These responses suggest that high urea combined with TiO₂.NPs and high nitrate combined with TiO₂ NPs induced oxidative stress in cyanobacteria. The peroxidase (POD) activity of M. aeruginosa decreased by 17.7% with increasing urea concentrations. Our findings suggest that TiO₂ NPs combined with changing nutrient (urea and nitrate) concentrations may adversely affect cyanobacterial development and antioxidant defense enzymes.

Interactions between Phosphorus Enrichment and Nitrification Accelerate Relative Nitrogen Deficiency during Cyanobacterial Blooms in a Large Shallow Eutrophic Lake

Regime shifts between nitrogen (N) and phosphorus (P) limitation, which trigger cyanobacterial succession, occur in shallow eutrophic lakes seasonally. However, the underlying mechanism is not yet fully illustrated. We provide a novel insight to address this from interactions between sediment P and nitrification through monthly field investigations including 204 samples and microcosm experiments in Lake Chaohu. Total N to P mass ratios (TN/TP) varied significantly across seasons especially during algal bloom in summer, with the average value being 26.1 in June and descending to 7.8 in September gradually, triggering dominant cyanobacterial succession from Microcystis to Dolichospermum. The regulation effect of sediment N/P on water column TN/TP was stronger in summer than in other seasons. Iron-bound P and alkaline phosphatase activity in sediment, rather than ammonium, contributed to the higher part of nitrification. Furthermore, our microcosm experiments confirmed that soluble active P and enzymatic hydrolysis of organic P, accumulating during algal bloom, fueled nitrifiers and nitrification in sediments. These processes promoted lake N removal and led to relative N deficiency in turn. Our results highlight that N and P cycles do not exist independently but rather interact with each other during lake eutrophication, supporting the dual N and P reduction program to mitigate eutrophication in shallow eutrophic lakes.

The growth, biochemical composition, and antioxidant response of Microcystis and Chlorella are influenced by Ibuprofen

Non-steroidal anti-inflammatory drugs like ibuprofen (IBU) are extensively used, causing substantial amounts to end up in aquatic ecosystems. Unfortunately, little research has been done on how these medications influence the physiology of phytoplankton. This study aimed to investigate the toxicological and physiological effects of IBU on the cyanobacteria Microcystis aeruginosa LE3 and Microcystis aeruginosa EAWAG 198, and the chlorophyte Chlorella sorokiniana. Exponential growth phase cultures were exposed to IBU at 10 to 10,000 μg/L for 96 h. The medium effect concentrations revealed varied sensitivity to IBU in the order Chlorella sorokiniana > Microcystis aeruginosa LE3 > Microcystis aeruginosa EAWAG 198. The drug caused a significant difference from control in cell density and chlorophyll-a of the three strains, except for chlorophyll-a in M. aeruginosa EAWAG 198 cultures where a significant difference occurred at 100 μg/L. The cell density of M. aeruginosa LE3 cultures exposed to 10 μg/L IBU increased 24 h post-exposure. Increasing concentrations of IBU induced higher total microcystins content of the Microcystis aeruginosa. Intracellular hydrogen peroxide content, peroxidase, and glutathione S-transferase activities, and lipid peroxidation increased as a function of IBU exposure. Total lipid, carbohydrate, and protein content of Chlorella sorokiniana were stimulated following IBU exposure. We conclude that the increasing presence of IBU in aquatic ecosystems could significantly alter the population dynamics of the investigated and other phytoplankton species.

Nitrogen and phosphorus significantly alter growth, nitrogen fixation, anatoxin-a content, and the transcriptome of the bloom-forming cyanobacterium, Dolichospermum

While freshwater cyanobacteria are traditionally thought to be limited by the availability of phosphorus (P), fixed nitrogen (N) supply can promote the growth and/or toxin production of some genera. This study characterizes how growth on N2 (control), nitrate (NO3 -), ammonium (NH4 +), and urea as well as P limitation altered the growth, toxin production, N2 fixation, and gene expression of an anatoxin-a (ATX-A) - producing strain of Dolichospermum sp. 54. The transcriptomes of fixed N and P-limited cultures differed significantly from those of fixed N-deplete, P-replete (control) cultures, while the transcriptomes of P-replete cultures amended with either NH4 + or NO3 - were not significantly different relative to those of the control. Growth rates of Dolichospermum (sp. 54) were significantly higher when grown on fixed N relative to without fixed N; growth on NH4 + was also significantly greater than growth on NO3 -. NH4 + and urea significantly lowered N2 fixation and nifD gene transcript abundance relative to the control while cultures amended with NO3 - exhibited N2 fixation and nifD gene transcript abundance that was not different from the control. Cultures grown on NH4 + exhibited the lowest ATX-A content per cell and lower transcript abundance of genes associated ATX-A synthesis (ana), while the abundance of transcripts of several ana genes were highest under fixed N and P - limited conditions. The significant negative correlation between growth rate and cellular anatoxin quota as well as the significantly higher number of transcripts of ana genes in cultures deprived of fixed N and P relative to P-replete cultures amended with NH4 + suggests ATX-A was being actively synthesized under P limitation. Collectively, these findings indicate that management strategies that do not regulate fixed N loading will leave eutrophic water bodies vulnerable to more intense and toxic (due to increased biomass) blooms of Dolichospermum.

