Dinoflagellate-cyanobacterium communication may determine the composition of phytoplankton assemblage in a mesotrophic lake

Multimodal detection and analysis of microplastics in human thrombi from multiple anatomically distinct sites

BACKGROUND

Microplastic (MP) pollution has emerged as a significant environmental concern worldwide. While extensive research has focused on their presence in marine organisms and ecosystems, their potential impact on human health, particularly on the circulatory system, remains understudied. This project aimed to identify and quantify the mass concentrations, polymer types, and physical properties of MPs in human thrombi surgically retrieved from both arterial and venous systems at three anatomically distinct sites, namely, cerebral arteries in the brain, coronary arteries in the heart, and deep veins in the lower extremities. Furthermore, this study aimed to investigate the potential association between the levels of MPs and disease severity.

METHODS

Thrombus samples were collected from 30 patients who underwent thrombectomy procedures due to ischaemic stroke (IS), myocardial infarction (MI), or deep vein thrombosis (DVT). Pyrolysis-gas chromatography mass spectrometry (Py-GC/MS) was employed to identify and quantify the mass concentrations of the MPs. Laser direct infrared (LDIR) spectroscopy and scanning electron microscopy (SEM) were used to analyse the physical properties of the MPs. Demographic and clinical information were also examined. A rigorous quality control system was used to eliminate potential environmental contamination.

FINDINGS

MPs were detected by Py-GC/MS in 80% (24/30) of the thrombi obtained from patients with IS, MI, or DVT, with median concentrations of 61.75 μg/g, 141.80 μg/g, and 69.62 μg/g, respectively. Among the 10 target types of MP polymers, polyamide 66 (PA66), polyvinyl chloride (PVC), and polyethylene (PE) were identified. Further analyses suggested that higher concentrations of MPs may be associated with greater disease severity (adjusted β = 7.72, 95% CI: 2.01-13.43, p < 0.05). The level of D-dimer in the MP-detected group was significantly higher than that in the MP-undetected group (8.3 ± 1.5 μg/L vs 6.6 ± 0.5 μg/L, p < 0.001). Additionally, LDIR analysis showed that PE was dominant among the 15 types of identified MPs, accounting for 53.6% of all MPs, with a mean diameter of 35.6 μm. The shapes of the polymers detected using LDIR and SEM were found to be heterogeneous.

INTERPRETATION

This study presents both qualitative and quantitative evidence of the presence of MPs, and their mass concentrations, polymer types, and physical properties in thrombotic diseases through the use of multimodal detection methods. Higher concentrations of MPs may be associated with increased disease severity. Future research with a larger sample size is urgently needed to identify the sources of exposure and validate the observed trends in the study.

FUNDING

This study was funded by the SUMC Scientific Research Initiation Grant (SRIG, No. 009-510858038), Postdoctoral Research Initiation Grant (No. 202205230031-3), and the 2020 Li Ka Shing Foundation Cross-Disciplinary Research Grant (No. 2020LKSFG02C).

SLC24A-mediated calcium exchange as an indispensable component of the diatom cell density-driven signaling pathway

Diatom bloom is characterized by a rapid increase of population density. Perception of population density and physiological responses can significantly influence their survival strategies, subsequently impacting bloom fate. The population density itself can serve as a signal, which is perceived through chemical signals or chlorophyll fluorescence signals triggered by high cell density, and their intracellular signaling mechanisms remain to be elucidated. In this study, we focused on the model diatom, Phaeodactylum tricornutum, and designed an orthogonal experiment involving varying cell densities and light conditions, to stimulate the release of chemical signals and light-induced chlorophyll fluorescence signals. Utilizing RNA-Seq and Weighted Gene Co-expression Network Analysis, we identified four gene clusters displaying density-dependent expression patterns. Within these, a potential hub gene, PtSLC24A, encoding a Na+/Ca2+ exchanger, was identified. Based on molecular genetics, cellular physiology, computational structural biology, and in situ oceanic data, we propose a potential intracellular signaling mechanism related to cell density in marine diatoms using Ca2+: upon sensing population density signals mediated by chemical cues, the membrane-bound PtSLC24A facilitates the efflux of Ca2+ to maintain specific intracellular calcium levels, allowing the transduction of intracellular density signals, subsequently regulating physiological responses, including cell apoptosis, ultimately affecting algal blooms fate. These findings shed light on the calcium-mediated intracellular signaling mechanism of marine diatoms to changing population densities, and enhances our understanding of diatom bloom dynamics and their ecological implications.

Symbiodiniaceae photophysiology and stress resilience is enhanced by microbial associations

Symbiodiniaceae form associations with extra- and intracellular bacterial symbionts, both in culture and in symbiosis with corals. Bacterial associates can regulate Symbiodiniaceae fitness in terms of growth, calcification and photophysiology. However, the influence of these bacteria on interactive stressors, such as temperature and light, which are known to influence Symbiodiniaceae physiology, remains unclear. Here, we examined the photophysiological response of two Symbiodiniaceae species (Symbiodinium microadriaticum and Breviolum minutum) cultured under acute temperature and light stress with specific bacterial partners from their microbiome (Labrenzia (Roseibium) alexandrii, Marinobacter adhaerens or Muricauda aquimarina). Overall, bacterial presence positively impacted Symbiodiniaceae core photosynthetic health (photosystem II [PSII] quantum yield) and photoprotective capacity (non-photochemical quenching; NPQ) compared to cultures with all extracellular bacteria removed, although specific benefits were variable across Symbiodiniaceae genera and growth phase. Symbiodiniaceae co-cultured with M. aquimarina displayed an inverse NPQ response under high temperatures and light, and those with L. alexandrii demonstrated a lowered threshold for induction of NPQ, potentially through the provision of antioxidant compounds such as zeaxanthin (produced by Muricauda spp.) and dimethylsulfoniopropionate (DMSP; produced by this strain of L. alexandrii). Our co-culture approach empirically demonstrates the benefits bacteria can deliver to Symbiodiniaceae photochemical performance, providing evidence that bacterial associates can play important functional roles for Symbiodiniaceae.

Elevated pCO2 changes community structure and function by affecting phytoplankton group-specific mortality

The rise of atmospheric pCO₂ has created a number of problems for marine ecosystem. In this study, we initially quantified the effects of elevated pCO₂ on the group-specific mortality of phytoplankton in a natural community based on the results of mesocosm experiments. Diatoms dominated the phytoplankton community, and the concentration of chlorophyll a was significantly higher in the high-pCO₂ treatment than the low-pCO₂ treatment. Phytoplankton mortality (percentage of dead cells) decreased during the exponential growth phase. Although the mortality of dinoflagellates did not differ significantly between the two pCO₂ treatments, that of diatoms was lower in the high-pCO₂ treatment. Small diatoms dominated the diatom community. Although the mortality of large diatoms did not differ significantly between the two treatments, that of small diatoms was lower in the high-pCO₂ treatment. These results suggested that elevated pCO₂ might enhance dominance by small diatoms and thereby change the community structure of coastal ecosystems.

Pelagic diatoms communicate through synchronized beacon natural fluorescence signaling

Communication between conspecific individuals is an essential part of life both in terrestrial and marine realms. Until recently, social behavior in marine phytoplankton was assumed to rely mainly on the secretion of a variety of infochemicals that allowed population-scale collective responses. Here, we demonstrate that pelagic diatoms also use Sun-stimulated fluorescence signals for synchronizing their behavior. These unicellular microorganisms, playing a key biogeochemical role in the ocean, use photoreceptor proteins and red–far-red fluorescent radiation to communicate. A characteristic beaconing signal is generated by rhythmic organelle displacement within the cell cytoplasm, triggering coordinated population behavior. These light-based communication networks could critically determine major facets of diatom ecology and fitness and regulate the dynamics of larger-scale ocean processes.

