Insights into the interaction mechanisms between Microcystin-degrading bacteria and Microcystis aeruginosa.

Interactions between bacteria and cyanobacteria influence the occurrence and development of harmful cyanobacterial blooms (HCBs). Bloom-forming cyanobacteria and cyanotoxin-degrading bacteria are essential in HCBs, nonetheless, their interactions and the underlying mechanisms remain unclear. To address this gap, a typical microcystin-LR (MC-LR)-degrading bacterium and a toxic Microcystis aeruginosa strain were co-cultivated to investigate their interactions. The cyanobacterial growth was enhanced by 24.8 %-44.3 % in the presence of the bacterium in the first 7 days, and the cyanobacterium enhanced the bacterial growth by 59.2 %-117.5 % throughout the growth phases, suggesting a mutualistic relationship between them. The presence of the bacterium increased cyanobacterial intracellular MC-LR content on days 4, 8, and 10 while reducing the extracellular MC-LR concentration, revealing the dual roles of the bacterium in enhancing cyanotoxin production and degrading cyanotoxins. The bacterium alleviated the oxidative stress, which may be crucial in promoting cyanobacterial growth. Critical functional genes related to cyanobacterial photosynthesis and MC-LR synthesis, and bacterial MC-LR degradation were up-regulated in the presence of the bacterium and cyanobacterium, respectively. Moreover, extracellular polymeric substances (EPS) were produced at the cell interface, implying EPS play a role in cyanobacterial-bacterial interactions. This study is the first to unveil the interaction mechanisms between cyanotoxin-degrading bacteria and bloom-forming cyanobacteria, shedding light on the dynamics of HCBs.

Intensified effect of nitrogen forms on dominant phytoplankton species succession by climate change.

Nutrient proportion, light intensity, and temperature affect the succession of dominant phytoplankton species. Despite these insights, this transformation mechanism in highly turbid lakes remains a research gap, especially in response to climate change. To fill this gap, we investigated the mechanism by which multi-environmental factors influence the succession of dominant phytoplankton species in Lake Chagan. This investigation deployed the structural equation model (SEM) and the hydrodynamic-water quality-water ecology mechanism model. Results demonstrated that the dominant phytoplankton species in Lake Chagan transformed from diatom to cyanobacteria during 2012 and 2022. Notably, Microcystis was detected in 2022. SEM revealed the primary environment variables for this succession, including water temperature (Tw), nutrients (total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH4N)), and total suspended solids (TSS). Moreover, this event was not the consequence of zooplankton grazing. An integrated hydrodynamic-water quality-bloom mechanism model was built to explore the mechanism driving phytoplankton succession and its response to climate change. Nutrients determined the phytoplankton biomass and dominant species succession based on various proportions. High NH4N:NO3N ratios favored cyanobacteria and inhibited diatom under high TSS. Additionally, the biomass proportions of diatom (30.77 % vs. 22.28 %) and green (30.56 % vs. 23.30 %) decreased dramatically. In contrast, cyanobacteria abundance remarkably increased (35.78 % to 51.71 %) with the increasing NH4-N:NO3-N ratios. In addition, the proportion of non-nitrogen-fixing cyanobacteria was higher than that of the nitrogen-fixing cyanobacteria counterparts when TN:TP≥20 and NH4N:NO3N ≥ 10. Light-limitation phenotypes also experienced an increase with the rising NH4N:NO3N ratios. Notably, the cyanobacterial biomass reached 3-6 times that in the baseline scenario when the air temperature escalated by 3.0 °C until 2061 under the SSP585 scenario. We highlighted the effect of nitrogen forms on the succession of dominant phytoplankton species. Climate warming will increase nitrogen proportion, providing an insightful reference for controlling cyanobacterial blooms.

Community Structure and Toxicity Potential of Cyanobacteria during Summer and Winter in a Temperate-Zone Lake Susceptible to Phytoplankton Blooms.

