Trophic transfer of microcystins through the lake pelagic food web: evidence for the role of zooplankton as a vector in fish contamination

Suspended particulate matter-biofilm aggregates benefit microcystin removal in turbulent water but trigger toxicity toward Daphnia magna

Suspended particulate matter (SPM) and biofilm are critical in removing contaminants in aquatic environments, but the environmental behavior and ecological toxicity of SPM-biofilm aggregates modulated by turbulence intensities are largely unknown. This study determined the removal pathways of microcystin-LR (MC-LR) by SPM and its biofilm under different turbulence intensities (2.25 × 10-3, 1.01 × 10-2, and 1.80 × 10-2 m2/s3). Then, we evaluated the toxicity of SPM-biofilm aggregates to Daphnia magna. The results revealed that SPM contributed to the adsorption of MC-LR, and the removal of MC-LR can be accelerated with biofilm formation on SPM, with 95.66 % to 97.45 % reduction in MC-LR concentration under the studied turbulence intensities. Higher turbulence intensity triggered more frequent contact of SPM and MC-LR, formed compact but smaller clusters of SPM-biofilm aggregates, and enhanced the abundance of mlrA and mlrB; thus benefiting the adsorption, biosorption, and biodegradation of MC-LR. Furthermore, the SPM-biofilm aggregates formed in turbulent water triggered oxidative stress to Daphnia magna, while a weak lethal toxic effect was identified under moderate turbulence intensity. The results indicate that the toxicity of SPM-biofilm aggregates fail to display a linear relationship with turbulence intensity. These findings offer new perspectives on understanding the environmental behavior and ecological outcomes of SPM and its biofilms in turbulent aquatic environments.

Zooplankton functional diversity as a bioindicator of freshwater ecosystem health across land use gradient

Zooplankton are critical indicators of pressures impacting freshwater ecosystems. We analyzed the response of zooplankton communities across different sub-catchment types-headwaters, natural, urban, urban-agricultural, and agricultural-within the Łyna river-lake system in Northern Poland. Using taxonomic groups and functional traits (body size, feeding strategies), we applied Partial Least Squares Regression (PLS-R) to elucidate the relationships between environmental conditions, land use, and zooplankton metacommunity structure. Two-Way Cluster Analysis (TWCA) identified local subsets with characteristic patterns, while Indicator Species Analysis (ISA) determined area-specific taxa. The natural river zone exhibited significant habitat heterogeneity and feeding niches, whereas urban areas created functional homogenization of zooplankton, dominated by small, broad-diet microphages. Agricultural areas promoted diversity among large filter feeders (Crustacea), active suctors (Rotifera), and amoebae (Protozoa). However, intensified agricultural activities, substantially diminished the zooplankton population, biomass, taxonomic richness, and overall ecosystem functionality. The impact of land cover change is more pronounced at small-scale sub-catchments than at the catchment level as a whole. Therefore, assessing these impacts requires detailed spatial and temporal analysis at the sub-catchment level to identify the most affected areas. This study introduces a new sub-catchment-based perspective on ecosystem health assessment and underscores the zooplankton's role as robust indicators of ecological change.

Fate of Planktothrix-derived toxins in aquatic food webs: A case study in Lake Mindelsee (Germany)

