Effects of rising atmospheric CO2 levels on physiological response of cyanobacteria and cyanobacterial bloom development: A review

Sulfate assimilation regulates antioxidant defense response of the cyanobacterium Synechococcus elongatus PCC 7942 to high concentrations of carbon dioxide

The adaptive evolution of cyanobacteria over a prolonged period has allowed them to utilize carbon dioxide (CO2) at the low concentrations found in the atmosphere (0.04% CO2) for growth. However, whether the exposure of cyanobacteria to high concentrations of CO2 results in oxidative stress and the activation of antioxidant defense response remains unknown, albeit fluctuations in other culture conditions have been reported to exert these effects. The current study reveals the physiological regulation of the model cyanobacterium Synechococcus elongatus PCC 7942 upon exposure to 1% CO2 and the underlying mechanism. Exposure to 1% CO2 was demonstrated to induce oxidative stress and activate antioxidant defense responses in S. elongatus. Further analysis of variations in metabolism between S. elongatus cells grown at 0.04% CO2 and exposed to 1% CO2 revealed that sulfate assimilation was enhanced after the exposure to 1% CO2. A strain of S. elongatus lacking the gene cysR, encoding a global transcriptional regulator for genes involved in sulfate assimilation, was generated by deleting the gene from the genomic DNA. A comparative analysis of the wild-type and cysR-null strains indicated the regulation of the antioxidant response by sulfate assimilation. In addition, lines of evidence were presented that suggest a role of degradation of phycobilisome in the antioxidant response of S. elongatus under conditions of 1% CO2 and sulfate limitation. This study sheds light on the in situ effects of high CO2-induced stress on the ecophysiology of cyanobacteria upon exposure to diverse scenarios from a biotechnological and ecological perspective.IMPORTANCECyanobacteria that grow autotrophically with CO2 as the sole carbon source can be subject to high-CO2 stress in a variety of biotechnological and ecological scenarios. However, physiological regulation of cyanobacteria in response to high-CO2 stress remains elusive. Here, we employed microbial physiological, biochemical, and genetic techniques to reveal the regulatory strategies of cyanobacteria in response to high-CO2 stress. This study, albeit physiological, provides a biotechnological enterprise for manipulating cyanobacteria as the chassis for CO2 conversion and sheds light on the in situ ecological effects of high CO2 on cyanobacteria.

Effects of elevated atmospheric carbon dioxide to Microcystis aeruginosa under different forms of phosphorus sources

Human activities have led to an increase in atmospheric carbon dioxide (CO2) concentration, which can enhance the flux of CO2 from air to water, thus impacting algal growth. Phosphorus (P) is a key factor influencing the formation of cyanobacteria blooms. Nutrient utilization is closely related to carbon (C) metabolism, but the effects of elevated CO2 on microalgae under different P sources are rarely studied. In this study, we investigated the growth and physiological and biochemical responses of Microcystis aeruginosa (M. aeruginosa) under ambient (400 ppm) and elevated (550 ppm) CO2 levels in P-free, dissolved inorganic P (DIP, 1 mg P/L), and dissolved organic P (DOP, 1 mg P/L) groups. The bioavailability of DIP to M. aeruginosa was greater than that of DOP, and elevated CO2 increased both the uptake of DIP and DOP. Elevated CO2 promoted the growth (increasing by 9.0%-14.2%), photosynthesis, and CO2 fixation of M. aeruginosa under different P sources (P-free, DIP, DOP), and increased total microcystin-LR content (increasing by 5.4%-12.6%), which increased the risk of microcystin-LR release into the environment. Furthermore, elevated CO2 aggravated the stress effect of DOP, leading to an increase in protein content and proportion of humic acid substances in the extracellular polymeric substances. Our study provides a theoretical basis for understanding the impact of elevated CO2 on cyanobacteria bloom under different P sources, and provides a new insight for the control of eutrophic waters under the background of climate change.