Cyanobacterial blooms are increasingly common during winters, especially when they are mild. The goal of this study was to determine the summer and winter phytoplankton community structure, cyanotoxin presence, and toxigenicity in a eutrophic lake susceptible to cyanobacterial blooms throughout the year, using classical microscopy, an analysis of toxic cyanometabolites, and an analysis of genes involved in biosynthesis of cyanotoxins. We also assessed whether cyanobacterial diversity in the studied lake has changed compared to what was reported in previous reports conducted several years ago. Moreover, the bloom-forming cyanobacterial strains were isolated from the lake and screened for cyanotoxin presence and toxigenicity. Cyanobacteria were the main component of the phytoplankton community in both sampling times, and, in particular, Oscillatoriales were predominant in both summer (Planktothrix/Limnothrix) and winter (Limnothrix) sampling. Compared to the winter community, the summer community was denser; richer in species; and contained alien and invasive Nostocales, including Sphaerospermopsis aphanizomenoides, Raphidiopsis raciborskii, and Raphidiopsis mediterranea. In both sampling times, the blooms contained toxigenic species with genetic determinants for the production of cylindrospermopsin and microcystins. Toxicological screening revealed the presence of microcystins in the lake in summer but no cyanotoxins in the winter period of sampling. However, several cyanobacterial strains isolated from the lake during winter and summer produced anabaenopeptins and microcystins. This study indicates that summer and winter blooms of cyanobacteria in the temperate zone can differ in biomass, structure, and toxicity, and that the toxic hazards associated with cyanobacterial blooms may potentially exist during winter.

The ice phenology as a predictor of Planktothrix rubescens bloom in vegetation season in temperate lakes.

Introduction: Global warming affects air and water temperatures, which impacts the phenology of lakes and aquatic ecosystems. These changes are most noticeable during winter, when the potentially toxic Planktothrix rubescens forms its inoculum for annual blooms. Mostly, research has been conducted on alpine lakes, where blooms have persisted for decades, while a few have focused on temperate lakes. Our study aimed to determine the factors influencing the dynamics of the development of P. rubescens in temperate lakes where blooms occasionally occur, with a particular emphasis on the role of ice phenology. Methods: We investigated the vertical distribution of P. rubescens in an annual cycle in three temperate lakes. Samples were collected monthly in the winter and biweekly during the vegetative seasons. Overall, 434 samples were collected and analyzed according to biological and chemical parameters. Physical parameters were measured in situ. Results: The vegetation seasons in temperate lakes showed a similar development pattern in the P. rubescens population as that in alpine lakes. Our results also show the influence of physical and chemical factors on the vertical distribution of this cyanobacterium. These results revealed the significant impact of P. rubescens filaments on phytoplankton biodiversity and biomass. Our data show the role of ice phenology in the establishment of the winter inoculum of P. rubescens and its further mass development until its disappearance in autumn. Conclusion: A climate-zone-independent pattern of P. rubescens blooms was observed during the vegetation periods. The population of P. rubescens was more influenced by physical factors than by the availability of dissolved nutrients in the water. Despite the same etiology, global warming has been shown to cause different responses in aquatic ecosystems, which affect the different nature of P. rubescens appearances. We associated blooms in temperate lakes, in contrast to alpine lakes, mainly with the presence of ice cover during severe winters, when the species establishes its inoculum. Hence, blooms in temperate lakes occur at different time intervals. Therefore, the dynamics of periodic blooms of P. rubescens in temperate lakes provide novel knowledge to the case study and a counterpoint to permanent blooms found in deep alpine lakes.

Enhanced inhibitory efficiency against toxic bloom forming Raphidiopsis raciborskii by Streptomyces sp. HY through triple algicidal modes: Direct and indirect attacks combined with bioflocculation.