Blooms of the red, filamentous cyanobacterium Planktothrix rubescens occur frequently in pre-alpine lakes in Europe, often with concomitant toxic microcystin (MC) production. Trophic transfer of MCs has been observed in bivalves, fish, and zooplankton species, while uptake of MCs into Diptera species could facilitate distribution of MCs into terrestrial food webs and habitats. In this study, we characterized a Planktothrix bloom in summer 2019 in Lake Mindelsee and tracked possible trophic transfer and/or bioaccumulation of MCs via analysis of phytoplankton, zooplankton (Daphnia) and emergent aquatic insects (Chaoborus, Chironomidae and Trichoptera). Using 16 S rRNA gene amplicon sequencing, we found that five sequence variants of Planktothrix spp. were responsible for bloom formation in September and October of 2019, and these MC-producing variants, provisionally identified as P. isothrix and/or P. serta, occurred exclusively in Lake Mindelsee (Germany), while other variants were also detected in nearby Lake Constance. The remaining cyanobacterial community was dominated by Cyanobiaceae species with high species overlap with Lake Constance, suggesting a well-established exchange of cyanobacteria species between the adjacent lakes. With targeted LC-HRMS/MS we identified two MC-congeners, MC-LR and [Asp3]MC-RR with maximum concentrations of 45 ng [Asp3]MC-RR/L in lake water in September. Both MC congeners displayed different predominance patterns, suggesting that two different MC-producing species occurred in a time-dependent manner, whereby [Asp3]MC-RR was clearly associated with the Planktothrix spp. bloom. We demonstrate an exclusive transfer of MC-LR, but not [Asp3]MC-RR, from phytoplankton into zooplankton reaching a 10-fold bioconcentration, yet complete absence of these MC congeners or their conjugates in aquatic insects. The latter demonstrated a limited trophic transfer of MCs from zooplankton to zooplanktivorous insect larvae (e.g., Chaoborus), or direct transfer into other aquatic insects (e.g. Chironomidae and Trichoptera), whether due to avoidance or limited uptake and/or rapid excretion of MCs by higher trophic emergent aquatic insects.

The response of zooplankton network indicators to winter water warming using shallow artificial reservoirs as model case study

To predict the most likely scenarios, the consequences of the rise in water surface temperature have been studied using various methods. We tested the hypothesis that winter water warming significantly alters the importance and nature of the relationships in zooplankton communities in shallow reservoirs. These relationships were investigated using network graph analysis for three thermal variants: warm winters (WW), moderate winters (MW) and cold winters (CW). The CW network was the most cohesive and was controlled by eutrophic Rotifera and Copepoda, with a corresponding number of positive and negative interspecific relationships. An increase in water temperature in winter led to a decrease in the centrality of MW and WW networks, and an increase in the importance of species that communicated with the highest number of species in the subnetworks. The WW network was the least cohesive, controlled by psammophilous and phytophilous rotifers, and littoral cladocerans. Adult copepods were not identified in the network and the importance of antagonistic relationships decreased, indicating that the WW network structure was weak and unstable. This study can serve as a model for generalisations of zooplankton community response to the disappearance of long winter periods of low temperatures, as predicted in global climate change projections.

Microcystin-LR induces lactate production disruption via altering the m6A modification in Sertoli cells

We have previously reported the toxicity of microcystin-LR (MC-LR) to the male reproductive system, which results in functional changes in mouse testes. In this study, mice were orally exposed to MC-LR at 1, 7.5, 15, or 30 μg/L daily for 180 days. We found an increase in germ cell apoptosis in the seminiferous tubules and low-quality sperm in the epididymis. A decrease in lactate dehydrogenase A (Ldha) expression in testes through high-throughput sequencing was observed. We validated that MC-LR disrupted lactate production in Sertoli cells by suppressing the expression of Ldha. Further studies identified that methyltransferase 3 (Mettl3) catalysed N6-methyladenosine (m6A) methylation of Ldha mRNA. Mettl3 was downregulated in Sertoli cells following exposure to MC-LR, decreasing m6A levels of Ldha. The stability of Ldha mRNA decreased when m6A levels of Ldha were inhibited. In conclusion, these results showed that MC-LR inhibits the expression of Ldha in an m6A-dependent manner, which might result in the apoptosis of spermatogenic cells and a decline in sperm quality. Our work provides a new perspective to understanding MC-LR-induced male infertility.

Microcystin concentrations and congener composition in relation to environmental variables across 440 north-temperate and boreal lakes