The substantial generation of photochemically produced reactive intermediates (PPRIs) in algae-type zones from one large shallow lake promoted the removal of organic pollutants

Photochemically produced reactive intermediates (PPRIs) are ubiquitously present in aquatic systems and hold significant importance in biogeochemical cycles. The photochemical reaction of dissolved organic matter (DOM), known as photosensitizers upon irradiation, is the main pathway for PPRIs generation. However, the PPRIs produced by algal-derived organic matter (ADOM) and their environmental effects remains elusive. This study confirmed that substantial PPRIs were generated by ADOM in the algal-derived areas. UV absorption spectra, fluorescence spectra and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were then indicated a significant correlation between the molecular weight of DOM and the quantum yield of PPRIs, with lower molecular weight of DOM exhibiting a higher potential for PPRIs generation. Orthogonal partial least squares (OPLS) were used to build novel multivariate predictive models for indicating the PPRIs production in algae-type zone. Also, the higher concentrations of PPRIs could significantly removal different kinds of organic pollutants, such as bisphenol A (BPA), sulfadiazine (SDZ) and 17α-ethinylestradiol (EE2). Quenching experiments further elucidated that 3DOM⁎ was the key specie for pollutants degradation, serving as the precursor to generate a series of PPRIs. This study highlighted the importance of PPRIs generated from ADOM in the natural attenuation of pollutants and provided a new insight for understanding the self-purification in aquatic system.

Hydrogen peroxide concentration as an indicator of cyanobacterial response to diurnal variation in light intensity

We measured diurnal variations in oxidative stress conditions of cyanobacteria utilizing field observations and laboratory experiments in order to evaluate photoinhibition effects. On clear summer days, transparent bottles filled with surface water were set up at several depths and were collected every three hours together with the measurement of the photosynthetically active radiation (PAR). In the laboratory experiment, two cyanobacterial species were exposed to gradually increasing and then decreasing light intensities. The samples were analyzed with the PAR-induced (H2O2), along with the total hydrogen peroxide concentrations (total H2O2), the catalase activities (CAT), OD730, protein (Protein), and chlorophyll a (Chl a) contents, and so on. Protein was significantly proportionate with OD730 and Chl a, and was used as an indicator of cell biomass. Increasing PAR, H2O2 concentration increased proportionately with the PAR intensity. Then, an oxidative stress indicator in a cell, H2O2/Protein is given by the PAR divided by cell volume, evaluated by Protein. CAT activity in a cell, far largest among antioxidant activities, solely followed total H2O2/Protein. The prediction model for H2O2/Protein was developed with the sufficient agreement with the experimental and field observation results. The model elucidated that the maximum H2O2/Protein in a day was larger with lower cell density even at the water surface, indicating that the higher photoinhibition was imposed at low density, in addition to the lower attenuation of PAR. These results indicate that H2O2/Protein is an effective biomarker to indicate the stress level of cyanobacteria; the observed levels of H2O2 to freshwater may prove useful in designing the criteria for cyanobacteria management.

Impact of elevated CO2 on soil microbiota: A meta-analytical review of carbon and nitrogen metabolism

In the face of 21st-century challenges driven by population growth and resource depletion, understanding the intricacies of climate change is crucial for environmental sustainability. This review systematically explores the interaction between rising atmospheric CO2 concentrations and soil microbial populations, with possible feedback effects on climate change and terrestrial carbon (C) cycling through a meta-analytical approach. Furthermore, it investigates the enzymatic activities related to carbon acquisition, gene expression patterns governing carbon and nitrogen metabolism, and metagenomic and meta-transcriptomic dynamics in response to elevated CO2 levels. The study reveals that elevated CO2 levels substantially influence soil microbial communities, increasing microbial biomass C and respiration rate by 15 % and upregulating genes involved in carbon and nitrogen metabolism by 12 %. Despite a 14 % increase in C-acquiring enzyme activity, there is a 5 % decrease in N-acquiring enzyme activity, indicating complex microbial responses to CO2 changes. Additionally, fungal marker ratios increase by 14 % compared to bacterial markers, indicating potential ecosystem changes. However, the current inadequacy of data on metagenomic and meta-transcriptomic processes underscores the need for further research. Understanding soil microbial feedback mechanisms is crucial for elucidating the role of rising CO2 levels in carbon sequestration and climate regulation. Consequently, future research should prioritize a comprehensive elucidation of soil microbial carbon cycling, greenhouse gas emission dynamics, and their underlying drivers.