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.

Cell Death and Metabolic Stress in Gymnodinium catenatum Induced by Allelopathy

Allelopathy between phytoplankton species can promote cellular stress and programmed cell death (PCD). The raphidophyte Chattonella marina var. marina, and the dinoflagellates Margalefidinium polykrikoides and Gymnodinium impudicum have allelopathic effects on Gymnodinium catenatum; however, the physiological mechanisms are unknown. We evaluated whether the allelopathic effect promotes cellular stress and activates PCD in G. catenatum. Cultures of G. catenatum were exposed to cell-free media of C. marina var. marina, M. polykrikoides and G. impudicum. The mortality, superoxide radical (O₂^●-) production, thiobarbituric acid reactive substances (TBARS) levels, superoxide dismutase (SOD) activity, protein content, and caspase-3 activity were quantified. Mortality (between 57 and 79%) was registered in G. catenatum after exposure to cell-free media of the three species. The maximal O₂^●- production occurred with C. marina var. marina cell-free media. The highest TBARS levels and SOD activity in G. catenatum were recorded with cell-free media from G. impudicum. The highest protein content was recorded with cell-free media from M. polykrikoides. All cell-free media caused an increase in the activity of caspase-3. These results indicate that the allelopathic effect in G. catenatum promotes cell stress and caspase-3 activation, as a signal for the induction of programmed cell death.

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.

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.

Cell density-dependent suppression on the development and photosynthetic activities of Sargassum fusiformis embryos by dinoflagellate Karenia mikimotoi

Lots of research has demonstrated that macroalgae can strongly inhibit the growth of harmful algal bloom (HAB) species in general. However, the effects of HABs or HAB-forming species on macrophytes are still largely uncharacterized until now. In the present study, the effects of the dinoflagellate Karenia mikimotoi cell density gradient, live cell suspension (LC), ruptured cell suspension (RC) as well as the cell-free supernatant (FC) of K. mikimotoi at 1000 μg Chla l-1 (~1.0 × 105 cells ml-1) on the development and photosynthesis of Sargassum fusiforme embryos were investigated in a series of laboratory experiments. The results showed that co-cultivation with K. mikimotoi at 500 μg Chla l-1(~5.0 × 104 cells ml-1) and higher cell densities significantly (P<0.05) inhibited the development, pigment content and photosynthetic activities of the embryos. In addition, the inhibitory effects increased with increased cell densities and prolonged exposure time. Compared to the embryos cultured with the F/2 medium (Control), exposure to LC, RC and FC of K. mikimotoi at 1000 μg Chla l-1for 2 weeks all led to decreased relative growth rate (RGR), chlorophyll (Chl) a content, carotenoids (Car) content and photosynthetic activities of the embryos, with LC and RC exhibiting the maximal and the minimal suppression. The dominant inhibitory effects of FC on the embryos indicated that the suppression was mainly caused by the allelochemicals, while the slightest inhibitory effects of RC on the embryos suggested that some intracellular growth-promoting substances were synchronously released when K. mikimotoi cells lyzed. In addition, the most severe growth suppression of embryos by LC indicated that intact cell contact by K. mikimotoi probably also contributed to the inhibitory effects. These results indicated that a dense HAB formed by K. mikimotoi could seriously suppress the development and photosynthesis of S. fusiforme embryos and eventually reduce the seedlings stock.

Interspecies interactions between Microcystis aeruginosa PCC 7806 and Desmodesmus subspicatus SAG 86.81 in a co-cultivation system at various growth phases

In lakes, cyanobacterial blooms are frequently associated with green algae and dominate the phytoplankton community in successive waves. In the present study, the interactions between Microcystis aeruginosa PCC 7806 and Desmodesmus subspicatus were studied to clarify the probable ecological significance of algal secondary metabolites; focusing on the role of cyanotoxin 'microcystin-LR' (MC-LR). A dialysis co-cultivation technique was applied where M. aeruginosa was grown inside and D. subspicatus was cultured outside of the dialysis tubing. The concentration of the intra- and extracellular MC-LR and the growth of two species were measured at different time points over a period of one month. Additionally, the growth of the two species in the culture filtrate of one another and the effect of the purified MC-LR on the growth of the green alga were studied. The results indicated that the co-existing species could affect each other depending on the growth phases. Despite the early dominance of D. subspicatus during the logarithmic phase, M. aeruginosa suppressed the growth of the green alga at the stationary phase, which coincided with increased MC production and release. However, the inhibitory effects of Microcystis might be related to its other extracellular metabolites rather than, or possibly in addition to, MC.

Desmodesmus subspicatus co-cultured with microcystin producing (PCC 7806) and the non-producing (PCC 7005) strains of Microcystis aeruginosa

Although microcystins (MCs) are the most commonly studied cyanotoxins, their significance to the producing organisms remains unclear. MCs are known as endotoxins, but they can be found in the surrounding environment due to cell lysis, designated as extracellular MCs. In the present study, the interactions between MC producing and the non-producing strains of Microcystis aeruginosa, PCC 7806 and PCC 7005, respectively, and a green alga, Desmodesmus subspicatus, were studied to better understand the probable ecological importance of MCs at the collapse phase of cyanobacterial blooms. We applied a dialysis co-cultivation system where M. aeruginosa was grown inside dialysis tubing for one month. Then, D. subspicatus was added to the culture system on the outside of the membrane. Consequently, the growth of D. subspicatus and MC contents were measured over a 14-day co-exposure period. The results showed that Microcystis negatively affected the green alga as the growth of D. subspicatus was significantly inhibited in co-cultivation with both the MC-producing and -deficient strains. However, the inhibitory effect of the MC-producing strain was greater and observed earlier compared to the MC-deficient strain. Thus, MCs might be considered as an assistant factor that, in combination with other secondary metabolites of Microcystis, reinforce the ability to outcompete co-existing species.

Light-dependent single-cell heterogeneity in the chloroplast redox state regulates cell fate in a marine diatom

Diatoms are photosynthetic microorganisms of great ecological and biogeochemical importance, forming vast blooms in aquatic ecosystems. However, we are still lacking fundamental understanding of how individual cells sense and respond to diverse stress conditions, and what acclimation strategies are employed during bloom dynamics. We investigated cellular responses to environmental stress at the single-cell level using the redox sensor roGFP targeted to various organelles in the diatom Phaeodactylum tricornutum. We detected cell-to-cell variability using flow cytometry cell sorting and a microfluidics system for live imaging of oxidation dynamics. Chloroplast-targeted roGFP exhibited a light-dependent, bi-stable oxidation pattern in response to H₂O₂ and high light, revealing distinct subpopulations of sensitive oxidized cells and resilient reduced cells. Early oxidation in the chloroplast preceded commitment to cell death, and can be used for sensing stress cues and regulating cell fate. We propose that light-dependent metabolic heterogeneity regulates diatoms’ sensitivity to environmental stressors in the ocean.

Microscopic algae, such as diatoms, are widely spread throughout the oceans, and are responsible for half of the oxygen we breathe. At certain times of the year these algae grow very rapidly to form large “blooms” that can be detected by satellites in space. These blooms are generally short-lived because the algae are either eaten by other marine organisms, run out of nutrients, or die as a result of being infected by viruses or bacteria. However, some diatom cells survive the end of the bloom and go on to generate new blooms in the future, but it is still not clear how.