Ultrasound assisted cyanotoxin extraction for nematode inhibition in soil

Root-knot nematodes are one of the plant damaging nematodes in agriculture causing a projected annual yield loss of ∼12 % (∼$160 billion) worldwide. Conventional solutions to control these plant-parasitic nematodes involve chemical nematicides. To reduce the use of harmful chemicals, microalgal extracts can be used as greener alternatives for nematode management. Microalgae produce valuable metabolites, including cyanotoxins which can aid in nematode suppression. In this study, two microalgae species, Trichormus variabilis and Nostoc punctiforme, were treated with ultrasound for intensified recovery of secondary metabolites. Ultrasound results in cell wall disruption of the microalgal species, thus resulting in enhanced release of secondary metabolites. Microalgal biomass was treated with an ultrasound probe at 50 % amplitude, 20 kHz frequency, using water as the extraction medium, for 5-30 min. The extraction efficiency was determined in terms of the total chlorophyll (Chl) content of the extract. Microscopic images of the treated cells were also investigated to gain insight into the effect of the ultrasonication time on the cell morphology. Our results suggest that ultrasonication resulted in the intensified release of secondary metabolites, as established through the total chlorophyll content of the ultrasonicated microalgal samples as well as the microscopic images of the ruptured cells. The best extraction for Trichormus variabilis was achieved with 15 min extraction time where the Total Chl content increased by 29 times (compared to the non-ultrasonicated sample), and for the Nostoc punctiforme, 30 min extraction time gave the highest metabolite recovery of 6.4 times higher than the non-ultrasonicated sample. Ultrasonicated algal extracts were then tested for their nematicidal potential against root-knot nematode, Meloidogyne hapla, in infested field soil samples. Experimental study was conducted using different concentrations of each microalga, Trichormus sp. and Nostoc sp., individually, as well as in combination. The nematode count for the treated soil was compared with that of the control (untreated soil). Ultrasonicated microalgal extracts showed 66% to 100% inhibition on root-knot nematodes in the soil samples tested.

Microscope-based light gradient generation for quantitative growth studies of photosynthetic micro-organisms

Photosynthetic micro-organisms are equipped with molecular machineries that are designed to transform light into chemical or bioenergy, and help shape and balance the ecosystem of all life forms on earth. Recently, aquatic ecosystems have been disrupted by climate change, which leads to the frequent occurrence of harmful algal blooms (HABs). HABs endanger drinking water resources and harm the fishing and coastal recreation industries. Despite its urgency, mechanistic understanding of how key biophysical and biochemical parameters impact algal growth is largely unexplored. In this article, we developed a microscope-based light gradient generator for studies of photosynthetic micro-organisms under well-defined light intensity gradients. This technology utilized a commercially available microscope, allowed for controlled light exposure and imaging of cells on the same microscope platform, and can be integrated with any micrometer-scale device. Using this technology, we studied the role of light intensity in the growth of photosynthetic micro-organisms. A parallel study was also carried out using a 96-well plate. Our work revealed that the growth rate of the microalgae/cyanobacteria was significantly regulated by the light intensity and followed Monod or van Oorschot kinetic models. The measured half-saturation constants were compared with those obtained in macro-scale devices, and indicated that shading, light spectrum, and temperature may all play important roles in the light sensitivity of photosynthetic micro-organisms. This work highlighted the importance of analytical tools for quantitative understanding of biophysical parameters in the growth of photosynthetic micro-organisms, and knowledge learned will be critical in the design of future technologies for managing algal blooms or optimizing bioenergy production.

Maximization of growth and lipid production of a toxic isolate of Anabaena circinalis by optimization of various parameters with mathematical modeling and computational validation

Toxic cyanobacterial blooms are recurrent for few decades throughout the globe, due to climate change, atmospheric warming and various anthropogenic activities with severe impacts of potential toxins on various ecosystems finally affecting the entire environment. These cyanobacteria are merely unexplored regarding their biochemical components except toxins. Variable influences and interactions of different factors including nitrogen, carbon, and availability of light are well known to crucially regulate cyanobacterial growth and metabolism. Thus, current research work is motivated for the evaluation and optimization of the effects of the aforementioned vital factors for improvement of biomass and lipid production of a freshwater, toxic strain of Anabaena circinalis. The modelling and optimization of factors such as nitrogen, light intensity and bicarbonate concentration (source of carbon) to maximize growth and lipid production were based on 20 design point experiments by Response Surface Methodology (RSM) and optimized values were further improved and validated by Particle Swarm Optimization (PSO) algorithm. The maximum optima were obtained 1.829 g L^-1 and 39.64 % for biomass production and lipid content respectively from PSO optimization with two different sets of optimal values of factors. It shows 0.44 % and 2.77 % higher values of responses than that of RSM optimization. These asynchronous findings pioneered the enhanced lipid accumulation as well as the growth of a toxic cyanobacterium by optimizing interaction effects of culture conditions through various statistical and computational approaches.

Cyanotoxin-encoding genes as powerful predictors of cyanotoxin production during harmful cyanobacterial blooms in an inland freshwater lake: Evaluating a novel early-warning system

Freshwater harmful cyanobacterial blooms (HCBs) potentially produce excessive cyanotoxins, mainly microcystins (MCs), significantly threatening aquatic ecosystems and public health. Accurately predicting HCBs is thus essential to developing effective HCB mitigation and prevention strategies. We previously developed a novel early-warning system that uses cyanotoxin-encoding genes to predict cyanotoxin production in Harsha Lake, Ohio, USA, in 2015. In this study, we evaluated the efficacy of the early-warning system in forecasting the 2016 HCB in the same lake. We also examined potential HCB drivers and cyanobacterial community composition. Our results revealed that the cyanobacterial community was stable at the phylum level but changed dynamically at the genus level over time. Microcystis and Planktothrix were the major MC-producing genera that thrived in June and July and produced high concentrations of MCs (peak level 10.22 μg·L^-1). The abundances of the MC-encoding gene cluster mcy and its transcript levels significantly correlated with total MC concentrations (before the MC concentrations peaked) and accurately predicted MC production as revealed by logistic equations. When the Microcystis-specific gene mcyG reached approximately 1.5 × 10³ copies·mL^-1 or when its transcript level reached approximately 2.4 copies·mL^-1, total MC level exceeded 0.3 μg L^-1 (a health advisory limit) approximately one week later (weekly sampling scheme). This study suggested that cyanotoxin-encoding genes are promising predictors of MC production in inland freshwater lakes, such as Harsha Lake. The evaluated early-warning system can be a useful tool to assist lake managers in predicting, mitigating, and/or preventing HCBs.