Raphidiopsis raciborskii (R. raciborskii) forms harmful cyanobacterial blooms globally, and poses a great threat to the safety of drinking water and public health. There is a great need to develop eco-friendly biological alternative measures to mitigate mass blooms of R. raciborskii. However, previous rare studies on algicidal microorganisms against R. raciborskii restricted this aim. Recently, an algicidal bacterium Streptomyces sp. HY (designated HY) was identified with flavones producing ability, and could remove up to 98.73 % of R. raciborskii biomass within 48 h by directly attacking the cyanobacterium and release of algicidal substances (i.e., flavonoids) with a inoculum ratio of 5 %. Algicidal rate of HY was enhanced by 88.05 %, 89.33 % under dark and light, and full-light conditions respectively, when compared with the dark condition. Its algicidal substances were stable in a broad range of temperature (-80-55 °C) and pH (3-11) conditions, and all treated groups exhibited ≈ 100 % algicidal rate at day 3. HY treatment disrupted the photosynthesis system and triggered serious oxidative stress resulting in severe morphological injury. Thereby, HY treatment significantly affected expression levels of several essential genes (i.e., psbA, psaB, rbcL, ftsZ, recA, grpE), and simultaneously inhibited the biosynthesis and release of cylindrospermopsin. Yet, HY treatment didn't show any toxicity to zebrafish test embryos. Such results indicate that HY is a promising algicidal candidate strain to control global R. raciborskii blooms, and holds great promises for an effective biological measure to sustain water safety.

Exudates of Microcystis aeruginosa on oxidative stress and inflammatory responses in gills of Sinocyclocheilus grahami.

Early cyanobacterial blooms studies observed that exposure to blue-green algae led to fish gills impairment. The objective of this work was to evaluate the toxic mechanisms of exudates of Microcystis aeruginosa (MaE) on fish gills. In this study, the toxic mechanism of MaE (2×106 cells/mL) and one of its main components phytosphingosine (PHS) with two concentrations 2.9 ng/mL and 145 ng/mL were conducted by integrating histopathology, biochemical biomarkers, and transcriptomics techniques in Sinocyclocheilus grahami (S. grahami) for 96 h exposure. Damaged gill tissue with epithelial hyperplasia and hypertrophy, remarkable Na+/K+-ATPase (NKA) enzyme activity, disrupted the redox homeostats including lipid peroxidation and inflammatory responses were observed in the fish of MaE exposure group. Compare to MaE exposure, two concentrations of PHS exposure appeared to be a trend of lower degree of tissue damage, NKA activity and oxidative stress, but induced obviously lipid metabolism disorder with higher triglycerides, total cholesterol and total bile acid, which might be responsible for inflammation responses in fish gill. By transcriptome analysis, MaE exposure were primarily enriched in pathways related to gill function and immune response. PHS exposure, with higher number of differentially expressed genes (DEGs), were enriched in Toll-like receptor (TLR), Mitogen-Activated Protein Kinase (MAPK) and NOD-like receptor protein 3 (NLRP3) pathways. We concluded that MaE and PHS were induced the inflammatory responses, with oxidative stress-induced inflammation for MaE exposure but lipid metabolism disorder-induced inflammation for PHS exposure. The present study provided two toxin-induced gill inflammation response pathways under cyanobacterial blooms, which could be a scientific basis for the ecological and health risk assessment in the aquatic environment.

A non-microcystin-producing Microcystis wesenbergii strain alters fish food intake by disturbing neuro-endocrine appetite regulation.

Cyanobacterial harmful algal blooms (cHABs) are pervasive sources of stress resulting in neurotoxicity in fish. A member of the widely distributed Microcystis genus of bloom-forming cyanobacteria, Microcystis wesenbergii can be found in many freshwater lakes, including Dianchi Lake (China), where it has become one of the dominant contributors to the lake's recurrent blooms. However, unlike its more well-known counterpart M. aeruginosa, the effects of dense non-microcystin-containing M. wesenbergii blooms are seldom studied. The disturbance of appetite regulation and feeding behaviour can have downstream effects on the growth of teleost fish, posing a significant challenge to aquaculture and conservation efforts. Here we examined the effects of M. wesenbergii blooms on the food intake of Acrossocheilus yunnanensis, a native cyprinid in southern China. This fish species has disappeared in Dianchi Lake, and its reintroduction might be negatively affected by the presence of this newly-dominant Microcystis species. We co-cultured juvenile A. yunnanensis with a non-microcystin-producing strain of M. wesenbergii at initial densities between 5 × 104 and 1 × 106 cells/mL and monitored fish feeding behaviour and changes in neurotransmitter and hormone protein levels. High-density M. wesenbergii cultures increased the feeding rate of co-cultured fish, elevating concentrations of appetite-stimulating signalling molecules (Agouti-related protein and γ-aminobutyric acid), while decreasing inhibitory ones (POMC). These changes coincided with histopathological alterations and reduced somatic indices in brain and intestinal tissues. Given this potential for detrimental effects and dysregulation of food intake, further studies are necessary to determine the impacts of chronic exposure of M. wesenbergii in wild fish.