Regional and Longitudinal Dynamics of Cyanobacterial Blooms/Cyanobiome and Cyanotoxin Production in the Great Lakes Area

Cyanobacteria (blue-green algae) are a diverse group of prokaryotic microorganisms that impact global biogeochemical cycles. Under eutrophic conditions, cyanobacterial species can produce cyanotoxins, resulting in harmful algal blooms (cHABs) that degrade water quality and result in economic and recreational losses. The Laurentian Great Lakes, a key global freshwater source, are increasingly affected by these blooms. To understand the underlying mechanisms in cHAB formation, we investigated microcystin levels, cyanotoxin genes/transcripts, and taxonomic/microcystin metabarcoding across three sampling locations in the Canadian Great Lakes region, including Hamilton Harbour, Bay of Quinte, and Three Mile Lake (Muskoka), to observe the regional and longitudinal cyanobacterial dynamics. The results revealed a positive correlation between microcystin levels, the occurrence of cyanobacterial taxonomic/cyanotoxin molecular markers, and the relative widespread abundance of specific dominant cyanobacterial taxa, including Planktothrix, Microcystis, and Dolichospermum. The Cyanobium genus was not observed in Hamilton Harbor samples during late summer (August to September), while it was consistently observed in the Three Mile Lake and Bay of Quinte samples. Notably, Dolichospermum and saxitoxin genes were predominantly higher in Three Mile Lake (an inland lake), suggesting site-specific characteristics influencing saxitoxin production. Additionally, among the potential microcystin producers, in addition to Microcystis, Hamilton Harbour and Bay of Quinte samples showed consistent presence of less dominant microcystin-producing taxa, including Phormidium and Dolichospermum. This study highlights the complexity of cHAB formation and the variability in cyanotoxin production in specific environments. The findings highlight regional and site-specific factors that can influence cyanobacterial taxonomic and molecular profiles, necessitating the integration of advanced molecular technologies for effective monitoring and targeted management strategies.

Carbon transportation, transformation, and sedimentation processes at the land-river-estuary continuum

In terrestrial ecosystems, carbon (C) transportation and C pool transformation processes both occur at the land-river-estuary continuum. Moreover, C budget and C balance processes are generally critical in achieving the C neutrality of terrestrial ecosystems. This study analyzes key C transportation processes at multiple interfaces that collectively constitute the land-river-estuary continuum, discusses C transportation and sedimentation processes at the land-river interface, and reveals aquatic plant C sequestration coupling processes and associated productivity. Transformation mechanisms of inorganic-organic C pools are also investigated here as well as a systematic evaluation of C transport flux within the different interfaces that constitute the land-river-estuary continuum. Results show that the net C sink of terrestrial ecosystems was 1.70 Pg C yr-1, wherein the gross primary productivity (GPP) of global terrestrial vegetation reached 123 Pg C yr-1, while rock weathering also consumed 0.30 Pg C yr-1 of atmospheric carbon dioxide (CO2). Subsequently, the C transported by the land-river-estuary continuum reached 1.70 Pg C yr-1. During this process, 0.20 Pg C is deposited and buried in inland water and 1.00 Pg C escapes from inland water systems each year. Therefore, only 0.85 Pg C is transported to the estuary. Finally, this study clarifies control mechanisms of C transportation and transformation processes at the land-river-estuary continuum. The aim of this study is to provide an important scientific basis for the quantitative analysis of C sources and sinks at the land-river-estuary continuum and C neutrality of the biosphere.

Effects of carbon limitation and carbon fertilization on karst lake-reservoir productivity

Nitrogen and phosphorus are universally recognized as limiting elements in the eutrophication processes affecting the majority of the world's lakes, reservoirs, and coastal ecosystems. However, despite extensive research spanning several decades, critical questions in eutrophication science remain unanswered. For example, there is still much to understand about the interactions between carbon limitation and ecosystem stability, and the availability of carbon components adds significant complexity to aquatic resource management. Mounting evidence suggests that aqueous CO2 could be a limiting factor, influencing the structure and succession of aquatic plant communities, especially in karstic lake and reservoir ecosystems. Moreover, the fertilization effect of aqueous CO2 has the potential to enhance carbon sequestration and phosphorus removal. Therefore, it is important to address these uncertainties to achieve multiple positive outcomes, including improved water quality and increased carbon sinks in karst lakes and reservoirs.