Cyanophage infections reduce photosynthetic activity and expression of CO2 fixation genes in the freshwater bloom-forming cyanobacterium Aphanizomenon flos-aquae

Cyanobacteria play a significant role in ecosystem functioning as photosynthetic and CO₂ fixing microorganisms. Whether and to what extent cyanophages alter these carbon and energy cycles in their cyanobacterial hosts is still poorly understood. In this study, we investigated changes in photosynthetic activity (PSII), expression of genes associated with the light phase of photosynthesis (psbA, petA, ndhK) and carbon metabolism (rbcL, zwf) as well as intracellular ATP and NADHP concentrations in freshwater bloom-forming filamentous cyanobacterium Aphanizomenon flos-aquae infected by cyanophage vB_AphaS-CL131. We found that PSII activity and expression level of rbcL genes, indicating potential for CO₂ fixation, had decreased in response to cyanophage adsorption and DNA injection. During the period of viral DNA replication and assembly, PSII performance and gene expression remained at this decreased level and did not change significantly, indicating lack of transcriptional shutdown by the cyanophage. Combined, these observations suggest that although there is little to no interference between cyanophage DNA replication, host transcription and cellular metabolism, A. flos-aquae underwent a physiological state-shift toward lower efficiency of carbon and energy cycling. This further suggest potential cascading effect for co-occurring non-infected members of the microbial community.

Algicidal Bacteria: A Review of Current Knowledge and Applications to Control Harmful Algal Blooms

Interactions between bacteria and phytoplankton in aqueous ecosystems are both complex and dynamic, with associations that range from mutualism to parasitism. This review focuses on algicidal interactions, in which bacteria are capable of controlling algal growth through physical association or the production of algicidal compounds. While there is some evidence for bacterial control of algal growth in the field, our understanding of these interactions is largely based on laboratory culture experiments. Here, the range of these algicidal interactions is discussed, including specificity of bacterial control, mechanisms for activity, and insights into the chemical and biochemical analysis of these interactions. The development of algicidal bacteria or compounds derived from bacteria for control of harmful algal blooms is reviewed with a focus on environmentally friendly or sustainable methods of application. Potential avenues for future research and further development and application of bacterial algicides for the control of algal blooms are presented.

Dynamic Responses of Endosymbiotic Microbial Communities Within Microcystis Colonies in North American Lakes to Altered Nitrogen, Phosphorus, and Temperature Levels

The toxic cyanobacterium, Microcystis, is a pervasive cyanobacterial harmful algal bloom (CHAB) - forming genus that naturally occurs in colonies that harbor diverse microbiomes of heterotrophic bacteria. While the effects of nutrient loading and climatic warming on CHABs are well-known, little is known regarding how these environmental drivers alter the structural and functional potential of the microbial assemblages associated with blooms that, in turn, may impact cyanobacterial growth. Here, we used next-generation sequencing of 16S ribosomal rRNA genes to characterize the dynamics of the bacterial assemblages within Microcystis colonies in two temperate North American lakes: Lake Erie and Lake Agawam (NY, United States) and quantified their responses to experimentally increased levels of nitrogen (N), phosphorus (P) and temperature. Across experiments, Microcystis populations were consistently and significantly promoted by N and, to a lesser extent, elevated temperature (p < 0.05). In contrast, bacterial assemblages within Microcystis colonies were more resilient to environmental perturbations, with the relative abundance of 7–16% of amplicon sequence variants changing and several individual taxa displaying significant (p < 0.05) increases and decreases in relative abundance, primarily in response to elevated temperature and to a lesser extent, N. In contrast to individual taxa, community diversity was not significantly altered by individual treatments during experiments but rather was inversely correlated with the intensity of Microcystis blooms (p < 0.001). While predicted metabolic function was even less impacted by environmental drivers than microbial diversity, the predicted abundance of nitrogenase (nifH), alkaline phosphatase (phoX), and urease (ure) genes significantly increased in response to N but decreased in response to increased temperature (p < 0.05). Collectively, the resilience of microbial community structure and function within colonies suggests they may support the ability of Microcystis to persist through short-term fluctuations in environmental conditions by supplying essential nutrients.

[Elucidation of Phenomena Involving Cyanobacteria in Freshwater Ecosystem by Chemically Ecological Approach]

Lakes Sagami and Tsukui are reservoirs constructed by connecting to the Sagami River. Because of eutrophication of the lakes, cyanobacteria have appeared every year. This review deals with phenomena related to occurrence of cyanobacteria that have been observed for 40 years since 1974 at the lakes. These 40 years of observations raised three interesting issues including the retention of cyanobacteria on their surfaces. These phenomena have been attributed to the usual factors, such as illuminance, nutrition and water temperature, but our research results suggested that they cannot be resolved without the introduction of another factor. We have attempted to elucidate various phenomena involving cyanobacteria in lake ecosystems by chemical ecological methods using volatile organic compounds (VOCs) produced by the cyanobacteria as indicators. One of the VOCs, β-cyclocitral, was significantly involved in the above phenomena, which was considered to be produced by the carotenoid cleavage dioxygenase (CCD) of the cyanobacteria. β-Cyclocitral was not produced in the two known CCDs, but two additional CCDs to Microcystis aeruginosa participated to produce the β-cyclocitral. These CCDs did not directly produce β-cyclocitral, but it was accumulated in cells as their precursors. The released β-cyclocitral underwent a Baeyer-Villiger-like oxidation. It was speculated that Microcystis activated the CCD genes through density stress and produced β-cyclocitral, which acted as an allelopathic substance. As a result, the number of cells of cyanobacteria decreased, and the resulting nitrogen and phosphorus were fed to the living cyanobacteria. It is postulated that this "quorum sensing" was functioning in the above-mentioned issues.

Non-targeted metabolomic profiling of filamentous cyanobacteria Aphanizomenon flos-aquae exposed to a concentrated culture filtrate of Microcystis aeruginosa

Microcystis and Aphanizomenon are two toxic cyanobacteria genera, which frequently cause blooms in freshwater lakes. In some cases, succession of these two genera was observed in natural water bodies. Among the diverse factors contributing to such succession of dominant cyanobacterial genera, an allelopathic effect was proposed to be involved after the growth inhibitory effect of several Microcystis species on A. flos-aquae was investigated. However, the response of target species exposed to Microcystis are poorly described. In the present study, we used two toxic cyanobacteria strains, Aphanizomenon flos-aquae (Aph1395) and Microcystis aeruginosa strain 905 (Ma905) as research subjects. Aph1395 was inhibited with a necessarily concentrated culture filtrate of Ma905 (MA905-SPE), and the response of the inhibited Aph1395 cells was explored via non-targeted metabolomic profiling. In total, 3735 features were significantly different in the Aph1395 treated with Ma905-SPE vs. those treated with BG11 medium. Among them, the annotations of 146 differential features were considered to be confident via MS/MS spectrum matching analysis. Based on the reported physiological functions of the annotated differential features, we proposed a putative model that in the growth-inhibited Aph1395, a suite of increased or decreased features with activities in apoptosis, growth inhibition, and stress response processes contributed to, or defended against, the allelopathic effect caused by Ma905. Our findings provide insights into the interaction between the bloom forming cyanobacterial species that share the same ecological environment.