Differences in environmental microbial community responses under rice-crab co-culture and crab monoculture models under cyanobacterial bloom.

Cyanobacterial blooms (CBs) present significant challenges to Chinese mitten crab (CMC) culture, posing hazards to the aquatic microbial ecology. However, the current focus on the microbial ecological changes within the CMC culture system under the influence of CBs is somewhat insufficient. There's an urgent need to analyze the microbial ecosystem of the CMC culture system under CBs. This study employed 16S rRNA gene amplicon sequencing to investigate the dynamics of the environmental microbial community in both the rice-crab co-culture (RC) and crab monoculture (CM) models. The results revealed that cyanobacteria reached high levels in the CM water in July, while they began to increase in the RC water in August. Notably, OTU147 (uncultured bacterium g_Planktothrix NIVA-CYA 15), identified as the dominant taxon associated with CBs, showed a significant linear relationship with TP, NO2 --N, and the N:P ratio. TP, TN, NO2 --N, and CODMn had a more pronounced impact on the structure of bacterial communities and cyanobacterial taxa in the water. The bacterial community structure involved in carbon metabolism displayed temporal succession in the water. The co-occurrence network of the bacterial community primarily consisted of Chloroflexi, Proteobacteria, and Firnicutes in the sediment, and Actinobacteria, Proteobacteria, Chloroflexi, and Bacteroidota in the water. In contrast, the co-occurrence network included different peripheral species in the sediment and water. Keystone species were predominantly represented by OTU22 (uncultured actinobacterium g_ hgcI clade) and OTU12 (uncultured Opitutae bacterium g_ norank) in the RC water, and by OTU25 (uncultured bacterium g_ Limnohabitans) in the CM water. TP, TN, NO2 --N, and CODMn were identified as the primary environmental factors influencing these keystone taxa within the culture water. In conclusion, this study on the microbial ecology of the CMC culture system under the influence of CBs provides valuable insights that can be instrumental in subsequent management efforts.

Cyanobacterial blooms in Lake Taihu: Temporal trends and potential drivers.

Lake Taihu, an inland lake, frequently experiences Cyanobacterial blooms that have historically posed severe threats to its aquatic ecosystem. Combining field observations and satellite remote-sensed data, factors that influence algal bloom intensity in Lake Taihu over an eight-year period, from 2016 to 2023, are examined, and changes in phytoplankton community composition, climate, water quality, economic activities, and food web dynamics are reported. Sentinel-2 MSI data analysis reveals a dramatic decrease in Cyanobacterial blooms in 2023, with a reduction in the annual maximum bloom area of 76.90 % from 2016 values. From 2016 to 2022, the ratio of Cyanobacteria to other phytoplankton ranged 82.09 %-98.29 %, but in 2023, this dropped to 60.98 %. Concurrently, Cyanobacteria density dropped to an historic low of 2.29 × 107 cells/L (16.4 % of 2021 peak values). Redundancy and random forest analyses indicated that nitrogen has a greater influence on phytoplankton than phosphorus, with temperature and permanganate index being the important parameters to affect phytoplankton community structure. We attribute the decrease in Cyanobacterial blooms in Lake Taihu in 2023 to be largely caused by shifts in phytoplankton community structure, particularly the sharp decline in Microcystis sp. density, a genus often linked to bloom formation. Meteorological conditions such as reduced rainfall and wind speed during the winter and spring of 2023 also contributed to diminishing Cyanobacterial blooms. Ongoing improvements in water quality, reduced economic activities because of pandemic restrictions, and implementation of a fishing ban since 2020 have likely further contributed to reductions in bloom frequency. These results improve our understanding of the processes that affect algal blooms in Lake Taihu, and potentially other eutrophic inland lakes.

Ecotoxicological assessment of cyclic peptides produced by a Planktothrix rubescens bloom: Impact on aquatic model organisms.