Understanding the environmental conditions and taxa that promote the occurrence of cyanobacterial toxins is imperative for effective management of lake ecosystems. Herein, we modeled total microcystin presence and concentrations with a broad suite of environmental predictors and cyanobacteria community data collected across 440 Canadian lakes using standardized methods. We also conducted a focused analysis targeting 14 microcystin congeners across 190 lakes, to examine how abiotic and biotic factors influence their relative proportions. Microcystins were detected in 30 % of lakes, with the highest total concentrations occurring in the most eutrophic lakes located in ecozones of central Canada. The two most commonly detected congeners were MC-LR (61 % of lakes) and MC-LA (37 % of lakes), while 11 others were detected more sporadically across waterbodies. Congener diversity peaked in central Canada where cyanobacteria biomass was highest. Using a zero-altered hurdle model, the probability of detecting microcystin was best explained by increasing Microcystis biomass, Daphnia and cyclopoid biomass, soluble reactive phosphorus, pH and wind. Microcystin concentrations increased with the biomass of Microcystis and other less dominant cyanobacteria taxa, as well as total phosphorus, cyclopoid copepod biomass, dissolved inorganic carbon and water temperature. Collectively, these models accounted for 34 % and 70 % of the variability, respectively. Based on a multiple factor analysis of microcystin congeners, cyanobacteria community data, environmental and zooplankton data, we found that the relative abundance of most congeners varied according to trophic state and were related to a combination of cyanobacteria genera biomasses and environmental variables.

Transmission of Microcystins in Natural Systems and Resource Processes: A Review of Potential Risks to Humans Health

The rapid rise of microcystins (MCs) poses a serious threat to global freshwater ecosystems and has become an important issue of global public health. MCs have considerable stability and are the most widely distributed hepatotoxins. It cannot only accumulate in aquatic organisms and transfer to higher nutrients and levels, but also be degraded or transferred during the resource utilization of cyanobacteria. No matter which enrichment method, it will lead to the risk of human exposure. This review summarizes the research status of MCs, and introduces the distribution of MCs in different components of aquatic ecosystems. The distribution of MCs in different aquatic organisms was summarized, and the potential risks of MCs in the environment to human safety were summarized. MCs have polluted all areas of aquatic ecosystems. In order to protect human life from the health threats caused by MCs, this paper also proposes some future research directions to promote MCs control and reduce human exposure to MCs.

Detection of numerous phycotoxins in young bull sharks (Carcharhinus leucas) collected from an estuary of national significance

Florida's Indian River Lagoon (IRL) has experienced large-scale, frequent blooms of toxic harmful algae in recent decades. Sentinel, or indicator, species can provide an integrated picture of contaminants in the environment and may be useful to understanding phycotoxin prevalence in the IRL. This study evaluated the presence of phycotoxins in the IRL ecosystem by using the bull shark (Carcharhinus leucas) as a sentinel species. Concentrations of phycotoxins were measured in samples collected from 50 immature bull sharks captured in the IRL between 2018 and 2020. Ultra-performance liquid chromatography/tandem mass spectrometry was used to measure toxins in shark gut contents, plasma, and liver. Analysis of samples (n = 123) demonstrated the presence of multiple phycotoxins (microcystin, nodularin, teleocidin, cylindrospermopsin, domoic acid, okadaic acid, and brevetoxin) in 82 % of sampled bull sharks. However, most detected toxins were in low prevalence (≤25 % of samples, per sample type). This study provides valuable baseline information on presence of multiple phycotoxins in a species occupying a high trophic position in this estuary of national significance.

Rapid in situ assessment of high-resolution spatial and temporal distribution of cyanobacterial blooms using fishery echosounder

Cyanobacterial blooms are increasing in frequency, magnitude, and duration globally because of enhanced eutrophication and climate change. Thus, comprehensive investigation and systematic monitoring of the spatial and temporal distribution of cyanobacteria in aquatic environments are urgently needed to better understand bloom development and complex interactions within a dynamic environment. Various methods have been used to investigate the distribution of cyanobacteria, however, none of them can provide high-resolution data for the three-dimensional spatial structure of the bloom and its dynamics in real time. In the present study, we investigated the applicability of a high-frequency (200 kHz) fishery echosounder, a type widely used in fisheries acoustics, to detect and estimate the cyanobacterial genus Microcystis bloom distribution and biomass in a shallow lake (Sulejów Reservoir, Poland). Verification of the usefulness of in situ acoustic quantification of bloom-forming cyanobacteria was based on a comparison of acoustic estimates of cyanobacterial biomass with the ground truth-that is, fluorometric measurements and chlorophyll a concentrations. We compared the acoustic estimates with other methods for continuous measurements along 10 predetermined parallel transects and point samples at 14 stations situated on the transects. In vertical hydroacoustic measurements at night, we observed that cyanobacterial biomass was highest in the uppermost layer and diminished continuously with depth. For both horizontal and vertical continuous measurements, we found significant positive correlations between acoustic and fluorometric estimates of cyanobacterial biomass. The traditional point samples measurements, however, did not agree equally well with the acoustic estimates, especially for vertical beam. We argue that the point measurements have more stochastic character and less adequately describe dynamic changes in the cyanobacteria distribution than continuous acoustic estimates. More studies are required to explore the cyanobacteria distribution patterns under different biological, physical, and meteorological conditions.