As the bloom collapses, diatoms experience many stressful conditions which can cause active molecules known as reactive oxygen species, or ROS for short, to accumulate inside cells. Normally growing cells also produce low amounts of ROS, which regulate various processes that are important for maintaining a cell’s health. However, high amounts of ROS can cause damage, which may lead to a cell’s death. Now, Mizrachi et al. investigated why some algae survive while others die in response to stressful conditions, focusing on the amount of ROS that accumulates within the diatom Phaeodactylum tricornutum.

Laboratory experiments showed that individual cells of P. tricornutum respond differently to environmental stress, forming two distinct groups of either sensitive or resilient cells. Sensitive cells accumulated high levels of ROS within a cell compartment known as the chloroplast and eventually died. Whereas resilient cells were able to maintain low levels of ROS in the chloroplast and survived long after the other cells perished. Populations of genetically identical diatom cells also formed distinct groups of sensitive and resilient cells, demonstrating that these two opposing reactions to stress are not caused by genetic differences between cells.

Lastly, Mizrachi et al. showed that how diatoms acclimate to stress depends on the amount of light they are exposed to. When in the dark, all cells became sensitive to oxidative stress, without forming distinct groups. But, when exposed to strong light that mimics the ocean surface, cells formed distinct groups within the population. This suggests that light regulates how susceptible these microscopic algae are to environmental stress.

The different responses within a population may serve as a “bet-hedging” strategy, enabling at least some of the cells to survive unpredicted stressful conditions. The next challenge will be to find out whether algae growing in the oceans also use the same strategy and investigate what impact this has on diatom blooms.

Karenia brevis allelopathy compromises the lipidome, membrane integrity, and photosynthesis of competitors

The formation, propagation, and maintenance of harmful algal blooms are of interest due to their negative effects on marine life and human health. Some bloom-forming algae utilize allelopathy, the release of compounds that inhibit competitors, to exclude other species dependent on a common pool of limiting resources. Allelopathy is hypothesized to affect bloom dynamics and is well established in the red tide dinoflagellate Karenia brevis. K. brevis typically suppresses competitor growth rather than being acutely toxic to other algae. When we investigated the effects of allelopathy on two competitors, Asterionellopsis glacialis and Thalassiosira pseudonana, using nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS)-based metabolomics, we found that the lipidomes of both species were significantly altered. However, A. glacialis maintained a more robust metabolism in response to K. brevis allelopathy whereas T. pseudonana exhibited significant alterations in lipid synthesis, cell membrane integrity, and photosynthesis. Membrane-associated lipids were significantly suppressed for T. pseudonana exposed to allelopathy such that membranes of living cells became permeable. K. brevis allelopathy appears to target lipid biosynthesis affecting multiple physiological pathways suggesting that exuded compounds have the ability to significantly alter competitor physiology, giving K. brevis an edge over sensitive species.

Diurnal fluctuations in chloroplast GSH redox state regulate susceptibility to oxidative stress and cell fate in a bloom-forming diatom

Diatoms are one of the key phytoplankton groups in the ocean, forming vast oceanic blooms and playing a significant part in global primary production. To shed light on the role of redox metabolism in diatom's acclimation to light-dark transition and its interplay with cell fate regulation, we generated transgenic lines of the diatom Thalassiosira pseudonana that express the redox-sensitive green fluorescent protein targeted to various subcellular organelles. We detected organelle-specific redox patterns in response to oxidative stress, indicating compartmentalized antioxidant capacities. Monitoring the GSH redox potential (E_GSH ) in the chloroplast over diurnal cycles revealed distinct rhythmic patterns. Intriguingly, in the dark, cells exhibited reduced basal chloroplast E_GSH but higher sensitivity to oxidative stress than cells in the light. This dark-dependent sensitivity to oxidative stress was a result of a depleted pool of reduced glutathione which accumulated during the light period. Interestingly, reduction in the chloroplast E_GSH was observed in the light phase prior to the transition to darkness, suggesting an anticipatory phase. Rapid chloroplast E_GSH re-oxidation was observed upon re-illumination, signifying an induction of an oxidative signaling during transition to light that may regulate downstream metabolic processes. Since light-dark transitions can dictate metabolic capabilities and susceptibility to a range of environmental stress conditions, deepening our understanding of the molecular components mediating the light-dependent redox signals may provide novel insights into cell fate regulation and its impact on oceanic bloom successions.

The allelopathic effect and safety evaluation of 3,4-Dihydroxybenzalacetone on Microcystis aeruginosa

Harmful algal blooms (HABs) has been a serious problem in recent years, because of large quantities of cyanobacterial in eutrophic water. We studied the effects of 3,4-Dihydroxybenzalacetone (DBL) and other four compounds (vanillic acid, ferulic acid, 3,5-Dichlorophenol and cupric sulfate) on Microcystis aeruginosa. The results showed that the growth of M. aeruginosa was significantly inhibited by all tested compounds with a half maximal effect concentration (EC₅₀) of 5.2, 22.8, 54.7, 1.5, 0.3μg/mL, respectively. Our data also demonstrated that DBL triggered the generation of superoxide anion radicals (O₂^-). The O₂^- might induce a lipid peroxidation which may change cell membrane penetrability, thereby leading to the eventual death of M. aeruginosa. In addition, DBL may has few toxic to aquatic species as indicated by its 96h half maximum lethal concentration value to zebrafish (Danio rerio) was far higher than 128μg/mL. Our current study further provides evidence that some phenolic acids such as DBL may be a potential effective solution for aquatic management.

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

Microcystis and Anabaena (Dolichospermum) are among the most toxic cyanobacterial genera and often succeed each other during harmful algal blooms. The role allelopathy plays in the succession of these genera is not fully understood. The allelopathic interactions of six strains of Microcystis and Anabaena under different nutrient conditions in co-culture and in culture-filtrate experiments were investigated. Microcystis strains significantly reduced the growth of Anabaena strains in mixed cultures with direct cell-to-cell contact and high nutrient levels. Cell-free filtrate from Microcystis cultures proved equally potent in suppressing the growth of nutrient replete Anabaena cultures while also significantly reducing anatoxin-a production. Allelopathic interactions between Microcystis and Anabaena were, however, partly dependent on ambient nutrient levels. Anabaena dominated under low N conditions and Microcystis dominated under nutrient replete and low P during which allelochemicals caused the complete suppression of nitrogen fixation by Anabaena and stimulated glutathione S-transferase activity. The microcystin content of Microcystis was lowered with decreasing N and the presence of Anabaena decreased it further under low P and high nutrient conditions. Collectively, these results indicate that strong allelopathic interactions between Microcystis and Anabaena are closely intertwined with the availability of nutrients and that allelopathy may contribute to the succession, nitrogen availability, and toxicity of cyanobacterial blooms.