Differences in susceptibility of cyanobacteria species to lytic volatile organic compounds and influence on seasonal succession

Cyanobacteria produce numerous volatile organic compounds (VOCs) that show a lytic activity against other cyanobacteria. We found the lytic phenomenon under natural conditions and during densification experiments, and also observed the species change of the cyanobacteria during the lysis processes, in which Microcystis finally became dominant. The species change of the cyanobacteria was strongly suggested to depend on the susceptibility of the cyanobacteria toward the VOCs. To verify this suggestion, the susceptibility of the species was evaluated by the minimal inhibitory concentration (MIC) using axenic cyanobacterial strains against β-cyclocitral, its oxidation products and β-ionone with the aid of log D. It was found that the difference depended on the susceptibility of the cyanobacteria toward the VOCs, in which β-cyclocitral played a crucial role and Microcystis had a significantly protective ability compared to the other cyanobacteria. In addition, the species change of cyanobacteria was consistent with the cyanobacterial seasonal succession in Lakes Sagami and Tsukui, based on data that had been accumulated for 10 years. Conventionally, although this phenomenon could be explained by nutrient availability or the physical structure of the environment, the results of this study revealed that it was controlled by the VOCs, particularly β-cyclocitral produced by the cyanobacteria.

Cyanobacterial community succession and associated cyanotoxin production in hypereutrophic and eutrophic freshwaters

Cyanobacterial harmful algal blooms (cyanoHABs) in freshwater bodies are mainly attributed to excess loading of nutrients [nitrogen (N) and phosphorus (P)]. This study provides a comprehensive review of how the existing nutrient (i.e., N and P) conditions and microbial ecological factors affect cyanobacterial community succession and cyanotoxin production in freshwaters. Different eutrophic scenarios (i.e., hypereutrophic vs. eutrophic conditions) in the presence of (i) high levels of N and P, (ii) a relatively high level of P but a low level of N, and (iii) a relatively high level of N but a low level of P, are discussed in association with cyanobacterial community succession and cyanotoxin production. The seasonal cyanobacterial community succession is mostly regulated by temperature in hypereutrophic freshwaters, where both temperature and nitrogen fixation play a critical role in eutrophic freshwaters. While the early cyanoHAB mitigation strategies focus on reducing P from water bodies, many more studies show that both N and P have a profound contribution to cyanobacterial blooms and toxin production. The availability of N often shapes the structure of the cyanobacterial community (e.g., the relative abundance of N₂-fixing and non-N₂-fixing cyanobacterial genera) and is positively linked to the levels of microcystin. Ecological aspects of cyanotoxin production and release, related functional genes, and corresponding nutrient and environmental conditions are also elucidated. Research perspectives on cyanoHABs and cyanobacterial community succession are discussed and presented with respect to the following: (i) role of internal nutrients and their species, (ii) P- and N-based control vs. solely P-based control of cyanoHABs, and (iii) molecular investigations and prediction of cyanotoxin production.

A critical review on operation and performance of source water control strategies for cyanobacterial blooms: Part II-mechanical and biological control methods

This review summarizes current knowledge on mechanical (artificial mixing, hypolimnetic aeration, dredging, and sonication) and biological (biomanipulation, macrophytes, and straws) methods for the management of cyanobacterial blooms in drinking water sources. Emphasis has been given to (i) the mechanism of cyanobacterial control, (ii) successful and unsuccessful case studies, and (iii) factors influencing successful implementation. Most mechanical and biological control strategies offer long-term control. However, their application can be cost-prohibitive and treatment efficacy is influenced by source water geometry and continual nutrient inputs from external sources. When artificial mixing and hypolimnetic oxygenation units are optimized based on source water characteristics, observed water quality benefits included increased dissolved oxygen contents, reduced internal loading of nutrients, and lower concentrations of reduced ions . Treatment efficacy during oxygenation and aeration was derailed by excessive sedimentation of organic matter and sediment characteristics such as low Fe/P ratios. Dredging is beneficial for contaminated sediment removal, but it is too costly to be a practical bloom control strategy for most systems. Sonication control methods have contradictory findings requiring further research to evaluate the efficacy and applicability for field-scale control of cyanobacteria. Biological control methods such as biomanipulation offer long-term treatment benefits; however, investigations on the mechanisms of field-scale cyanobacterial control are still limited, particularly with the use of macrophytes and straws. Each control method has site-specific strengths, limitations, and ecological impacts. Reduction of external nutrient inputs should still be a significant focus of restoration efforts as treatment benefits from mechanical and biological control were commonly offset by continued nutrient inputs.

Bioavailable Nutrients (N and P) and Precipitation Patterns Drive Cyanobacterial Blooms in Missisquoi Bay, Lake Champlain

Anthropogenic activities release large amounts of nitrogen (N) and phosphorus (P) nutrients into the environment. Sources of nutrients include surface and sub-surface runoffs from agricultural practices with the application of chemical fertilizers and manure as well as combined sewer overflows (CSOs). Nutrient runoffs contribute to the eutrophication of aquatic ecosystems and enhance the growth of cyanobacteria. Precipitation is an important driving force behind the runoff of nutrients from agricultural fields into surrounding water bodies. To understand the dynamics between nutrient input, precipitation and cyanobacterial growth in Missisquoi Bay, Lake Champlain (Quebec), one location in Pike River (a major tributary into the bay) and four locations in Missisquoi Bay were monitored from April to November in 2017 and 2018. Biweekly water samples were analyzed using chemical methods and high-throughput sequencing of 16S rRNA gene amplicons. High concentrations of N and P were typically measured in April and May. Three major spikes in nutrient concentrations were observed in early and mid-summer as well as early fall, all of which were associated with intense cumulative precipitation events of 40 to 100 mm within 7 days prior to sampling. Despite the high concentrations of nutrients in the spring and early summer, the cyanobacterial blooms appeared in mid to late summer as the water temperature increased. Dolichospermum sp. was the major bloom-forming cyanobacterium during both summers. A second intense bloom event of Microcystis was also observed in the fall (October and November) for both years. Variation in the cyanobacteria population was strongly associated with inorganic and readily available fractions of N and P such as nitrites and nitrates (NOx), ammonia (NH₃) and dissolved organic phosphorus (DOP). During blooms, total Kjeldahl nitrogen (TKN) and total particulate phosphorus (TPP) fractions had a substantial influence on total nitrogen (TN) and total phosphorus (TP) concentrations, respectively. The abundance of bacteria involved in the metabolism of nitrogen compared to that of phosphorus revealed the importance of nitrogen on overall microbial dynamics as well as CB formation in the bay. Our findings emphasize the combined influence of precipitation events, temperature and several bioavailable fractions of nitrogen and phosphorus on cyanobacterial bloom episodes.