Transcriptomic response analysis of ultraviolet mutagenesis combined with high carbon acclimation to promote photosynthetic carbon assimilation in Euglena gracilis

The potential of Euglena gracilis for carbon sequestration offers significant opportunities in the capture and utilization of carbon dioxide (CO2). In this study, a mutant LE-ZW of E. gracilis, capable of efficient growth and carbon sequestration, was obtained through ultraviolet mutagenesis combined with high carbon acclimation. Subsequently, the potential of LE-ZW for carbon assimilation was systematically analyzed. The results demonstrated that the cell density of the LE-ZW was 1.33 times that of the wild type and its carbon sequestration efficiency was 6.67 times that of the wild type when cultured at an optimal CO2 concentration of 5% until day 10. At this time, most key enzyme genes associated with the photosystem membrane protein complex, photosynthetic electron transport chain, antenna protein, and carbon fixation were up-regulated in mutant LE-ZW. Furthermore, after 10 days of culture under 10% CO2, the cell density and carbon sequestration efficiency of LE-ZW reached 1.10 times and 1.54 times of that under 5% CO2, respectively. Transcriptome analysis revealed significant up-regulation of key enzyme genes associated with carbon fixation, central carbon metabolism, and photosynthesis in LE-ZW under a 10% CO2 concentration. Physiological indices such as the amount of oxygen evolution, the values of Fv/Fm, the expression levels of photosynthetic protein genes and the enzyme activity of key enzymes related to photosynthetic carbon assimilation were corroborated by transcriptome data, elucidating that the mutant LE-ZW exhibited augmented photosynthetic carbon sequestration capacity and metabolic activity, thereby demonstrating robust adaptability to a high-carbon environment. This research contributes to a deeper understanding of the carbon assimilation mechanism in photosynthetic protists under elevated CO2 concentrations.

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.

UV radiation and temperature increase alter the PSII function and defense mechanisms in a bloom-forming cyanobacterium Microcystis aeruginosa

The aim was to determine the response of a bloom-forming Microcystis aeruginosa to climatic changes. Cultures of M. aeruginosa FACHB 905 were grown at two temperatures (25°C, 30°C) and exposed to high photosynthetically active radiation (PAR: 400-700 nm) alone or combined with UVR (PAR + UVR: 295-700 nm) for specified times. It was found that increased temperature enhanced M. aeruginosa sensitivity to both PAR and PAR + UVR as shown by reduced PSII quantum yields (Fv/Fm) in comparison with that at growth temperature (25°C), the presence of UVR significantly exacerbated the photoinhibition. M. aeruginosa cells grown at high temperature exhibited lower PSII repair rate (Krec) and sustained nonphotochemical quenching (NPQs) induction during the radiation exposure, particularly for PAR + UVR. Although high temperature alone or worked with UVR induced higher SOD and CAT activity and promoted the removal rate of PsbA, it seemed not enough to prevent the damage effect from them showing by the increased value of photoinactivation rate constant (Kpi). In addition, the energetic cost of microcystin synthesis at high temperature probably led to reduced materials and energy available for PsbA turnover, thus may partly account for the lower Krec and the declination of photosynthetic activity in cells following PAR and PAR + UVR exposure. Our findings suggest that increased temperature modulates the sensitivity of M. aeruginosa to UVR by affecting the PSII repair and defense capacity, thus influencing competitiveness and abundance in the future water environment.

Aerobic methane production by phytoplankton as an important methane source of aquatic ecosystems: Reconsidering the global methane budget

Biological methane, a major source of global methane budget, is traditionally thought to be produced in anaerobic environments. However, the recent reports about methane supersaturation occurring in oxygenated water layer, termed as "methane paradox", have challenged this prevailing paradigm. Significantly, growing evidence has indicated that phytoplankton including prokaryotic cyanobacteria and eukaryotic algae are capable of generating methane under aerobic conditions. In this regard, a systematic review of aerobic methane production by phytoplankton is expected to arouse the public attention, contributing to the understanding of methane paradox. Here, we comprehensively summarize the widespread phenomena of methane supersaturation in oxic layers. The remarkable correlation relationships between methane concentration and several key indicators (depth, chlorophyll a level and organic sulfide concentration) indicate the significance of phytoplankton in in-situ methane accumulation. Subsequently, four mechanisms of aerobic methane production by phytoplankton are illustrated in detail, including photosynthesis-driven metabolism, reactive oxygen species (ROS)-driven demethylation of methyl donors, methanogenesis catalyzed by nitrogenase and demethylation of phosphonates catalyzed by CP lyase. The first two pathways occur in various phytoplankton, while the latter two have been specially discovered in cyanobacteria. Additionally, the effects of four crucial factors on aerobic methane production by phytoplankton are also discussed, including phytoplankton species, light, temperature and crucial nutrients. Finally, the measures to control global methane emissions from phytoplankton, the precise intracellular mechanisms of methane production and a more complete global methane budget model are definitely required in the future research on methane production by phytoplankton. This review would provide guidance for future studies of aerobic methane production by phytoplankton and emphasize the potential contribution of aquatic ecosystems to global methane budget.