Influence of cyanobacteria, mixotrophic flagellates, and virioplankton size fraction on transcription of microcystin synthesis genes in the toxic cyanobacterium Microcystis aeruginosa

Toxic cyanobacteria such as Microcystis aeruginosa are a worldwide concern in freshwater reservoirs. Problems associated with their mass occurrence are predicted to increase in the future due to global warming. The hepatotoxic secondary metabolite microcystin is of particular concern in this context. This study aimed to determine whether co-occurring microorganisms influence the expression of microcystin biosynthesis genes. To this end, we performed cocultivation experiments and measured mcyB and mcyD transcripts in M. aeruginosa using RT-qPCR. We utilized representatives from three different plankton groups: the picocyanobacterium Synechococcus elongatus, the unicellular flagellate grazer Ochromonas danica, and virioplankton from two different lakes. The presence of S. elongatus significantly increased mcyB and mcyD transcription in M. aeruginosa. Cocultivation with the mixotrophic chrysophyte O. danica did not increase the transcription of mcyB and mcyD; in fact, mcyD transcripts decreased significantly. The virioplankton size fraction of environmental water samples induced a significant increase in mcyB and mcyD transcription when obtained from lakes with cyanobacterial blooms. Our results show that co-occurring microorganisms influence the expression of microcystin biosynthesis genes in M. aeruginosa.

Stepwise strategy for monitoring toxic cyanobacterial blooms in lentic water bodies

Climate change has been causing the increase in frequency, severity, and duration of harmful algal blooms, which makes the establishment of water management strategies indispensable. For cyanobacteria, several methods are currently used in monitoring programs. However, these methods are time-consuming and require specialists, and results are usually not provided within an adequate timeframe for taking timely mitigation actions. This work proposes a strategy for a faster, easier, and more cost-effective monitoring of cyanobacterial blooms, using a stepwise approach based on fluorometric determination of phycocyanin at an early stage. Complementary parameters (chlorophyll a, enumeration of dominant cyanobacterial species and cyanotoxin potential and quantification) are determined when necessary, thus progressively allocating human and financial resources within the monitoring program. This strategy was applied and validated using nine lentic eutrophic freshwater bodies prone to the occurrence of cyanobacterial blooms. Samples were sequentially evaluated, and the study ended up with two samples that showed high health risks. However, according to WHO guidelines, eight of the nine samples would be classified as having "moderate risk of adverse health effects" and could lead to preventive measures that would have an important regional economic impact. Therefore, the present approach proved to be a promising alternative to increase the effectiveness and accuracy of the risk assessment process in water bodies where cyanobacterial blooms occur.

Bactericidal metabolites from Phellinus noxius HN-1 against Microcystis aeruginosa

Harmful algal blooms cause serious problems worldwide due to large quantities of cyanotoxins produced by cyanobacteria in eutrophic water. In this study, a new compound named 2-(3, 4-dihydroxy-2-methoxyphenyl)-1, 3-benzodioxole-5-carbaldehyde (Compound 1), together with one known compound, 3, 4-dihydroxybenzalacetone (DBL), was purified from Phellinus noxius HN-1 (CCTCC M 2016242). Compound 1 and DBL displayed activity against the cyanobacteria Microcystis aeruginosa with a half maximal effective concentration of 21 and 5 μg/mL, respectively. Scanning electron and transmission electron microscopic observations showed that the compounds caused serious damage and significant lysis to M. aeruginosa cells. qRT-PCR assay indicated that compound 1 and DBL exposure up-regulated the expression of gene mcyB and down-regulated the expression of genes ftsZ, psbA1, and glmS in M. aeruginosa. This study provides the first evidence of bactericidal activity of a new compound and DBL. In summary, our results suggest that compound 1 and DBL might be developed as naturally-based biocontrol agents.

Electrochemical inactivation of cyanobacteria and microcystin degradation using a boron-doped diamond anode - A potential tool for cyanobacterial bloom control

Cyanobacterial blooms are global phenomena that can occur in calm and nutrient-rich (eutrophic) fresh and marine waters. Human exposure to cyanobacteria and their biologically active products is possible during water sports and various water activities, or by ingestion of contaminated water. Although the vast majority of harmful cyanobacterial products are confined to the interior of the cells, these are eventually released into the surrounding water following natural or artificially induced cell death. Electrochemical oxidation has been used here to damage cyanobacteria to halt their proliferation, and for microcystin degradation under in-vitro conditions. Partially spent Jaworski growth medium with no addition of supporting electrolytes was used. Electrochemical treatment resulted in the cyanobacterial loss of cell-buoyancy regulation, cell proliferation arrest, and eventual cell death. Microcystin degradation was studied separately in two basic modes of treatment: batch-wise flow, and constant flow, for electrolytic-cell exposure. Batch-wise exposure simulates treatment under environmental conditions, while constant flow is more appropriate for the study of boron-doped diamond electrode efficacy under laboratory conditions. The effectiveness of microcystin degradation was established using high-performance liquid chromatography-photodiode array detector analysis, while the biological activities of the products were estimated using a colorimetric protein phosphatase-1 inhibition assay. The results indicate potential for the application of electro-oxidation methods for the control of bloom events by taking advantage of specific intrinsic ecological characteristics of bloom-forming cyanobacteria. The applicability of the use of boron-doped diamond electrodes in remediation of water exposed to cyanobacteria bloom events is discussed.

What Underpins the Trophic Networks of the Plankton in Shallow Oxbow Lakes?

The aim of this study was to determine the relationships in the microbial trophic network underpinning them about communities of plankton ciliates in shallow oxbow lakes of the Vistula River in southern Poland (Jeziorzany 1, Jeziorzany 2, Piekary, Tyniec). The plankton components (phytoplankton, ciliates, zooplankton) were grouped by dietary preference. The studied oxbows differed in physicochemical parameters and in phytoplankton. Cyanobacteria dominated in the total biomass of phytoplankton in the Tyniec oxbow, big green algae (>30 μm) in Piekary and Jeziorzany 1, and euglenoids in Jeziorzany 2 oxbow. The dominance pattern of ciliates and zooplankton were similar in all oxbows. Algivorous ciliates were the main dominant ciliates, and among zooplankton the dominant ones were herbivores that feed on small algae (<30 μm). The oxbows differed significantly in total phytoplankton biomass, cyanobacteria biomass, euglenoid biomass, small green algae (<30 μm) biomass, total biomass of zooplankton, biomass of zooplankton feeding on bacteria + algae, and biomass of zooplankton feeding on big algae (>30 μm). There was no significant differences in ciliate biomass between oxbows. In redundancy analyses, the variability at the trophic groups of plankton was described by explanatory variables in 42.3 %, and positive relationships were found: e.g., between omnivorous zooplankton biomass, the biomass of ciliates feeding on bacteria + algae, and NH₄ level; between euglenoid biomass and dinoflagellate biomass; and between cyanobacteria biomass and bacterivorous ciliate biomass. Spearman correlation analysis revealed several relationships between different groups of plankton. In general, phytoplankton group shows more connection among themselves and with different zooplankton groups, e.g., phytoplankton biomass with herbivorous zooplankton biomass (-0.33); and cyanobacteria biomass with dinoflagellate biomass (0.65). Ciliates showed more connections among their trophic groups (e.g., algivorous ciliate biomass with omnivorous ciliate biomass, 0.78) and with zooplankton trophic groups (e.g., biomass of algivorous + bacterivorous ciliates with biomass of predator zooplankton, -0.36). Simple correlations analysis revealed the trophic food web network connectivity among plankton organisms, indicating the flow of organic matter from phytoplankton to zooplankton and from ciliates to zooplankton. Our study sheds light on the trophic relations among plankton ciliates, which are neglected in research but often form a large percentage of zooplankton biomass. In the studied oxbows, ciliate forms 6.7 % of total zooplankton biomass in Jeziorzany 1 and up to 44.5 % of it in the Piekary oxbow.