Structural insight into the substrate-binding mode and catalytic mechanism for MlrC enzyme of Sphingomonas sp. ACM-3962 in linearized microcystin biodegradation

Removing microcystins (MCs) safely and effectively has become an urgent global problem because of their extremely hazardous to the environment and public health. Microcystinases derived from indigenous microorganisms have received widespread attention due to their specific MC biodegradation function. However, linearized MCs are also very toxic and need to be removed from the water environment. How MlrC binds to linearized MCs and how it catalyzes the degradation process based on the actual three-dimensional structure have not been determined. In this study, the binding mode of MlrC with linearized MCs was explored using a combination of molecular docking and site-directed mutagenesis methods. A series of key substrate binding residues, including E70, W59, F67, F96, S392 and so on, were identified. Sodium dodecane sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to analyze samples of these variants. The activity of MlrC variants were measured using high performance liquid chromatography (HPLC). We used fluorescence spectroscopy experiments to research the relationship between MlrC enzyme (E), zinc ion (M), and substrate (S). The results showed that MlrC enzyme, zinc ion and substrate formed E-M-S intermediates during the catalytic process. The substrate-binding cavity was made up of N and C-terminal domains and the substrate-binding site mainly included N41, E70, D341, S392, Q468, S485, R492, W59, F67, and F96. The E70 residue involved in both substrate catalysis and substrate binding. In conclusion, a possible catalytic mechanism of the MlrC enzyme was further proposed based on the experimental results and a literature survey. These findings provided new insights into the molecular mechanisms of the MlrC enzyme to degrade linearized MCs, and laid a theoretical foundation for further biodegradation studies of MCs.

Microcystin‑leucine arginine promotes colorectal cancer cell proliferation by activating the PI3K/Akt/Wnt/β‑catenin pathway

Microcystin‑leucine arginine (MC‑LR) is an environmental toxin produced by cyanobacteria and is considered to be a potent carcinogen. However, to the best of our knowledge, the effect of MC‑LR on colorectal cancer (CRC) cell proliferation has never been studied. The aim of the present study was to investigate the effect of MC‑LR on CRC cell proliferation and the underlying mechanisms. Firstly, a Cell Counting Kit‑8 (CCK‑8) assay was conducted to determine cell viability at different concentrations, and 50 nM MC‑LR was chosen for further study. Subsequently, a longer CCK‑8 assay and a cell colony formation assay showed that MC‑LR promoted SW620 and HT29 cell proliferation. Furthermore, western blotting analysis showed that MC‑LR significantly upregulated protein expression of PI3K, p‑Akt (Ser473), p‑GSK3β (Ser9), β‑catenin, c‑myc and cyclin D1, suggesting that MC‑LR activated the PI3K/Akt and Wnt/β‑catenin pathways in SW620 and HT29 cells. Finally, the pathway inhibitors LY294002 and ICG001 were used to validate the role of the PI3K/Akt and Wnt/β‑catenin pathways in MC‑LR‑accelerated cell proliferation. The results revealed that MC‑LR activated Wnt/β‑catenin through the PI3K/Akt pathway to promote cell proliferation. Taken together, these data showed that MC‑LR promoted CRC cell proliferation by activating the PI3K/Akt/Wnt/β‑catenin pathway. The present study provided a novel insight into the toxicological mechanism of MC‑LR.