Removal of endocrine disruptor compounds, CO2 fixation, and macromolecules accumulation in Thermosynechococcus sp. CL-1 cultivation

Recently, concern on several environmental issues including the pollutant discharge and high concentration of CO2 have gained high interest due to its impact on ecosystem and global warming effect, respectively. Implementation of photosynthetic microorganism carries out numerous advantages including high efficiency of CO2 fixation, the great endurance under extreme conditions and generation of valuable bioproducts. Thermosynechococcus sp. CL-1 (TCL-1), a cyanobacterium, has the ability to perform CO2 fixation and accumulation of various byproducts under extreme conditions like high temperature and alkalinity, presence of estrogen, or even using swine wastewater. This study aimed to assess TCL-1 performance under various endocrine disruptor compounds (bisphenol-A, 17-β-estradiol/E2, and 17-α-ethynilestradiol/EE2), concentrations (0-10 mg/L), light intensities (500-2000 µE/m2/s), and dissolved inorganic carbon/DIC levels (0-113.2 mM). Addition of E2 content even until 10 mg/L carried out insignificant biomass growth interruption along with the improvement in CO2 fixation rate (79.8 ± 0.1 mg/L/h). Besides the influence of E2, application of higher DIC level and light intensity also enhanced the CO2 fixation rate and biomass growth. The highest biodegradation of E2 at 71% was achieved by TCL-1 in the end of 12 h cultivation period. TCL-1 dominantly produced protein (46.7% ± 0.2%), however, production of lipid and carbohydrate (39.5 ± 1.5 and 23.3 ± 0.9%, respectively) also could be considered as the potential source for biofuel production. Thus, this study can provide an efficient strategy in simultaneously dealing with environmental issues with side advantage in production of macromolecules.

Singular adaptations in the carbon assimilation mechanism of the polyextremophile cyanobacterium Chroococcidiopsis thermalis

Cyanobacteria largely contribute to the biogeochemical carbon cycle fixing ~ 25% of the inorganic carbon on Earth. However, the carbon acquisition and assimilation mechanisms in Cyanobacteria are still underexplored regardless of being of great importance for shedding light on the origins of autotropism on Earth and providing new bioengineering tools for crop yield improvement. Here, we fully characterized these mechanisms from the polyextremophile cyanobacterium Chroococcidiopsis thermalis KOMAREK 1964/111 in comparison with the model cyanobacterial strain, Synechococcus sp. PCC6301. In particular, we analyzed the Rubisco kinetics along with the in vivo photosynthetic CO₂ assimilation in response to external dissolved inorganic carbon, the effect of CO₂ concentrating mechanism (CCM) inhibitors on net photosynthesis and the anatomical particularities of their carboxysomes when grown under either ambient air (0.04% CO₂) or 2.5% CO₂-enriched air. Our results show that Rubisco from C. thermalis possess the highest specificity factor and carboxylation efficiency ever reported for Cyanobacteria, which were accompanied by a highly effective CCM, concentrating CO₂ around Rubisco more than 140-times the external CO₂ levels, when grown under ambient CO₂ conditions. Our findings provide new insights into the Rubisco kinetics of Cyanobacteria, suggesting that improved S_c/o values can still be compatible with a fast-catalyzing enzyme. The combination of Rubisco kinetics and CCM effectiveness in C. thermalis relative to other cyanobacterial species might indicate that the co-evolution between Rubisco and CCMs in Cyanobacteria is not as constrained as in other phylogenetic groups.

A smart and precise mixing strategy for efficient and cost-effective microalgae production in open ponds

Microalgae biotechnology is a great candidate for carbon neutralization, wastewater treatment and the sustainable production of biofuels and food. Efficient and cost-effective microalgae production depends on highly coordinating the resources used for algal growth. However, dynamic natural disturbances such as culture temperature and sunlight can lead to the poor coordination and waste of resources. Open ponds are the most commonly used commercial microalgal production systems, and enhanced mixing can significantly increase their productivity, but mixing energy can be seriously wasted due to dynamic disturbances, presenting a hindrance to further reducing production costs. Herein, a smart and precise mixing strategy was developed for open ponds in which a paddle wheel's stirring speed for an open pond was smartly and precisely controlled in real time based on dynamic variations in light intensity and culture temperature. The proposed technology achieved the same biomass productivity of Spirulina platensis (8.37 g m^-2 day^-1) as a control with a constant high mixing rate under dynamic disturbances while reducing mixing energy inputs by approximately 30 % compared to the control. This study provides a promising method to address serious resource waste and poor coordination due to dynamic natural disturbances, holding great potential for efficient and cost-effective microalgae production.