Cyanobacterial blooms, a natural phenomenon in freshwater ecosystems, have increased in frequency and severity due to climate change and eutrophication. Some cyanobacteria are able to produce harmful substances called cyanotoxins. These metabolites possess different chemical structures and action mechanisms representing a serious concern for human health and the environment. The most studied cyanotoxins belong to the group of microcystins which are potent hepatotoxins. Anabaenopeptins are another class of cyclic peptides produced by certain species of cyanobacteria, including Planktothrix spp. Despite limited knowledge regarding individual effects of anabaenopeptins on freshwater organisms, reports have identified in vivo toxicity in representatives of freshwater zooplankton by cyanobacterial extracts or mixtures containing anabaenopeptins. This study focused on the isolation and toxicity evaluation of the cyanotoxins produced in the 2022 Planktothrix rubescens bloom in Averno lake, Italy. The three main cyclic peptides have been isolated and identified by nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS) and optical analyses as anabaenopeptins A and B, and oscillamide Y. Ecotoxicological tests on the aquatic model organisms Daphnia magna (crustacean), Raphidocelis subcapitata (algae), and Aliivibrio fischeri (bacterium) revealed that anabaenopeptins A and B do not generate significant toxicity at environmentally relevant concentrations, being also found a stimulatory effect on R. subcapitata in the case of anabaenopeptin A. By contrast, oscillamide Y displayed toxicity. Ecological implications based on ECOSAR predictions align with experimental data. Moreover, long-term exposure bioassays on different green unicellular algae species showed that R. subcapitata was not significantly affected, while Scenedesmus obliquus and Chlorella vulgaris exhibited altered growth patterns. These results, together with the already-known background in literature, highlight the complexity of interactions between organisms and the tested compounds, which may be influenced by species-specific sensitivities, physiological differences, and modes of action, possibly affected by parameters like lipophilicity.

How do seasonal temperature variations influence interplay between toxic and non-toxic cyanobacterial blooms? Evidence from modeling and experimental data.

Summer cyanobacterial blooms exhibit a dynamic interplay between toxic and non-toxic genotypes, significantly influencing the cyanotoxin levels within a lake. The challenge lies in accurately predicting these toxin concentrations due to the significant temporal fluctuations in the proportions of toxic and non-toxic genotypes. Typically, the toxic genotypes dominate during the early and late summer periods, while the non-toxic variants prevail in mid-summer. To dissect this phenomenon, we propose a model that accounts for the competitive interaction between toxic and non-toxic genotypes, as well as seasonal temperature variations. Our numerical simulations suggest that the optimal temperature of the toxic genotypes is lower than that of the optimal temperatures of the non-toxic counterparts. This difference of optimal temperature may potentially contribute to explain the dominance of toxic genotypes at the early and late summer periods, situation often observed in the field. Experimental data from the laboratory align qualitatively with our simulation results, enabling a better understanding of complex interplays between toxic and non-toxic cyanobacteria.

Cyanotoxin cylindrospermopsin disrupts lipid homeostasis and metabolism in a 3D in vitro model of the human liver.

Cylindrospermopsin, a potent hepatotoxin produced by harmful cyanobacterial blooms, poses environmental and human health concerns. We used a 3D human liver in vitro model based on spheroids of HepG2 cells, in combination with molecular and biochemical assays, automated imaging, targeted LC-MS-based proteomics, and lipidomics, to explore cylindrospermopsin effects on lipid metabolism and the processes implicated in hepatic steatosis. Cylindrospermopsin (1 µM, 48 h) did not significantly affect cell viability but partially reduced albumin secretion. However, it increased neutral lipid accumulation in HepG2 spheroids while decreasing phospholipid levels. Simultaneously, cylindrospermopsin upregulated genes for lipogenesis regulation (SREBF1) and triacylglycerol synthesis (DGAT1/2) and downregulated genes for fatty acid synthesis (ACLY, ACCA, FASN, SCD1). Fatty acid uptake, oxidation, and lipid efflux genes were not significantly affected. Targeted proteomics revealed increased levels of perilipin 2 (adipophilin), a major hepatocyte lipid droplet-associated protein. Lipid profiling quantified 246 lipid species in the spheroids, with 28 significantly enriched and 15 downregulated by cylindrospermopsin. Upregulated species included neutral lipids, sphingolipids (e.g., ceramides and dihexosylceramides), and some glycerophospholipids (phosphatidylethanolamines, phosphatidylserines), while phosphatidylcholines and phosphatidylinositols were mostly reduced. It suggests that cylindrospermopsin exposures might contribute to developing and progressing towards hepatic steatosis or metabolic dysfunction-associated steatotic liver disease (MASLD).