Variability in the zooplankton assemblages in relation to environmental variables in the tidal creeks of Sundarbans estuarine system, India

The present study illustrates a holistic account of zooplankton community dynamics in relation to physicochemical variables in the tidal creeks of Indian Sundarbans estuarine system. Eleven water variables and zooplankton samples were collected in a monthly basis from the six sampling locations between July 2016 and June 2018. Out of the 11 water parameters, seven parameters (temperature, salinity, dissolved oxygen, turbidity, PO₄ - P, NO₃ - N, and NO₂ - N) varied significantly (p ≤ 0.05) among seasons. A total of 63 zooplankton taxa were recorded with the predominance of Copepoda. Permutational multivariate analysis of variance (PERMANOVA) depicted the significant variations of zooplankton population both spatially (F = 2.313; p = 0.001) and temporally (F = 6.107; p = 0.001). Out of the 41 species of Copepoda recorded, 14 species (Paracalanus parvus, Parvocalanus dubia, Bestiolina similis, Acrocalanus gibber, A. gracilis, Acartia erythraea, A. spinicauda, Pseudodiaptomus serricaudatus, P. annandalei, P. aurivilli, Oithona brevicornis, O. similis, Longipedia weberi and Microsetella norvegica) were indicated as "characterizing species" in the creek environment, which highlighted the euryhaline nature as well as broad range of thermal tolerance of these species. Acrocalanus longicornis, Eucalanus crassus, Candacia bradyi, and Acartia sewelli were specifically found in pre-monsoon; Euterpina acutiformis, Clytemnestrata rostrata, Acartia plumosa, and Canthocalanus pauper were specific to post-monsoon season, whereas Mesocyclops leuckarti, Temora turbinata, and Temora discaudata were found only during monsoon. Thus, it explains the limited period of existence, highlighting the temporal shift in species abundance in the creeks. Overall, the calculated mean value of α-diversity (d' = 4.07; H' = 2.31) indicated a "good" zooplankton diversity in the creeks. The biota-environmental relationship following canonical correspondence analysis revealed that variables, viz., water temperature, turbidity, and NO₃ - N, were the explanatory variables during monsoon; pH and DO in post-monsoon; PO₄ - P, SiO₄ - Si, and salinity in pre-monsoon that explained the variations of zooplankton community compositions in the creeks. The variables (PO₄ - P, NO₃ - N, and SiO₄ - Si) that are correlated to the zooplankton community implied their indirect effect on the distribution of zooplankton by supporting the phytoplankton succession and production in the mangrove-dominated creeks.

Zooplankton network conditioned by turbidity gradient in small anthropogenic reservoirs

Water turbidity can significantly influence interspecific interactions in aquatic ecosystems. We tested the hypothesis that the turbidity gradient significantly differentiates the dynamics, significance and type of relationships in the structure of zooplankton communities colonizing mine pit reservoirs. The interactions between zooplankton species were evaluated by network graph analysis for three water turbidity classes: high turbidity (HT), moderate turbidity (MT) and low turbidity (LT). The HT network was most cohesive, and it was controlled by taxa grazing on various food sources within one ecological niche (Polyarthra longiremis, Brachionus angularis, Cyclops vicinus, Codonella cratera) and the positive and negative relationships between them were balanced. The MT biocenotic network was composed of three sub-networks connected by nodes with high communication attributes (Polyarthra vulgaris, Bosmina longirostris, C. vicinus), and antagonistic interactions (predation and competition) were less important. The LT network was most heterogeneous, and Daphnia cuculllata exerted the strongest influence on the network's structure by forming numerous positive (coexistence with predators) and negative (interference competition with microphagous rotifers) interspecific relationships. The study provides new information about the ecology of aquatic ecosystems, that are disturbed by changes in water turbidity.

Dynamics of phytoplankton community in seasonally open and closed wetlands in the Teesta-Torsa basin, India, and management implications for sustainable utilization