2-Hydroxychalcone as a Novel Natural Photosynthesis Inhibitor against Bloom-Forming Cyanobacteria

The control of harmful cyanobacterial blooms has been becoming a global challenge. The development of eco-friendly algicides with strong specificity is urgently needed. The photosynthetic apparatus is a promising target site for algicides to minimize the possible harmful effects on animals and humans. In this study, biologically derived 2-hydroxychalcone efficiently inhibited the growth of bloom-forming M. aeruginosa by selectively interfering with photosynthesis. 2-Hydroxychalcone targeting Photosystem II (PSII) inhibited electron transfer between the primary and secondary electron acceptors (Q_A and Q_B) and the binding of plastoquinone (PQ) molecules to the Q_B binding pocket at the acceptor side of PSII, as revealed by polyphasic chlorophyll (Chl) a fluorescence induction and Q_A^- reoxidation kinetics. Molecular docking for 2-hydroxychalcone to D1 protein and the proteomic responses of M. aeruginosa suggested that 2-hydroxychalcone formed a stable monodentate ligand with the nonheme iron in D1 protein, provoking significant modulation of PSII proteins. The unique binding mode of 2-hydroxychalcone with PSII differentiated it from classical PSII inhibitors. Furthermore, 2-hydroxychalcone down-regulated the expression of microcystin (MC) synthesis-related genes to restrain MC synthesis and release. These results indicated the potential application of 2-hydroxychalcone as an algicide or a template scaffold for designing novel derivatives with superior algicidal activity.

Elevated CO2 significantly increases N2 fixation, growth rates, and alters microcystin, anatoxin, and saxitoxin cell quotas in strains of the bloom-forming cyanobacteria, Dolichospermum

The effect of rising CO₂ levels on cyanobacterial harmful algal blooms (CHABs) is an emerging concern, particularly within eutrophic ecosystems. While elevated pCO₂ has been associated with enhanced growth rates of some cyanobacteria, few studies have explored the effect of CO₂ and nitrogen availability on diazotrophic (N₂-fixing) cyanobacteria that produce cyanotoxins. Here, the effects of elevated CO₂ and fixed nitrogen (NO₃^-) availability on the growth rates, toxin production, and N₂ fixation of microcystin, saxitoxin, and anatoxin-a - producing strains of the genus Dolichospermum were quantified. Growth rates of all Dolichospermum spp. were significantly increased by CO₂ or both CO₂ and NO₃^- with rates being highest in treatments with the highest levels of CO₂ and NO₃^-for all strains. While NO₃^- suppressed N₂ fixation, diazotrophy significantly increased when NO₃^--enriched Dolichospermum spp. were supplied with higher CO₂ compared to cultures grown under lower CO₂ levels. This suggests that diazotrophy will play an increasingly important role in N cycling in CO₂-enriched, eutrophic lentic systems. NO₃^- significantly increased quotas of the N-rich cyanotoxins, microcystin and saxitoxin, at ambient and enriched CO₂ levels, respectively. In contrast, elevated CO₂ significantly decreased cell quotas of microcystin and saxitoxin, but significantly increased cell quotas of the N-poor cyanotoxin, anatoxin. N₂ fixation was significantly negatively and positively correlated with quotas of N-rich and N-poor cyanotoxins, respectively. Findings suggest cellular quotas of N-rich toxins (microcystin and saxitoxin) may be significantly reduced, or cellular quotas of N-poor toxins (anatoxin) may be significantly enhanced, under elevated CO₂ conditions during diazotrophic cyanobacterial blooms. Finally, in the future, ecosystems that experience combinations of excessive N loading and CO₂ enrichment may become more prone to toxic blooms of Dolichospermum.