Spatial and Temporal Resolution of Cyanobacterial Bloom Chemistry Reveals an Open-Ocean Trichodesmium thiebautii as a Talented Producer of Specialized Metabolites.

While the ecological role that Trichodesmium sp. play in nitrogen fixation has been widely studied, little information is available on potential specialized metabolites that are associated with blooms and standing stock Trichodesmium colonies. While a collection of biological material from a T. thiebautii bloom event from North Padre Island, Texas, in 2014 indicated that this species was a prolific producer of chlorinated specialized metabolites, additional spatial and temporal resolution was needed. We have completed these metabolite comparison studies, detailed in the current report, utilizing LC-MS/MS-based molecular networking to visualize and annotate the specialized metabolite composition of these Trichodesmium blooms and colonies in the Gulf of Mexico (GoM) and other waters. Our results showed that T. thiebautii blooms and colonies found in the GoM have a remarkably consistent specialized metabolome. Additionally, we isolated and characterized one new macrocyclic compound from T. thiebautii, trichothilone A (1), which was also detected in three independent cultures of T. erythraeum. Genome mining identified genes predicted to synthesize certain functional groups in the T. thiebautii metabolites. These results provoke intriguing questions of how these specialized metabolites affect Trichodesmium ecophysiology, symbioses with marine invertebrates, and niche development in the global oligotrophic ocean.

Eco-friendly management of harmful cyanobacterial blooms in eutrophic lakes through vertical flow multi-soil-layering technology.

Eutrophication has led to the widespread occurrence of cyanobacterial blooms. Toxic cyanobacterial blooms with high concentrations of microcystins (MCs) have been identified in the Lalla Takerkoust reservoir in Morocco. The objective of this study was to evaluate the efficiency of the Multi-Soil-Layering (MSL) ecotechnology in removing natural cyanobacterial blooms from the lake. Two MSL pilots were used in rectangular glass tanks (60 × 10 × 70 cm). They consisted of permeable layers (PLs) made of pozzolan and a soil mixture layer (SML) containing local soil, ferrous metal, charcoal and sawdust. The main difference between the two systems was the type of local soil used: sandy soil for MSL1 and clayey soil for MSL2. Both MSL pilots effectively reduced cyanobacterial cell concentrations in the treated water to very low levels (0.09 and 0.001 cells/mL). MSL1 showed a gradual improvement in MC removal from 52 % to 99 %, while MSL2 started higher at 90 % but dropped to 54% before reaching 86%. Both MSL systems significantly reduced organic matter levels (97.2 % for MSL1 and 95.8 % for MSL2). Both MSLs were shown to be effective in removing cyanobacteria, MCs, and organic matter with comparable performance.

The secondary outbreak risk and mechanisms of Microcystis aeruginosa after H2O2 treatment.

The secondary outbreak of cyanobacteria after algicide treatment has been a serious problem to water ecosystems. Hydrogen peroxide (H2O2) is an algaecide widely used in practice, but similar re-bloom problems are inevitably encountered. Our work found that Microcystis aeruginosa (M. aeruginosa) temporarily hibernates after H2O2 treatment, but there is still a risk of secondary outbreaks. Interestingly, the dormant period was as long as 20 and 28 days in 5 mg L-1 and 20 mg L-1 H2O2 treatment groups, respectively, but the photosynthetic activity was both restored much earlier (within 14 days). Subsequently, a quantitative imaging flow cytometry-based method was constructed and confirmed that the re-bloom had undergone two stages including first recovery and then re-division. The expression of ftsZ and fabZ genes showed that M. aeruginosa had active transcription processes related to cell division protein and fatty acid synthesis during the dormancy stat. Furthermore, metabolomics suggested that the recovery of M. aeruginosa was mainly by activating folate and salicylic acid synthesis pathways, which promoted environmental stress resistance, DNA synthesis, and cell membrane repair. This study reported the comprehensive mechanisms of secondary outbreak of M. aeruginosa after H2O2 treatment. The findings suggest that optimizing the dosage and frequency of H2O2, as well as exploring the potential use of salicylic acid and folic acid inhibitors, could be promising directions for future algal control strategies.