The present study deals with the broader understanding of phytoplankton assemblage pattern and their ecohydrological interactions in two ecologically distinct floodplain wetlands of Teesta - Torsa basin, India. Analyses of data revealed significant seasonal variations (p ≤ 0.05) of ten water variables (temperature, transparency, pH, conductivity, total dissolved solids, dissolved oxygen, total hardness, total alkalinity, PO₄ - P, and SiO₄ - Si) in both the wetlands; however, no significant variation was observed among the sampling stations. In total, 128 species of phytoplankton were recorded (118 species belonging to 94 genera in seasonally open; 103 species belonging to 86 genera in closed wetland). Four algal groups, viz. Cyanophyceae, Coscinodiscophyceae, Bacillariophyceae, and Chlorophyceae, were the dominant quantitative component, remarkably influencing the total phytoplankton population in both the wetlands, contributing ~ 87% of total phytoplankton. Species Aulacoseira granulata alone contributed 12 - 41% and 8 - 34% to the total phytoplankton in the seasonally open and closed wetland, respectively, and indicated high organic load in both the wetlands. Altogether thirty-six and thirty-one phytoplankton taxa appeared as major indicators across the seasons for seasonally open and closed wetland, respectively. The indicator taxa (Aulacoseira, Oscillatoria, Dolichospermum, Spirogyra, Synedra, Nitzschia, Navicula, Euglena, Phacus) in both the wetlands hinted that the wetlands are under pollution pressure. The assemblage structure of phytoplankton was related to transparency, NO₃ - N, PO₄ - P, SiO₄ - Si, total dissolved solids, and temperature as evident from BIO - ENV. Furthermore, the marginal test also selected similar variables (depth, transparency, conductivity, PO₄ - P, SiO₄ - Si) for seasonally open and the variables such as depth, conductivity, total dissolved solids, total alkalinity, and NO₃ - N for the closed wetland. The study showed that the seasonal riverine connectivity greatly influences the variations in phytoplankton community in the seasonally open wetland.

Harmful algal blooms and shellfish in the marine environment: an overview of the main molluscan responses, toxin dynamics, and risks for human health

Besides human health risks, phycotoxins may cause physiological injuries on molluscan shellfish and, consequently, damages to marine ecosystems and global fisheries production. In this way, this review aimed to present an overview of HABs impacts on marine shellfish by evaluating the effects of cultivated molluscs exposure to microalgae and cyanobacteria that form blooms and/or synthesize toxins. More specifically, it was assessed the main molluscan shellfish responses to harmful algae, trophic transfer and dynamics of phycotoxins, and the risks for human health. Of the 2420 results obtained from literature search, 150 scientific publications were selected after thorough inspections for subject adherence. In total, 70 molluscan species and 37 taxa of harmful algae were assessed from retrieved scientific publications. A significant positive correlation was found between the marine production of molluscs and the number of available studies by molluscan category. Molluscan responses to HABs and phycotoxins were categorized and discussed in three sub-sections: effects on grazing and behavior, metabolic and physiological reactions, and fitness consequences. The main histopathological injuries and toxin concentrations in molluscan tissues were also compiled and discussed. Bivalves often accumulate more toxins than gastropods and cephalopods, occasionally exceeding recommended levels for safe consumption, representing a risk for human health. Harmful algae impact on molluscan shellfish are complex to trace and predict; however, considering the perspective of increase in the occurrence and intensity of HABs, the intensification of efforts to expand the knowledge about HABs impacts on marine molluscs is crucial to mitigate the damages on economy and human health.

Marine algal toxins and their vectors in southern California cetaceans

Published baseline data on biotoxin exposure in cetaceans is sparse but critical for interpreting mortality events as harmful algal blooms increase in frequency and duration. We present the first synthesis of domoic acid (DA), saxitoxin (STX), okadaic acid (OA), and microcystin detections in the feces and urine of stranded and bycaught southern California cetaceans, over an 18 year period (2001-2018), along with corresponding stomach content data. DA was detected in 13 out of 19 cetacean species, most often in harbor porpoise (Phocoena phocoena) (81.8%, n = 22) and long-beaked common dolphins (Delphinus delphis bairdii) (74%, n = 231). Maximum DA concentrations of 324,000 ng/g in feces and 271, 967 ng/ml in urine were observed in D. d. bairdii. DA was detected more frequently and at higher concentrations in male vs. female D. d. bairdii. Higher fecal DA concentrations in D. d. bairdii were associated with a greater proportion of northern anchovy (Engraulis mordax) in the diet, indicating it may be a primary vector of DA. Fecal DA concentrations for D. d. bairdii off Point Conception were greater than those from animals sampled off Los Angeles and San Diego counties, reflecting greater primary productivity and higher Pseudo-nitzschia spp. abundance in that region and a greater abundance of E. mordax in the diet. STX was detected at low levels (fecal max = 7.5 ng/g, urine max = 17 ng/ml) in 3.6% (n = 165) of individuals from 3 out of 11 species. The occurrence of E. mordax in 100% of the 3 examined stomachs suggests this species could be a primary vector of the detected STX. OA was detected in 2.4% of tested individuals (n = 85) at a maximum fecal concentration of 422.8 ng/g. Microcystin was detected in 14.3% (n = 7) of tested individuals with a maximum liver concentration of 96.8 ppb.