Preliminary analysis of allelochemicals produced by the diatom Phaeodactylum tricornutum

Marine diatom Phaeodactylum tricornutum is known to exude allelochemicals with negative effects on Heterosigma akashiwo according to our previous study, while the information about the allelochemical compounds remains unknown. The present study dealt with isolation and analysis of the active substances released by P. tricornutum into the culture medium. Filtrate of P. tricornutum was extracted using ethyl acetate and chloroform respectively. The anti-algal fractions were isolated using high performance liquid chromatography (HPLC) and screened using activity-guided fraction methods. Results demonstrated that fraction Ⅱ and Ⅵ showed significant allelopathic effect on H. akashiwo growth. Then the anti-algal activity fractions were analyzed preliminary using gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography-electrospray time-of-flight mass spectrometry (HPLC-ESI-TOF-MS). An active compound was derived from fraction Ⅵ with the molecular weight of 578 and possible molecular formula of C₃₀H₃₈N₆O₆, which was speculated to be TYR-PRO-PHE-PRO-GLY-NH₂. a kind of glycinamides.

A Genetic and Chemical Perspective on Symbiotic Recruitment of Cyanobacteria of the Genus Nostoc into the Host Plant Blasia pusilla L

Liverwort Blasia pusilla L. recruits soil nitrogen-fixing cyanobacteria of genus Nostoc as symbiotic partners. In this work we compared Nostoc community composition inside the plants and in the soil around them from two distant locations in Northern Norway. STRR fingerprinting and 16S rDNA phylogeny reconstruction showed a remarkable local diversity among isolates assigned to several Nostoc clades. An extensive web of negative allelopathic interactions was recorded at an agricultural site, but not at the undisturbed natural site. The cell extracts of the cyanobacteria did not show antimicrobial activities, but four isolates were shown to be cytotoxic to human cells. The secondary metabolite profiles of the isolates were mapped by MALDI-TOF MS, and the most prominent ions were further analyzed by Q-TOF for MS/MS aided identification. Symbiotic isolates produced a great variety of small peptide-like substances, most of which lack any record in the databases. Among identified compounds we found microcystin and nodularin variants toxic to eukaryotic cells. Microcystin producing chemotypes were dominating as symbiotic recruits but not in the free-living community. In addition, we were able to identify several novel aeruginosins and banyaside-like compounds, as well as nostocyclopeptides and nosperin.

Inhibitory effect of polyunsaturated aldehydes (PUAs) on the growth of the toxic benthic dinoflagellate Ostreopsis cf. ovata

Diatoms have been shown to produce and release a wide range of secondary metabolites that mediate interactions between individuals of different species. Among these compounds, different types of fatty acid derived long-chained polyunsaturated aldehydes (PUAs) have been related to multiple functions such as intra- or interspecific signals and adverse effect on the reproduction of marine organisms. Several studies have reported changes on growth, cell membrane permeability, flow cytometric properties and cell morphology in phytoplankton organisms exposed to PUAs, but little information is available on the effect of these compounds on benthic microalgae. Ostreopsis cf. ovata is a toxic benthic dinoflagellate which causes massive blooms along the Mediterranean coasts typically during the late summer period. In this study the effects of three toxic PUAs known to be produced by several algae (2E,4E-decadienal, 2E,4E-octadienal and 2E,4E-heptadienal) on the growth, cytological features and cell morphology of O. cf. ovata were investigated. Our results show a clear decrease of O. cf. ovata growth with longer-chain molecules than with shorter-chain ones, confirmed also by EC50 values calculated at 48h for 2E,4E-decadienal and 2E,4E-octadienal (6.6±1.5, 17.9±2.6μmolL(-1) respectively) and at 72h for 2E,4E-heptadienal (18.4±0.7μmolL(-1)). Moreover, morphological analysis highlighted up to 79% of abnormal forms of O. cf. ovata at the highest concentrations of 2E,4E-decadienal tested (9, 18 and 36μmolL(-1)), a gradual DNA degradation and an increase of lipid droplets with all tested PUAs. Further studies are needed to better clarify the interactions between diatoms and O. cf. ovata, especially on bloom-forming dynamics.

Modulation of host ROS metabolism is essential for viral infection of a bloom-forming coccolithophore in the ocean

The cosmopolitan coccolithophore Emiliania huxleyi is a unicellular eukaryotic alga responsible for vast blooms in the ocean. These blooms have immense impact on large biogeochemical cycles and are terminated by a specific large double-stranded DNA E. huxleyi virus (EhV, Phycodnaviridae). EhV infection is accompanied by induction of hallmarks of programmed cell death and production of reactive oxygen species (ROS). Here we characterized alterations in ROS metabolism and explored its role during infection. Transcriptomic analysis of ROS-related genes predicted an increase in glutathione (GSH) and H2O2 production during infection. In accordance, using biochemical assays and specific fluorescent probes we demonstrated the overproduction of GSH during lytic infection. We also showed that H2O2 production, rather than superoxide, is the predominant ROS during the onset of the lytic phase of infection. Using flow cytometry, confocal microscopy and multispectral imaging flow cytometry, we showed that the profound co-production of H2O2 and GSH occurred in the same subpopulation of cells but at different subcellular localization. Positively stained cells for GSH and H2O2 were highly infected compared with negatively stained cells. Inhibition of ROS production by application of a peroxidase inhibitor or an H2O2 scavenger inhibited host cell death and reduced viral production. We conclude that viral infection induced remodeling of the host antioxidant network that is essential for a successful viral replication cycle. This study provides insight into viral replication strategy and suggests the use of specific cellular markers to identify and quantify the extent of active viral infection during E. huxleyi blooms in the ocean.

The common bloom-forming cyanobacterium Microcystis is prone to a wide array of microbial antagonists

Many degraded waterbodies around the world are subject to strong proliferations of cyanobacteria - notorious for their toxicity, high biomass build-up and negative impacts on aquatic food webs - the presence of which puts serious limits on the human use of affected water bodies. Cyanobacterial blooms are largely regarded as trophic dead ends since they are a relatively poor food source for zooplankton. As a consequence, their population dynamics are generally attributed to changes in abiotic conditions (bottom-up control). Blooms however generally contain a vast and diverse community of micro-organisms of which some have shown devastating effects on cyanobacterial biomass. For Microcystis, one of the most common bloom-forming cyanobacteria worldwide, a high number of micro-organisms (about 120 taxa) including viruses, bacteria, microfungi, different groups of heterotrophic protists, other cyanobacteria and several eukaryotic microalgal groups are currently known to negatively affect its growth by infection and predation or by the production of allelopathic compounds. Although many of these specifically target Microcystis, sharp declines of Microcystis biomass in nature are only rarely assigned to these antagonistic microbiota. The commonly found strain specificity of their interactions may largely preclude strong antagonistic effects on Microcystis population levels but may however induce compositional shifts that can change ecological properties such as bloom toxicity. These highly specific interactions may form the basis of a continuous arms race (co-evolution) between Microcystis and its antagonists which potentially limits the possibilities for (micro)biological bloom control.