Cyanobacterial blooms control with CaO2 in different stages: Inhibition efficiency, water quality optimization and microbial community changes.

This study explored the feasibility of calcium peroxide (CaO2) to inhibit cyanobacterial blooms of the outbreak and dormancy stages. Our previous studies have found that CaO2 has a high inhibitory effect on cyanobacteria. In order to explore the application effect of CaO2 in actual cyanobacteria lake water, we conducted this study to clarify the effect of CaO2 on inhibiting cyanobacteria in outbreak and dormancy stages. The results showed that CaO2 inhibited the growth of cyanobacteria in the outbreak and dormancy stages by 98.7% and 97.6%, respectively. The main inhibitory mechanism is: (1) destroy the cell structure and make the cells undergo programmed cell death by stimulating the oxidation balance of cyanobacteria cells; (2) EPS released by cyanobacteria resist stimulation and combine calcium to form colonies, and accelerate cell settlement. In addition to causing direct damage to cyanobacteria, CaO2 can also improve water quality and sediment microbial diversity, and reduce the release of sediment to phosphorus, so as to further contribute to cyanobacterial inhibition. Finally, the results of qRT-PCR analysis confirmed the promoting effect of CaO2 on the downregulation of photosynthesis-related genes (rbcL and psaB), microcystn (mcyA and mcyD) and peroxiredoxin (prx), and verified the mechanism of CaO2 inhibition of cyanobacteria. In conclusion, this study provides new findings for the future suppression of cyanobacterial bloom, by combining water quality, cyanobacterial inhibition mechanisms, and sediment microbial diversity.

Dynamics of Microcystis surface scum formation under different wind conditions: the role of hydrodynamic processes at the air-water interface.

Due to climate change, Microcystis blooms occur at increasing frequencies in aquatic ecosystems worldwide. Wind-generated turbulence is a crucial environmental stressor that can vertically disperse the Microcystis surface scum, reducing its light availability. Yet, the interactions of Microcystis scum with the wind-generated hydrodynamic processes, particularly those at the air-water interface, remain poorly understood. Here, we explore the response of Microcystis (including colony size and migration dynamics) to varying magnitudes and durations of intermittent wind disturbances in a mesocosm system. The flow velocities, size of Microcystis colonies, and the areal coverage of the water surface by scum were measured through video observations. Our results demonstrate that low wind speeds increase colony size by providing a stable condition where Microcystis forms a scum layer and aggregates into large colonies at the air-water interface. In contrast, wind disturbances disperse scum and generate turbulence, resulting in smaller colonies with higher magnitudes of wind disturbance. We observed that surface scum can form rapidly following a long period (6 h) of high-magnitude (4.5 m s-1) wind disturbance. Furthermore, our results indicate reduced water surface tension caused by the presence of Microcystis, which can decrease surface flow velocity and counteract wind-driven mixing. The reduced surface tension may also drive lateral convection at the water surface. These findings suggest that Microcystis reduces surface tension, likely by releasing surface-active materials, as an adaptive response to various wind conditions. This could result in an increased rate of surface scum re-formation under wind conditions and potentially facilitate the lateral expansion of scum patches during weak wind periods. This study reveals new insights into how Microcystis copes with different wind conditions and highlights the importance of the air-water interface for Microcystis scum dynamics.

Cycles of solar ultraviolet radiation favor periodic expansions of cyanobacterial blooms in global lakes.