Toxigenic Microcystis aeruginosa (Cyanobacteria) affects the population growth of two common green microalgae: Evidence of other allelopathic metabolites different to cyanotoxins

Agriculture runoffs and discharge of wastewaters are the major causes of eutrophication. Although eutrophication could promote the thriving of any phytoplankter, harmful algal blooms (HABs) are dominated frequently by cyanobacteria. Currently, HABs dominated by the toxigenic cyanobacterium Microcystis aeruginosa in lakes and reservoirs are the main environmental concerns worldwide. This study aimed to determine how M. aeruginosa (Ma) modifies the population growth of Pseudokirchneriella subcapitata (Ps) and Ankistrodesmus falcatus (Af). Growth kinetics were determined for each species and in the combinations: Ps-Ma, Af-Ma, Af-Ps, and Ps-Af-Ma. At the end of experiments, photosynthetic pigments, phycobiliproteins, and microcystins were quantified. A logistic equation significantly described the growth trend for all of the tested species, enabling the identification of negative effects on early stages in the population growth of co-cultures with the cyanobacterium; in addition, the interaction effects on the growth rate and in the maximum attainable population density were determined. The biomasses of A. falcatus and P. subcapitata were significantly higher when cultured individually than in all of the combinations with the cyanobacterium. The concentrations of chlorophyll a and b, as well as carotenoids, were lower in combined cultures, but phycobiliprotein content in the cultures with M. aeruginosa was not significantly affected. Microcystis aeruginosa negatively affected the growth of the microalgae, but A. falcatus was significantly more inhibited than P. subcapitata; however, microcystin concentrations were significantly reduced in the co-cultures with microalgae. These results could help to explain the displacements of microalgae when cyanobacteria are present, giving rise to cyanobacterial blooms in eutrophic freshwaters.

Grazer-Induced Chemical Defense in a Microcystin-Producing Microcystis aeruginosa (Cyanobacteria) Exposed to Daphnia gessneri Infochemicals

Cyanobacteria are photosynthetic microorganisms that compose phytoplankton and therefore have a trophic relationship with zooplankton, which represent an important link for energy flux in aquatic food webs. Several species can form blooms and produce bioactive metabolites known as cyanotoxins. However, the ecological and adaptative role of these toxins are still under debate. Many studies have addressed the cyanotoxins' function in defense against herbivory when grazing pressure by zooplankton plays a role in phytoplankton top-down control. Thus, the present study evaluated the ecophysiological responses of the cyanobacterial strain Microcystis aeruginosa NPLJ-4 underlying the chemical induced defense against the cladoceran Daphnia gessneri. Exposure to predator infochemicals consisted of cultures established in ASM-1 medium prepared in a filtrate from a culture of adults of D. gessneri at an environmentally relevant density. Daphnia infochemicals promoted a significant increase in toxin production by M. aeruginosa. However, no differences in growth were observed, despite a significant increase in both maximum photosynthetic efficiency and electron transport rate in response to zooplankton. Additionally, there was no significant variation in the production of exopolysaccharides. Overall, although a grazer-induced defense response was demonstrated, there were no effects on M. aeruginosa fitness, which maintained its growth in the presence of Daphnia alarm cues.

In-situ responses of phytoplankton to graphene photocatalysis in the eutrophic lake Xingyun, southwestern China

Graphene photocatalysis is receiving increased attention for its potential to be used as a novel green technology for mitigating harmful algae in highly eutrophic waters. However, graphene is seldom applied to in situ aquatic ecosystems for environmental applications. Here, the impacts of graphene photocatalysis on phytoplankton and environmental conditions were evaluated through an in situ macrocosm experiment in the eutrophic Lake Xingyun, southwestern China. The graphene photocatalysis treated area had significantly reduced conductivity, total nitrogen (TN), total phosphorus (TP) and dissolved phosphorus concentrations, as well as increased dissolved oxygen (DO) concentrations. The abundances of all species of the genus Microcystis were significantly reduced in the graphene photocatalysis-treated area; in contrast, the abundances of all species of the diazotrophic genera, including Anabaena and Aphanizomenon, greatly increased after treatment with graphene photocatalysis. Eukaryotic algae, especially Chlorophyta, Euglenophyta and Pyrrophyta, as well as Cryptophyta, had significantly higher abundances in the graphene photocatalysis-treated area, whereas most of the eutrophic diatom species had lower abundances in the treated area. These observed differences in eukaryotic algae between the two groups might be related to their sensitivity to graphene photocatalysis and their tolerance of nutrients. Generally, graphene photocatalysis can make a great contribution to the improvement of eutrophic water, as evidenced by the reduction in cyanobacteria abundance and phosphorus concentration, as well as the increase in species richness and the dissolved oxygen concentration in the treated area. However, the mechanisms underlying these differences in phytoplankton community structure and environmental conditions require further study.