The interaction between cyanobacteria and zooplankton in a more eutrophic world

As blooms of cyanobacteria expand and intensify in freshwater systems globally, there is increasing interest in their ecological effects. In addition to being public health hazards, cyanobacteria have long been considered a poor quality food for key zooplankton grazers that link phytoplankton to higher trophic levels. While past laboratory studies have found negative effects of nutritional constraints and defensive traits (i.e., toxicity and colonial or filamentous morphology) on the fitness of large generalist grazers (i.e., Daphnia), cyanobacterial blooms often co-exist with high biomass of small-bodied zooplankton in nature. Indeed, recent studies highlight the remarkable diversity and flexibility in zooplankton responses to cyanobacterial prey. Reviewed here are results from a wide range of laboratory and field experiments examining the interaction of cyanobacteria and a diverse zooplankton taxa including cladocerans, copepods, and heterotrophic protists from temperate to tropical freshwater systems. This synthesis shows that longer exposure to cyanobacteria can shift zooplankton communities toward better-adapted species, select for more tolerant genotypes within a species, and induce traits within the lifetime of individual zooplankton. In turn, the function of bloom-dominated plankton ecosystems, the coupling between primary producers and grazers, the stability of blooms, and the potential to use top down biomanipulation for controlling cyanobacteria depend largely on the species, abundance, and traits of interacting cyanobacteria and zooplankton. Understanding the drivers and consequences of zooplankton traits, such as physiological detoxification and selective vs. generalist grazing behavior, are therefore of major importance for future studies. Ultimately, co-evolutionary dynamics between cyanobacteria and their grazers may emerge as a critical regulator of blooms.

Distribution and Habitat Specificity of Potentially-Toxic Microcystis across Climate, Land, and Water Use Gradients

Toxic cyanobacterial blooms are a growing threat to freshwater bodies worldwide. In order for a toxic bloom to occur, a population of cells with the genetic capacity to produce toxins must be present together with the appropriate environmental conditions. In this study, we investigated the distribution patterns and phylogeny of potentially-toxic Microcystis (indicated by the presence and/or phylogeny of the mcyD and mcyA genes). Samples were collected from the water column of almost 60 water bodies across widely differing gradients of environmental conditions and land use in Israel. Potentially, toxic populations were common but not ubiquitous, detected in ~65% of the studied sites. Local environmental factors, including phosphorus and ammonia concentrations and pH, as well as regional conditions such as the distance from built areas and nature reserves, were correlated with the distribution of the mcyD gene. A specific phylogenetic clade of Microcystis, defined using the sequence of the mcyA gene, was preferentially associated with aquaculture facilities but not irrigation reservoirs. Our results reveal important environmental, geospatial, and land use parameters affecting the geographic distribution of toxinogenic Microcystis, suggesting non-random dispersal of these globally abundant toxic cyanobacteria.

Effect of the toxin (microcystin) content of Microcystis on copepod grazing

Although phytoplankton chemical defense may regulate plankton dynamics, demonstrating an ecologically relevant anti-grazer cue is challenging. Presented here is a novel approach to evaluate the quantitative effect of microcystin (MC), the most studied group of cyanobacterial metabolites, on grazing by the common copepod Eudiaptomus gracilis. A temperature-induced gradient in the intracellular MC concentration of three different Microcystis strains enabled the comparison of grazing pressure on cells of the same cyanobacterial strain producing different amounts of MC, in a diet with alternative food (Chlamydomonas). In all treatments, grazing pressure on Microcystis was inversely related to its MC-LR content, while selection for alternative prey was positively related to the MC-LR content of Microcystis. Moreover, grazing on Chlamydomonas also declined with increasing Microcystis MC-LR content, suggesting toxicity related inhibition of E. gracilis. The negative relation between cellular MC-LR concentration and feeding responses supported the anti-grazer hypothesis. Not all MC variants responded to temperature, and some were therefore not associated to grazing responses. Using an induced gradient in the concentration of a suspected phytoplankton defense metabolite to evaluate its quantitative relationship with grazing pressure offers an improved inference on the ecological roles of toxins. Results suggest that either MC-LR or a correlating trait may be inversely linked to the grazer pressure on Microcystis.

Study on the release routes of allelochemicals from Pistia stratiotes Linn., and its anti-cyanobacteria mechanisms on Microcystis aeruginosa

Allelochemicals in Pistia stratiotes Linn. have a strong anti-cyanobacteria effect on Microcystis aeruginosa. To further determine the release routes of allelochemicals in P. stratiotes and understand their anti-cyanobacteria mechanisms, we aimed to systematically investigate the allelopathic effects of leaf leachates, leaf volatilization, root exudates, and residue decomposition of P. stratiotes on M. aeruginosa. The influences of P. stratiotes allelochemicals on the physiological properties of M. aeruginosa were also studied. Root exudates of P. stratiotes exhibited the strongest inhibitory effect on M. aeruginosa growth. The residue decomposition and leaf leachates exhibited a relatively strong inhibitory effect on M. aeruginosa growth. By contrast, the leaf volatilization stimulated M. aeruginosa growth. Therefore, root exudation was determined to be the main release route of allelochemicals from P. stratiotes. The mixed culture experiment of P. stratiotes root exudates and M. aeruginosa showed that the allelochemicals released from root exudation had no effect on the electron transfer of M. aeruginosa photosynthetic system II. However, it reduced the phycocyanin (PC) content and phycocyanin to allophycocyanin (PC/APC) ratio in the photosynthetic system. As the root exudates concentration increased, the electrical conductivity (EC) and superoxide anion radical (O2(*-)) values in the M. aeruginosa culture fluid increased significantly, indicating that the allelochemicals released from the root of P. stratiotes inhibited algae growth by affecting the PC and PC/APC levels in photosynthesis, destroying the cell membrane, and increasing O2(*-) content to result in oxidative damage of M. aeruginosa.

Effects of Cylindrospermopsin Producing Cyanobacterium and Its Crude Extracts on a Benthic Green Alga-Competition or Allelopathy?

Cylindrospermopsin (CYN) is a toxic secondary metabolite produced by filamentous cyanobacteria which could work as an allelopathic substance, although its ecological role in cyanobacterial-algal assemblages is mostly unclear. The competition between the CYN-producing cyanobacterium Chrysosporum (Aphanizomenon) ovalisporum, and the benthic green alga Chlorococcum sp. was investigated in mixed cultures, and the effects of CYN-containing cyanobacterial crude extract on Chlorococcum sp. were tested by treatments with crude extracts containing total cell debris, and with cell debris free crude extracts, modelling the collapse of a cyanobacterial water bloom. The growth inhibition of Chlorococcum sp. increased with the increasing ratio of the cyanobacterium in mixed cultures (inhibition ranged from 26% to 87% compared to control). Interestingly, inhibition of the cyanobacterium growth also occurred in mixed cultures, and it was more pronounced than it was expected. The inhibitory effects of cyanobacterial crude extracts on Chlorococcum cultures were concentration-dependent. The presence of C. ovalisporum in mixed cultures did not cause significant differences in nutrient content compared to Chlorococcum control culture, so the growth inhibition of the green alga could be linked to the presence of CYN and/or other bioactive compounds.

Impairment of benthic diatom adhesion and photosynthetic activity by allelopathic compounds from a green alga: involvement of free fatty acids?

The role of chemical interactions in shaping microbial communities has raised increasing interest over the last decade. Many benthic microorganisms are known to develop chemical strategies to overcome competitors, but the real importance of chemical interactions within freshwater biofilm remains unknown. This study focused on the biological and chemical mechanisms of an interaction involving two benthic microorganisms, an allelopathic filamentous green alga, Uronema confervicolum, and a common diatom, Fistulifera saprophila. Our results showed that functions critical for benthic phototrophic microorganisms were inhibited by U. confervicolum extracts. Growth, cell motility, adhesion, and photosynthetic activity were impaired at extract concentrations ranging between 5 and 20 μg ml(-1). The adhesion inhibition was mediated by intracellular nitric oxide (NO) induction. A bioassay-guided fractionation of the extract with HPLC helped to identify two C18 fatty acids present in the growth-inhibiting fractions: linoleic (LA) and α-linolenic (LNA) acids. These compounds represented 77% of the total free fatty acids of U. confervicolum and were present in the culture medium (1.45 μg l(-1) in total). Both could inhibit the diatom growth at concentrations higher than 0.25 μg ml(-1), but had no effect on cell adhesion. The discrepancy between the effective concentrations of fatty acids and the concentration found in culture medium may be explained by the presence of high-concentration microenvironments. The compounds involved in adhesion inhibition remain to be identified. Though further experiments with complex biofilms are needed, our results suggest that U. confervicolum may participate to the control of biofilm composition by inhibiting diatom adhesion.