Global warming and eutrophication are known to increase the prevalence of cyanobacterial blooms, posing a severe threat to the ecological stability and sustainability of water bodies. The long-term (over an annual time frame) effect of UV radiation on cyanobacterial blooms in lakes are rarely discussed though the substantial effects of high-intensity UV radiation on the growth inhibition of marine phytoplankton were studied. Here, we employed the datasets on surface solar UV radiation, nitrogen and phosphorus concentrations, and the annual scales and frequencies of cyanobacterial blooms in lakes across long-term spatial scales to probe the relationship of UV radiation with cyanobacterial blooms. The results indicated that enhanced solar UV radiation may unintentionally stimulate cyanobacterial growth and favor the expansions of cyanobacterial blooms in lakes around the world. The fluctuating UV radiation significantly affects the annual scales of cyanobacterial blooms in both eutrophic and oligotrophic lakes. Solar UV radiation enhances the positive impact of rising phosphorus levels on cyanobacterial blooms because UV radiation prompts the synthesis of polyphosphate in cyanobacteria cells, which helps cyanobacteria to alleviate the stress of UV light. The scales of cyanobacterial blooms are significantly impacted by solar UV radiation intensities as opposed to the annual frequency of cyanobacterial blooms. Furthermore, solar UV radiation fluctuation with a 9-year period over a 14-year main cycles significantly affects the periodicities of cyanobacterial blooms in global lakes, which provides a basis for predicting the peak value of the scales of cyanobacterial blooms in lakes. These findings opened up new avenues of inquiry into the mechanism and management strategies of cyanobacterial blooms in lakes worldwide.

Detection of Anatoxins in Human Urine by Liquid Chromatography Triple Quadrupole Mass Spectrometry and ELISA.

Harmful cyanobacterial blooms are becoming more common and persistent around the world. When in bloom, various cyanobacterial strains can produce anatoxins in high concentrations, which, unlike other cyanobacterial toxins, may be present in clear water. Potential human and animal exposures to anatoxins occur mainly through unintentional ingestion of contaminated algal mats and water. To address this public health threat, we developed and validated an LC-MS/MS method to detect anatoxins in human urine to confirm exposures. Pooled urine was fortified with anatoxin-a and dihydroanatoxin at concentrations from 10.0 to 500 ng/mL to create calibrators and quality control samples. Samples were diluted with isotopically labeled anatoxin and solvent prior to LC-MS/MS analysis. This method can accurately quantitate anatoxin-a with inter- and intraday accuracies ranging from 98.5 to 103% and relative standard deviations < 15%, which is within analytical guidelines for mass spectrometry methods. Additionally, this method qualitatively detects a common degradation product of anatoxin, dihydroanatoxin, above 10 ng/mL. We also evaluated a commercial anatoxin-a ELISA kit for potential diagnostic use; however, numerous false positives were detected from unexposed individual human urine samples. In conclusion, we have developed a method to detect anatoxins precisely and accurately in urine samples, addressing a public health area of concern, which can be applied to future exposure events.

Experimental warming promotes phytoplankton species sorting towards cyanobacterial blooms and leads to potential changes in ecosystem functioning.

A positive feedback loop where climate warming enhances eutrophication and its manifestations (e.g., cyanobacterial blooms) has been recently highlighted, but its consequences for biodiversity and ecosystem functioning are not fully understood. We conducted a highly replicated indoor experiment with a species-rich subtropical freshwater phytoplankton community. The experiment tested the effects of three constant temperature scenarios (17, 20, and 23 °C) under high-nutrient supply conditions on community composition and proxies of ecosystem functioning, namely resource use efficiency (RUE) and CO2 fluxes. After 32 days, warming reduced species richness and promoted different community trajectories leading to a dominance by green algae in the intermediate temperature and by cyanobacteria in the highest temperature treatments. Warming promoted primary production, with a 10-fold increase in the mean biomass of green algae and cyanobacteria. The maximum RUE occurred under the warmest treatment. All treatments showed net CO2 influx, but the magnitude of influx decreased with warming. We experimentally demonstrated direct effects of warming on phytoplankton species sorting, with negative effects on diversity and direct positive effects on cyanobacteria, which could lead to potential changes in ecosystem functioning. Our results suggest potential positive feedback between the phytoplankton blooms and warming, via lower net CO2 sequestration in cyanobacteria-dominated, warmer systems, and add empirical evidence to the need for decreasing the likelihood of cyanobacterial dominance.