Reviewing Interspecies Interactions as a Driving Force Affecting the Community Structure in Lakes via Cyanotoxins

The escalating occurrence of toxic cyanobacterial blooms worldwide is a matter of concern. Global warming and eutrophication play a major role in the regularity of cyanobacterial blooms, which has noticeably shifted towards the predomination of toxic populations. Therefore, understanding the effects of cyanobacterial toxins in aquatic ecosystems and their advantages to the producers are of growing interest. In this paper, the current literature is critically reviewed to provide further insights into the ecological contribution of cyanotoxins in the variation of the lake community diversity and structure through interspecies interplay. The most commonly detected and studied cyanobacterial toxins, namely the microcystins, anatoxins, saxitoxins, cylindrospermopsins and β-N-methylamino-L-alanine, and their ecotoxicity on various trophic levels are discussed. This work addresses the environmental characterization of pure toxins, toxin-containing crude extracts and filtrates of single and mixed cultures in interspecies interactions by inducing different physiological and metabolic responses. More data on these interactions under natural conditions and laboratory-based studies using direct co-cultivation approaches will provide more substantial information on the consequences of cyanotoxins in the natural ecosystem. This review is beneficial for understanding cyanotoxin-mediated interspecies interactions, developing bloom mitigation technologies and robustly assessing the hazards posed by toxin-producing cyanobacteria to humans and other organisms.

Effect of ultraviolet radiation (type B) and titanium dioxide nanoparticles on the interspecific interaction between Microcystis flos-aquae and Pseudokirchneriella subcapitata

Anthropogenic activities have led to the depletion of the ultraviolet radiation screening ozone layer, exposing aquatic biota to its harmful effects. Also, the rising applications of nanotechnology are resulting in the release and contamination of aquatic ecosystems with engineered nanometals like titanium dioxide nanoparticles (nTiO₂). The rise in ultraviolet radiation interacts with nanometals, increasing their bioactivities to susceptible aquatic organisms such as algae and cyanobacteria. The effect of ultraviolet radiation B (UVB) and nTiO₂ on Microcystis flos-aquae and Pseudokirchneriella subcapitata during inter-specific interaction was investigated. The specific growth rate (d^-1) of M. flos-aquae exposed to nTiO₂ increased significantly under monoculture conditions but was suppressed during co-culture with P. subcapitata. Contrarily, UVB stimulated the growth of the cyanobacterium regardless of the presence or absence of the green microalgae. However, there was a general decline in the growth of P. subcapitata following cultivation with M. flos-aquae and exposure to UVB and nTiO₂. The chlorophyll-a and total chlorophyll content of the monocultures of M. flos-aquae exposed to nTiO₂ increased while other co-culture treatments significantly decreased these parameters. The experimental treatments, UVB, nTiO₂, and UVB + nTiO₂ had differential effects on the pigment content of P. subcapitata. The total protein content, intracellular H₂O₂, peroxidase (POD), and glutathione S-transferase (GST) activity of both M. flos-aquae and P. subcapitata increased at varying degrees as a function of the treatment condition. Microcystin content was highest in co-cultures exposed to UVB. The results of this study suggest that increasing levels of nTiO₂ and UVB significantly alter the growth and cellular metabolic activity of M. flos-aquae and P. subcapitata, but the cyanobacterium will probably be favored by increasing UVB levels and its interaction with nanometals like nTiO₂ in aquatic ecosystems.

Environmental factors associated with cyanobacterial assemblages in a mesotrophic subtropical plateau lake: A focus on bloom toxicity

Harmful algal blooms caused by cyanobacteria have been increasing in frequency worldwide. However, the main environmental drivers of this change are often difficult to identify because of the effects of the interaction between eutrophication and climate change. Recently, filamentous N₂-fixing cyanobacteria and non-diazotrophic Microcystis have been observed to be co-existing and undergoing succession in some eutrophic lakes. However, the succession patterns of dominant cyanobacteria and the factors driving this in mesotrophic lakes are not well understood. We hypothesized that the changes in cyanobacterial assemblages in mesotrophic lakes could result in a relatively high risks of toxic blooms, and that these changes are associated with the global climatic changes. We tested these hypotheses using data from the subtropical mesotrophic Lake Erhai. We found that the high spatiotemporal variability in the cyanobacterial community, and the increase in biomass were driven primarily by the growth of bloom-forming cyanobacterial taxa. Species-specific biomasses were related to a different environmental stressor; increases in dissolved organic carbon (DOC) concentrations were statistically associated with an increase of Microcystis biomass, whereas increases in surface water temperature favored higher biomass of Pseudanabaena at low transparency and high concentration of phosphorus. In addition, low nitrogen- to- phosphorus ratios were identified as potential determinants of the abundance of N₂-fixing Dolichospermum. Furthermore, changes in the concentration of DOC, total nitrogen, pH and water transparency levels were found to affect the composition of Microcystis morphotypes and genotypes mostly. This study highlights that the toxic to non-toxic Microcystis ratio might increase with the water darkening and browning (which occurs in many subtropical plateau lakes). Lake management strategies, therefore, need to consider the toxicity of cyanobacterial assemblages in mesotrophic lakes over the intensity of the cyanobacterial blooms.

Cyanobacterial Harmful Algal Blooms in Aquatic Ecosystems: A Comprehensive Outlook on Current and Emerging Mitigation and Control Approaches

An intensification of toxic cyanobacteria blooms has occurred over the last three decades, severely affecting coastal and lake water quality in many parts of the world. Extensive research is being conducted in an attempt to gain a better understanding of the driving forces that alter the ecological balance in water bodies and of the biological role of the secondary metabolites, toxins included, produced by the cyanobacteria. In the long-term, such knowledge may help to develop the needed procedures to restore the phytoplankton community to the pre-toxic blooms era. In the short-term, the mission of the scientific community is to develop novel approaches to mitigate the blooms and thereby restore the ability of affected communities to enjoy coastal and lake waters. Here, we critically review some of the recently proposed, currently leading, and potentially emerging mitigation approaches in-lake novel methodologies and applications relevant to drinking-water treatment.

The genetic and ecophysiological diversity of Microcystis

Microcystis is a cyanobacterium that forms toxic blooms in freshwater ecosystems around the world. Biological variation among taxa within the genus is apparent through genetic and phenotypic differences between strains and via the spatial and temporal distribution of strains in the environment, and this fine-scale diversity exerts strong influence over bloom toxicity. Yet we do not know how varying traits of Microcystis strains govern their environmental distribution, the tradeoffs and links between these traits, or how they are encoded at the genomic level. Here we synthesize current knowledge on the importance of diversity within Microcystis and on the genes and traits that likely underpin ecological differentiation of taxa. We briefly review spatial and environmental patterns of Microcystis diversity in the field and genetic evidence for cohesive groups within Microcystis. We then compile data on strain-level diversity regarding growth responses to environmental conditions and explore evidence for variation of community interactions across Microcystis strains. Potential links and tradeoffs between traits are identified and discussed. The resulting picture, while incomplete, highlights key knowledge gaps that need to be filled to enable new models for predicting strain-level dynamics, which influence the development, toxicity and cosmopolitan nature of Microcystis blooms.