Toxicity alleviation and metabolism enhancement of nonylphenol in green algae Dictyosphaerium sp. by NaHCO3

Nonylphenol (NP) toxicity limits the improvements in its algal remediation efficiency. This study comprehensively investigated the performance and mechanism of NaHCO₃-driving effects on NP-exposed algae. The results showed that NaHCO₃ enhanced algal resistance to NP and the corresponding EC₅₀ values increased 1.31-4.25 times. Further, the toxicological effects of NP reduced with increasing pyrenoid volume and chlorophyll and carotenoids production, and decreasing cellular damage degree. Moreover, the concentration of extracellular polymeric substances was enhanced and more NP adsorption sites were formed. Consistently, RNA-seq demonstrated significant expression alterations in genes related to energy metabolism, cellular synthesis, photosynthesis, and carbon fixation. Besides, NP biodegradation rate was increased by 15.2 % and 11.1 % in the 1, and 4 mg/L NP treatments, respectively. Identification of degradation intermediates and their toxicity via Ecological Structure Activity Relationship program showed that NaHCO₃ accelerated sequential α-C removal from NP in algae with faster generation of less toxic metabolites, namely, 4-ethylphenol, 4-cresol and 4-hydroxybenzoic acid. This study provides new insights into the role of NaHCO₃ in toxicity alleviation and metabolism enhancement of NP in algae and can assist NP bioremediation efforts in aquatic environment.

Synergistic promoting effect of increasing aquatic ammonium and CO2 on Microcystis aeruginosa

Owing to climate change and intensive agricultural development, freshwater bodies have been affected by increases in both CO₂ levels and chemically-reduced forms of N. However, little is known about how these changes affect cyanobacterial growth and blooms. This study explored a range of light conditions (30, 80, 130, or 200 μmol photons/m²/s) wherein Microcystis aeruginosa, a widespread bloom-forming species, was exposed to different concentrations of CO₂ (400 parts per million (ppm) and 1000 ppm) in a medium containing NH₄⁺ or NO₃－. The interactive effects of N sources and CO₂ levels on the C/N metabolic balance and energy balance were examined to assess changes in the growth of M. aeruginosa. When the light intensity was 80 μmol photons/m²/s, elevated CO₂ could reduce intracellular reactive oxygen species (ROS) in NH₄⁺-grown M. aeruginosa. Meanwhile, cell density and chlorophyll a (Chl a) increased with increasing CO₂ levels, and the increase in Chl a was significantly greater in NH₄⁺-grown M. aeruginosa than in NO₃－-grown M. aeruginosa. Under light conditions of 200 μmol photons/m²/s, elevated CO₂ concentration caused NO₃－-grown M. aeruginosa to be affected by a large amount of ROS, and the growth of NO₃－-grown M. aeruginosa was finally suppressed. However, NH₄⁺-grown M. aeruginosa had a smaller amount of ROS and showed improved growth as CO₂ was elevated. This difference can be attributed to the faster metabolic pathways in the NH₄⁺ environment, which manifested in a lower accumulation of 2-oxoglutarate and fatty acids as CO₂ was elevated. These findings suggest that the simultaneous increase in ammonium and CO₂ in aquatic ecosystems confers cyanobacteria with greater advantages than the combination of nitrate and CO₂, which may aggravate cyanobacterial blooms.

Rapidly Measuring Scattered Polarization Parameters of the Individual Suspended Particle with Continuously Large Angular Range

Suspended particles play a vital role in aquatic environments. We propose a method to rapidly measure the scattered polarization parameters of individual suspended particles with continuously large angular range (PCLAR), from 60° to 120° in one shot. A conceptual setup is built to measure PCLAR with 20 kHz; to verify the setup, 10 μm-diameter silica microspheres suspended in water, whose PCLAR are consistent with those simulated by Mie theory, are measured. PCLAR of 6 categories of particles are measured, which enables high-accuracy classification with the help of a convolutional neural network algorithm. PCLAR of different mixtures of Cyclotella stelligera and silica microspheres are measured to successfully identify particulate components. Furthermore, classification ability comparisons of different angular-selection strategies show that PCLAR enables the best classification beyond the single angle, discrete angles and small-ranged angles. Simulated PCLAR of particles with different size, refractive index, and structure show explicit discriminations between them. Inversely, the measured PCLAR are able to estimate the effective size and refractive index of individual Cyclotella cells. Results demonstrate the method’s power, which intrinsically takes the advantage of the optical polarization and the angular coverage. Future prototypes based on this concept would be a promising biosensor for particles in environmental monitoring.