Temperature Effect on Exploitation and Interference Competition among Microcystis aeruginosa, Planktothrix agardhii and, Cyclotella meneghiniana

We studied the effect of temperature (18 and 30°C) on growth and on the exploitation and interference competition of three species: Microcystis aeruginosa (MIJAC), Planktothrix agardhii (PAT), and Cyclotella meneghiniana (CCAP). Coculturing the organisms in batch systems allowed for the examination of both competitive interactions, while the interference competition was studied in cross-cultures. The experiments were done during 10–12 days, and samples were taken for chlorophyll-a analysis, using PHYTO-PAM. The temperature did not influence exploitation competition between MIJAC and other competitors and it was the best competitor in both temperatures. PAT presented higher growth rates than CCAP in competition at 18 and 30°C. The temperature influenced the interference competition. The growth of MIJAC was favored in strains exudates at 30°C, while CCAP was favored at 18°C, revealing that the optimum growth temperature was important to establish the competitive superiority. Therefore, we can propose two hypotheses: (i) different temperatures may results in production of distinct compounds that influence the competition among phytoplankton species and (ii) the target species may have different vulnerability to these compounds depending on the temperature. At last, we suggest that both the sensitivity and the physiological status of competing species can determine their lasting coexistence.

Spatial dynamics of a nutrient-phytoplankton system with toxic effect on phytoplankton

The production of toxins by some species of phytoplankton is known to have several economic, ecological, and human health impacts. However, the role of toxins on the spatial distribution of phytoplankton is not well understood. In the present study, the spatial dynamics of a nutrient-phytoplankton system with toxic effect on phytoplankton is investigated. We analyze the linear stability of the system and obtain the condition for Turing instability. In the presence of toxic effect, we find that the distribution of nutrient and phytoplankton becomes inhomogeneous in space and results in different patterns, like stripes, spots, and the mixture of them depending on the toxicity level. We also observe that the distribution of nutrient and phytoplankton shows spatiotemporal oscillation for certain toxicity level.

Long-term changes in cyanobacteria populations in lake kinneret (sea of galilee), Israel: an eco-physiological outlook

The long-term record of cyanobacteria abundance in Lake Kinneret (Sea of Galilee), Israel, demonstrates changes in cyanobacteria abundance and composition in the last five decades. New invasive species of the order Nostocales (Aphanizomenon ovalisporum and Cylindrospermopsis raciborskii) became part of the annual phytoplankton assemblage during summer-autumn. Concomitantly, bloom events of Microcystis sp. (Chroococcales) during winter-spring intensified. These changes in cyanobacteria pattern may be partly attributed to the management policy in Lake Kinneret’s vicinity and watershed aimed to reduce effluent discharge to the lake and partly to climate changes in the region; i.e., increased water column temperature, less wind and reduced precipitation. The gradual decrease in the concentration of total and dissolved phosphorus and total and dissolved nitrogen and an increase in alkalinity, pH and salinity, combined with the physiological features of cyanobacteria, probably contributed to the success of cyanobacteria. The data presented here indicate that the trend of the continuous decline of nutrients may not be sufficient to reduce and to control the abundance and proliferation of toxic and non-toxic cyanobacteria.

Comprehensive insights into the response of Alexandrium tamarense to algicidal component secreted by a marine bacterium

Harmful algal blooms occur throughout the world, threatening human health, and destroying marine ecosystems. Alexandrium tamarense is a globally distributed and notoriously toxic dinoflagellate that is responsible for most paralytic shellfish poisoning incidents. The culture supernatant of the marine algicidal bacterium BS02 showed potent algicidal effects on A. tamarense ATGD98-006. In this study, we investigated the effects of this supernatant on A. tamarense at physiological and biochemical levels to elucidate the mechanism involved in the inhibition of algal growth by the supernatant of the strain BS02. Reactive oxygen species (ROS) levels increased following exposure to the BS02 supernatant, indicating that the algal cells had suffered from oxidative damage. The levels of cellular pigments, including chlorophyll a and carotenoids, were significantly decreased, which indicated that the accumulation of ROS destroyed pigment synthesis. The decline of the maximum photochemical quantum yield (Fv/Fm) and relative electron transport rate (rETR) suggested that the photosynthesis systems of algal cells were attacked by the BS02 supernatant. To eliminate the ROS, the activities of antioxidant enzymes, including superoxide dismutase (SOD) and catalase (CAT), increased significantly within a short period of time. Real-time PCR revealed changes in the transcript abundances of two target photosynthesis-related genes (psbA and psbD) and two target respiration-related genes (cob and cox). The transcription of the respiration-related genes was significantly inhibited by the treatments, which indicated that the respiratory system was disturbed. Our results demonstrate that the BS02 supernatant can affect the photosynthesis process and might block the PS II electron transport chain, leading to the production of excessive ROS. The increased ROS can further destroy membrane integrity and pigments, ultimately inducing algal cell death.

The molecular ecophysiology of programmed cell death in marine phytoplankton

Planktonic, prokaryotic, and eukaryotic photoautotrophs (phytoplankton) share a diverse and ancient evolutionary history, during which time they have played key roles in regulating marine food webs, biogeochemical cycles, and Earth's climate. Because phytoplankton represent the basis of marine ecosystems, the manner in which they die critically determines the flow and fate of photosynthetically fixed organic matter (and associated elements), ultimately constraining upper-ocean biogeochemistry. Programmed cell death (PCD) and associated pathway genes, which are triggered by a variety of nutrient stressors and are employed by parasitic viruses, play an integral role in determining the cell fate of diverse photoautotrophs in the modern ocean. Indeed, these multifaceted death pathways continue to shape the success and evolutionary trajectory of diverse phytoplankton lineages at sea. Research over the past two decades has employed physiological, biochemical, and genetic techniques to provide a novel, comprehensive, mechanistic understanding of the factors controlling this key process. Here, I discuss the current understanding of the genetics, activation, and regulation of PCD pathways in marine model systems; how PCD evolved in unicellular photoautotrophs; how it mechanistically interfaces with viral infection pathways; how stress signals are sensed and transduced into cellular responses; and how novel molecular and biochemical tools are revealing the impact of PCD genes on the fate of natural phytoplankton assemblages.