Factors controlling phytoplankton dynamics in an arid reservoir in Tunisia (case of Sidi Saad dam)

Temporal variation of the phytoplankton community and environmental factors were investigated over 8 months: from January 2018 to November 2018 in the Sidi Saad reservoir (central Tunisia): May, June (spring season), July, August (summer season) September, October and November (autumn season), and January (winter season). The relationships between phytoplankton and environmental factors were explored using Canonical Correspondence Analysis (CCA). General linear models (GLMs) were used to predict the phytoplankton abundance. Analysis of variance (ANOVA) was used to test the hypothesis that the abundance of each group of taxa differed between sampling months. The results of chemical analysis of the reservoir showed that the environment was enriched in nitrates, nitrites, ammonium, and orthophosphate, especially in May. Carlson's trophic index using average chemical variables showed that Sidi Saad has a mesotrophic statute. There is temporal phytoplankton succession in the Sidi Saad reservoir. Cyanophyceae dominated over the year except in winter and late autumn (November and January). Chlorophyceae was the dominant group in winter month. The CCA results showed that six environmental factors, orthophosphates (PO₄^3-), total phosphates (TP), Secchi disc depth (SD), total nitrogen (TN), temperature (T), and nitrite (NO₂^-) had significant influences on the changes in phytoplankton. GLM showed that PO₄^3-, TP, TN, SD, and T were the significant predictors of phytoplankton abundance. Phytoplankton composition is largely dominated by the species Microcystis aeruginosa which formed a bloom with excessive abundance (up to 89.76 billion cell l^-1 in spring). We recommend banning the fishing and their consumption during the period of Microcystis bloom and installing a system of biomonitoring of the levels of toxins in the water.

Microfluidic and mathematical modeling of aquatic microbial communities

Aquatic microbial communities contribute fundamentally to biogeochemical transformations in natural ecosystems, and disruption of these communities can lead to ecological disasters such as harmful algal blooms. Microbial communities are highly dynamic, and their composition and function are tightly controlled by the biophysical (e.g., light, fluid flow, and temperature) and biochemical (e.g., chemical gradients and cell concentration) parameters of the surrounding environment. Due to the large number of environmental factors involved, a systematic understanding of the microbial community-environment interactions is lacking. In this article, we show that microfluidic platforms present a unique opportunity to recreate well-defined environmental factors in a laboratory setting in a high throughput way, enabling quantitative studies of microbial communities that are amenable to theoretical modeling. The focus of this article is on aquatic microbial communities, but the microfluidic and mathematical models discussed here can be readily applied to investigate other microbiomes.

Community composition and functional genes explain different ecological roles of heterotrophic bacteria attached to two bloom-forming cyanobacterial genera

Eutrophication leads to frequent outbreaks of cyanobacterial blooms, however, the effect of heterotrophic bacteria attached to cyanobacterial cells is unclear. Field investigations were carried out to gain a deeper understanding of the community composition and functional role of heterotrophic bacteria attached to Dolichospermum and Microcystins cells. The significantly positive relationships between Dolichospermum density and total nitrogen (TN) and between Microcystins density and particle nitrogen (PN) indicated the strong nitrogen (N) demand of these two species. The lack of functional genes that mediate the nitrification process in bacteria attached to both Microcystins and Dolichospermum cells indicated that these two genera preferred ammonium (NH₄⁺-N). Dolichospermum cells obtained more available N through N₂ fixation, which was expressed by high nitrogenase gene abundance. Bacteria attached to Microcystins cells showed a higher activity of leucine aminopeptidase and a significantly higher abundance of functional genes that mediate dissimilatory nitrate reduction to ammonium (DNRA) than those attached to Dolichospermum cells. The significantly higher abundance of carbon degradation genes and β-glucosidase activity of bacteria attached to Microcystins cells compared with those of bacteria attached to Dolichospermum cells suggested that abundant organic carbon was bound to Microcystins cells, which is a prerequisite for DNRA. In addition, Microcystins cells exhibited a great advantage in soluble reactive phosphorus (SRP) production through high levels of organic phosphorus (P) hydrolysis associated with high levels of phosphatase genes of attached bacteria. In conclusion, bacteria attached to Microcystins cells performed more important functions on NH₄⁺-N and SRP production through ammonification and DNRA, as well as phosphatase hydrolysis respectively, compared to those attached to Dolichospermum. Thus, algal growth is the result of different variables such as nutrient concentration, their ratio and the microbial ability.

The combined effect of clethodim (herbicide) and nitrogen variation on allelopathic interactions between Microcystis aeruginosa and Raphidiopsis raciborskii

The large-scale use of herbicides deteriorates water quality and threatens aquatic biodiversity. Unfortunately, there are few studies on the ecological effects of herbicides on toxin-producing strains of cyanobacteria under changing nutrient conditions. The objective of the present study was to investigate the effects of the herbicide clethodim and nitrogen variation on the allelopathic interactions and toxin production of Microcystis aeruginosa BCCUSP232 and Raphidiopsis raciborskii (formerly known as Cylindrospermopsis raciborskii) ITEPA1. M. aeruginosa had increased cell density when exposed to the clethodim (H +) (23.55 mg/L), whereas the highest cell density of R. raciborskii was observed in the treatment with clethodim plus limited nitrogen. Also, the cell-free exudate of R. raciborskii significantly stimulated the growth of M. aeruginosa on day 3 of the experiment. The concentration of chlorophyll-a in M. aeruginosa cultures generally increased in all the treatments, while in R. raciborskii cultures, the opposite occurred. Total microcystins (MCs) content of M. aeruginosa in the mixed cultures was 68% higher in nitrogen-enriched conditions than the control. A similar increase in MC content occurred in M. aeruginosa unialgal culture treated with R. raciborskii exudate. Total saxitoxin concentration was 81% higher in mixed cultures of R. raciborskii simultaneously exposed to high nitrogen and clethodim. Similarly, unialgal cultures of R. raciborskii exposed to either high nitrogen or clethodim had higher saxitoxins concentrations than the control. The intracellular H₂O₂ content of M. aeruginosa cultures decreased, whereas, in R. raciborskii cultures, it increased during exposure to high nitrogen and clethodim. Only R. raciborskii had a significant variation in peroxidase activity. The activities of glutathione S-transferase of both strains were higher in the presence of clethodim. These results revealed that nitrogen enrichment and the presence of clethodim might lead to the excessive proliferation of M. aeruginosa and R. raciborskii and increased production of cyanotoxins in aquatic environments.