Exploring the photocatalytic inactivation mechanism of Microcystis aeruginosa under visible light using Ag3PO4/g-C3N4

In this work, a series of Ag₃PO₄/g-C₃N₄ (AG) photocatalysts were synthesized. After characterizing the properties, the effects of mass ratio, light intensity, and material dosages on photodegradation were investigated. The material with a 1/2 mass ratio of Ag₃PO₄/g-C₃N₄ showed the highest photocatalytic activity under visible light, and the removal efficiency reached 90.22% for an initial suspended algae concentration of 2.7 × 10⁶ cells/mL, 0.1 g of AG, and 3 h of irradiation. These results showed that the conductivity was increased while the total protein and COD contents of the algae suspension were declined rapidly. In contrast, the variations in the malondialdehyde (MDA) level suggested that the algae cell wall was severely damaged and that selective permeability of the membrane was significantly affected. A possible photocatalytic mechanism was proposed and •O₂^- was shown to be the major reactive oxygen species in the photocatalysis. In summary, during the visible light photocatalytic process, the cell structure was destroyed, which caused the leakage of electrolyte, the inactivation of protein, and the inhibition of photosynthesis; finally, the cells died. This study provides a reference for photodegradation of algae pollution in water bodies.

Insignificant Response of Bacterioplankton Community to Elevated pCO2 During a Short-Term Microcosm Experiment in a Subtropical Eutrophic Coastal Ecosystem

Ocean acidification, as one of the major consequences of global climate change, markedly affects multiple ecosystem functions in disparate marine environments from coastal habitats to the deep ocean. Evaluation of the responses of marine microbial community to the increasing partial pressure of CO₂ (pCO₂) is crucial to explore the microbe-driven biogeochemical processes in the future ocean. In this study, a microcosm incubation of eutrophic coastal water from Xiamen Bay under elevated pCO₂ (about 1,000 μatm) and control (ambient air, about 380-410 μatm) conditions was conducted to investigate the effect of ocean acidification on the natural bacterioplankton community. During the 5-day incubation period, the chlorophyll a concentration and bacterioplankton abundance were not significantly affected by increased pCO₂. Hierarchical clustering and non-metric multidimensional scaling analysis based on Bray-Curtis similarity among the bacterioplankton community derived from the 16S rRNA genes revealed an inconspicuous impact of elevated pCO₂ on the bacterial community. During the incubation period, Proteobacteria, Bacteroidetes, Actinobacteria, Cyanobacteria, and Epsilonbacteraeota were predominant in all microcosms. Despite the distinct temporal variation in the composition of the bacterioplankton community during the experimental period, statistical analyses showed that no significant difference was found on bacterioplankton taxa between elevated pCO₂ and control, indicating that the bacterioplankton at the population-level were also insensitive to elevated pCO₂. Our results therefore suggest that the bacterioplankton communities in the fluctuating and eutrophic coastal ecosystems appear to be adaptable to the short-term elevated pCO₂.

Northern High-Latitude Organic Soils As a Vital Source of River-Borne Dissolved Iron to the Ocean

Organic soils in the Arctic-boreal region produce small aquatic humic ligands (SAHLs), a category of naturally occurring complexing agents for iron. Every year, large amounts of SAHLs-loaded with iron mobilized in river basins-reach the oceans via river runoff. Recent studies have shown that a fraction of SAHLs belong to the group of strong iron-binding ligands in the ocean. That means, their Fe(III) complexes withstand dissociation even under the conditions of extremely high dilution in the open ocean. Fe(III)-loaded SAHLs are prone to UV-photoinduced ligand-to-metal charge-transfer which leads to disintegration of the complex and, as a consequence, to enhanced concentrations of bioavailable dissolved Fe(II) in sunlit upper water layers. On the other hand, in water depths below the penetration depth of UV, the Fe(III)-loaded SAHLs are fairly resistant to degradation which makes them ideally suited as long-lived molecular transport vehicles for river-derived iron in ocean currents. At locations where SAHLs are present in excess, they can bind to iron originating from various sources. For example, SAHLs were proposed to contribute substantially to the stabilization of hydrothermal iron in deep North Atlantic waters. Recent discoveries have shown that SAHLs, supplied by the Arctic Great Rivers, greatly improve dissolved iron concentrations in the Arctic Ocean and the North Atlantic Ocean. In these regions, SAHLs play a critical role in relieving iron limitation of phytoplankton, thereby supporting the oceanic sink for anthropogenic CO₂. The present Critical Review describes the most recent findings and highlights future research directions.