Metabolomics and proteomics reveal impacts of chemically mediated competition on marine plankton

Competition is a major force structuring marine planktonic communities. The release of compounds that inhibit competitors, a process known as allelopathy, may play a role in the maintenance of large blooms of the red-tide dinoflagellate Karenia brevis, which produces potent neurotoxins that negatively impact coastal marine ecosystems. K. brevis is variably allelopathic to multiple competitors, typically causing sublethal suppression of growth. We used metabolomic and proteomic analyses to investigate the role of chemically mediated ecological interactions between K. brevis and two diatom competitors, Asterionellopsis glacialis and Thalassiosira pseudonana. The impact of K. brevis allelopathy on competitor physiology was reflected in the metabolomes and expressed proteomes of both diatoms, although the diatom that co-occurs with K. brevis blooms (A. glacialis) exhibited more robust metabolism in response to K. brevis. The observed partial resistance of A. glacialis to allelopathy may be a result of its frequent exposure to K. brevis blooms in the Gulf of Mexico. For the more sensitive diatom, T. pseudonana, which may not have had opportunity to evolve resistance to K. brevis, allelopathy disrupted energy metabolism and impeded cellular protection mechanisms including altered cell membrane components, inhibited osmoregulation, and increased oxidative stress. Allelopathic compounds appear to target multiple physiological pathways in sensitive competitors, demonstrating that chemical cues in the plankton have the potential to alter large-scale ecosystem processes including primary production and nutrient cycling.

Mapping the diatom redox-sensitive proteome provides insight into response to nitrogen stress in the marine environment

Diatoms are ubiquitous marine photosynthetic eukaryotes responsible for approximately 20% of global photosynthesis. Little is known about the redox-based mechanisms that mediate diatom sensing and acclimation to environmental stress. Here we used a quantitative mass spectrometry-based approach to elucidate the redox-sensitive signaling network (redoxome) mediating the response of diatoms to oxidative stress. We quantified the degree of oxidation of 3,845 cysteines in the Phaeodactylum tricornutum proteome and identified approximately 300 redox-sensitive proteins. Intriguingly, we found redox-sensitive thiols in numerous enzymes composing the nitrogen assimilation pathway and the recently discovered diatom urea cycle. In agreement with this finding, the flux from nitrate into glutamine and glutamate, measured by the incorporation of (15)N, was strongly inhibited under oxidative stress conditions. Furthermore, by targeting the redox-sensitive GFP sensor to various subcellular localizations, we mapped organelle-specific oxidation patterns in response to variations in nitrogen quota and quality. We propose that redox regulation of nitrogen metabolism allows rapid metabolic plasticity to ensure cellular homeostasis, and thus is essential for the ecological success of diatoms in the marine ecosystem.

Grazers and phytoplankton growth in the oceans: an experimental and evolutionary perspective

The taxonomic composition of phytoplankton responsible for primary production on continental shelves has changed episodically through Earth history. Geological correlations suggest that major changes in phytoplankton composition correspond in time to changes in grazing and seawater chemistry. Testing hypotheses that arise from these correlations requires experimentation, and so we carried out a series of experiments in which selected phytoplankton species were grown in treatments that differed with respect to the presence or absence of grazers as well as seawater chemistry. Both protistan (Euplotes sp.) and microarthropod (Acartia tonsa) grazers changed the growth dynamics and biochemical composition of the green alga Tetraselmis suecica, the diatom Thalassiosira weissflogii, and the cyanobacterium Synechococcus sp., increasing the specific growth rate and palatability of the eukaryotic algae, while decreasing or leaving unchanged both parameters in the cyanobacteria. Synechococcus (especially) and Thalassiosira produced toxins effective against the copepod, but ciliate growth was unaffected. Acartia induced a 4-6 fold increase of Si cell quota in the diatom, but Euplotes had no similar effect. The differential growth responses of the eukaryotic algae and cyanobacteria to ciliate grazing may help to explain the apparently coeval radiation of eukaryophagic protists and rise of eukaryotes to ecological prominence as primary producers in Neoproterozoic oceans. The experimental results suggest that phytoplankton responses to the later radiation of microarthropod grazers were clade-specific, and included changes in growth dynamics, toxin synthesis, encystment, and (in diatoms) enhanced Si uptake.

Interactions between Scenedesmus and Microcystis may be used to clarify the role of secondary metabolites

Microcystis sp. are major players in the global intensification of toxic cyanobacterial blooms endangering the water quality of freshwater bodies. A novel green alga identified as Scenedesmus sp., designated strain huji (hereafter S. huji), was isolated from water samples containing toxic Microcystis sp. withdrawn from Lake Kinneret (Sea of Galilee), Israel, suggesting that it produces secondary metabolites that help it withstand the Microcystis toxins. Competition experiments suggested complex interaction between these two organisms and use of spent cell-free media from S. huji caused severe cell lysis in various Microcystis strains. We have isolated active metabolites from the spent S. huji medium. Application of the concentrated allelochemicals interfered with the functionality and perhaps the integrity of the Microcystis cell membrane, as indicated by the rapid effect on the photosynthetic variable fluorescence and leakage of phycobilins and ions. Although some activity was observed towards various bacteria, it did not alter growth of eukaryotic organisms such as the green alga Chlamydomonas reinhardtii.

The contamination and control of biological pollutants in mass cultivation of microalgae

The potential of microalgae as a biomass feedstock for biofuels, bioproducts and as a technological solution for CO(2) fixation is subject to intense academic and industrial researches. However, current microalgal mass culture technologies have failed to produce bulk volume of microalgal biomass at low cost, because the contaminations of biological pollutants become a big constraint in mass cultivation and impede the industrial process. Here the transmission routes, contamination mechanisms of biological pollutants both in open ponds and photobioreactors are described and recent attempts to overcome the barrier are reviewed. What worth noting, unlike conventional microbial fermentation which uses a pure monoculture, the cultivation of microalgae is a complicated symbiotic system of microalgae-bacterial-zooplankton where the target microalgae dominate, cross infection or contamination by biological pollutants is inevitable and it will require much further research. Further investigation and development of control methods are necessary, particularly microalgal strain selection.

Stability of toxin gene proportion in red-pigmented populations of the cyanobacterium Planktothrix during 29 years of re-oligotrophication of Lake Zürich

BACKGROUND

Harmful algal blooms deteriorate the services of aquatic ecosystems. They are often formed by cyanobacteria composed of genotypes able to produce a certain toxin, for example, the hepatotoxin microcystin (MC), but also of nontoxic genotypes that either carry mutations in the genes encoding toxin synthesis or that lost those genes during evolution. In general, cyanobacterial blooms are favored by eutrophication. Very little is known about the stability of the toxic/nontoxic genotype composition during trophic change.

RESULTS

Archived samples of preserved phytoplankton on filters from aquatic ecosystems that underwent changes in the trophic state provide a so far unrealized possibility to analyze the response of toxic/nontoxic genotype composition to the environment. During a period of 29 years of re-oligotrophication of the deep, physically stratified Lake Zürich (1980 to 2008), the population of the stratifying cyanobacterium Planktothrix was at a minimum during the most eutrophic years (1980 to 1984), but increased and dominated the phytoplankton during the past two decades. Quantitative polymerase chain reaction revealed that during the whole observation period the proportion of the toxic genotype was strikingly stable, that is, close to 100%. Inactive MC genotypes carrying mutations within the MC synthesis genes never became abundant. Unexpectedly, a nontoxic genotype, which lost its MC genes during evolution, and which could be shown to be dominant under eutrophic conditions in shallow polymictic lakes, also co-occurred in Lake Zürich but was never abundant. As it is most likely that this nontoxic genotype contains relatively weak gas vesicles unable to withstand the high water pressure in deep lakes, it is concluded that regular deep mixing selectively reduced its abundance through the destruction of gas vesicles.

CONCLUSIONS

The stability in toxic genotype dominance gives evidence for the adaptation to deep mixing of a genotype that retained the MC gene cluster during evolution. Such a long-term dominance of a toxic genotype draws attention to the need to integrate phylogenetics into ecological research as well as ecosystem management.