Effect of Chlorination on Microbiological Quality of Effluent of a Full-Scale Wastewater Treatment Plant

The evaluation of effluent wastewater quality mainly relies on the assessment of conventional bacterial indicators, such as fecal coliforms and enterococci; however, little is known about opportunistic pathogens, which can resist chlorination and may be transmitted in aquatic environments. In contrast to conventional microbiological methods, high-throughput molecular techniques can provide an accurate evaluation of effluent quality, although a limited number of studies have been performed in this direction. In this work, high-throughput amplicon sequencing was employed to assess the effectiveness of chlorination as a disinfection method for secondary effluents. Common inhabitants of the intestinal tract, such as Bacteroides, Arcobacter and Clostridium, and activated sludge denitrifiers capable of forming biofilms, such as Acidovorax, Pseudomonas and Thauera, were identified in the chlorinated effluent. Chloroflexi with dechlorination capability and the bacteria involved in enhanced biological phosphorus removal, i.e., Candidatus Accumulibacter and Candidatus Competibacter, were also found to resist chlorination. No detection of Escherichia indicates the lack of fecal coliform contamination. Mycobacterium spp. were absent in the chlorinated effluent, whereas toxin-producing cyanobacteria of the genera Anabaena and Microcystis were identified in low abundances. Chlorination significantly affected the filamentous bacteria Nocardioides and Gordonia, whereas Zoogloea proliferated in the disinfected effluent. Moreover, perchlorate/chlorate- and organochlorine-reducing bacteria resisted chlorination.

Cyanobacterial blooms in wastewater treatment facilities: Significance and emerging monitoring strategies

Municipal wastewater treatment facilities (WWTFs) are prone to the proliferation of cyanobacterial species which thrive in stable, nutrient-rich environments. Dense cyanobacterial blooms frequently disrupt treatment processes and the supply of recycled water due to their production of extracellular polymeric substances, which hinder microfiltration, and toxins, which pose a health risk to end-users. A variety of methods are employed by water utilities for the identification and monitoring of cyanobacteria and their toxins in WWTFs, including microscopy, flow cytometry, ELISA, chemoanalytical methods, and more recently, molecular methods. Here we review the literature on the occurrence and significance of cyanobacterial blooms in WWTFs and discuss the pros and cons of the various strategies for monitoring these potentially hazardous events. Particular focus is directed towards next-generation metagenomic sequencing technologies for the development of site-specific cyanobacterial bloom management strategies. Long-term multi-omic observations will enable the identification of indicator species and the development of site-specific bloom dynamics models for the mitigation and management of cyanobacterial blooms in WWTFs. While emerging metagenomic tools could potentially provide deep insight into the diversity and flux of problematic cyanobacterial species in these systems, they should be considered a complement to, rather than a replacement of, quantitative chemoanalytical approaches.

Diversity Assessment of Toxic Cyanobacterial Blooms during Oxidation

Fresh-water sources of drinking water are experiencing toxic cyanobacterial blooms more frequently. Chemical oxidation is a common approach to treat cyanobacteria and their toxins. This study systematically investigates the bacterial/cyanobacterial community following chemical oxidation (Cl₂, KMnO₄, O₃, H₂O₂) using high throughput sequencing. Raw water results from high throughput sequencing show that Proteobacteria, Actinobacteria, Cyanobacteria and Bacteroidetes were the most abundant phyla. Dolichospermum, Synechococcus, Microcystis and Nostoc were the most dominant genera. In terms of species, Dolichospermum sp.90 and Microcystis aeruginosa were the most abundant species at the beginning and end of the sampling, respectively. A comparison between the results of high throughput sequencing and taxonomic cell counts highlighted the robustness of high throughput sequencing to thoroughly reveal a wide diversity of bacterial and cyanobacterial communities. Principal component analysis of the oxidation samples results showed a progressive shift in the composition of bacterial/cyanobacterial communities following soft-chlorination with increasing common exposure units (CTs) (0–3.8 mg·min/L). Close cyanobacterial community composition (Dolichospermum dominant genus) was observed following low chlorine and mid-KMnO₄ (287.7 mg·min/L) exposure. Our results showed that some toxin producing species may persist after oxidation whether they were dominant species or not. Relative persistence of Dolichospermum sp.90 was observed following soft-chlorination (0.2–0.6 mg/L) and permanganate (5 mg/L) oxidation with increasing oxidant exposure. Pre-oxidation using H₂O₂ (10 mg/L and one day contact time) caused a clear decrease in the relative abundance of all the taxa and some species including the toxin producing taxa. These observations suggest selectivity of H₂O₂ to provide an efficient barrier against toxin producing cyanobacteria entering a water treatment plant.

Stringent Response Regulates Stress Resistance in Cyanobacterium Microcystis aeruginosa

Cyanobacterial blooms are serious environmental issues in global freshwater ecosystems. Nitrogen limitation is one of the most important strategies to control cyanobacterial blooms. However, recent researches showed that N limitation does not effectively control the bloom; oppositely, N limitation induces N-fixing cyanobacterial blooms. The mechanism underlying this ecological event is elusive. In this study, we found that N limitation enhances stress tolerance of Microcystis aeruginosa by triggering stringent response (SR), one of the most important bacterial adaptive responses to environmental stresses. Initiation of SR exerted protective effects on the cells against salt and oxidative stresses by promoting colony formation, maintaining membrane integrity, increasing photosynthetic performance, reducing ROS production, upregulating stress-related genes, etc. These protections possibly help M. aeruginosa maintain their population number during seasonal N limitation. As SR has been proven to be involved in nitrogen fixing under N limitation conditions, the potential role of SR in driving the shift and succession of cyanobacterial blooms was discussed. Our findings provide cellular evidence and possible mechanisms that reducing N input is ineffective for bloom control.