Impedimetric Microcystin-LR Aptasensor Prepared with Sulfonated Poly(2,5-dimethoxyaniline)–Silver Nanocomposite

This paper presents a novel impedimetric aptasensor for cyanobacterial microcystin-LR (L, l-leucine; R, l-arginine) (MC-LR) containing a 5′ thiolated 60-mer DNA aptamer (i.e., 5′-SH-(CH2)6GGCGCCAAACAGGACCACCATGACAATTACCCATACCACCTCATTATGCCCCATCT CCGC-3′). A nanocomposite electrode platform comprising biocompatible poly(2,5-dimethoxyaniline) (PDMA)-poly(vinylsulfonate) (PVS) and silver nanoparticle (Ag0) on a glassy carbon electrode (GCE), i.e., (GCE/PDMA–PVS–Ag0) was used in the biosensor development. Small-angle X-ray scattering (SAXS) spectroscopic analysis revealed that the PDMA–PVS–Ag0 nanocomposites were polydispersed and contained embedded Ag0. Electrochemical impedance spectroscopy (EIS) responses of the aptasensor gave a dynamic linear range (DLR) and limit of detection (LOD) values of 0.01–0.1 ng L−1 MC-LR and 0.003 ng L−1 MC-LR, respectively. The cross-reactivity studies, which was validated with enzyme-linked immunosorbent assay (ELISA), showed that the aptasensor possesses excellent selectivity for MC-LR.

Treatment of microcystin (MC-LR) and nutrients in eutrophic water by constructed wetlands: Performance and microbial community

Cyanobacterial harmful algal blooms and microcystins (MCs) pollution pose serious threat to aquatic ecosystem and public health. Planted and unplanted constructed wetlands (CWs) filled with four substrates (i.e., gravel (G-CWs), ceramsite (C-CWs), iron-carbon (I-CWs) and slag (S-CWs)) were established to evaluate nutrients and a typical MCs variant (i.e., MC-LR) removal efficiency from eutrophic water affected by the presence of plant and different substrate. The response of the microbial community to the above factors was also analyzed in this study. The results indicate that the presence of plant can generally enhance nutrients and MC-LR removal efficiency in CWs, except for I-CWs. Throughout the experiment, all CWs exhibited good nitrogen removal efficiency with removal percentages exceeding 90%; TP and MC-LR average removal efficiency of C-CWs and I-CWs were greater than G-CWs and S-CWs irrespective of the presence of plant. The best MC-LR removal efficiency under different MC-LR loads was observed in planted C-CWs (ranged from 91.56% to 95.16%). Except for I-CWs, the presence of plant can enhance relative abundances of functional microorganisms involved in nutrients removal (e.g., Comamonadaceae and Planctomycetaceae) and MCs degradation (e.g., Burkholderiaceae). The microbial community diversity of I-CWs was simplified, while the relative abundance of Proteobacteria was highest in this study. The highest relative abundances of Comamonadaceae, Planctomycetaceae and Burkholderiaceae were observed in planted C-CWs. Overall, ceramisite and iron-carbon were more suitable to be applied in CWs for nutrients and MC-LR removal. This study provides a theoretical basis for practical application of CWs in eutrophication and MCs pollution control.

Advances in forecasting harmful algal blooms using machine learning models: A case study with Planktothrix rubescens in Lake Geneva

The development of anthropic activities during the 20th century increased the nutrient fluxes in freshwater ecosystems, leading to the eutrophication phenomenon that most often promotes harmful algal blooms (HABs). Recent years have witnessed the regular and massive development of some filamentous algae or cyanobacteria in Lake Geneva. Consequently, important blooms could result in detrimental impacts on economic issues and human health. In this study, we tried to lay the foundation of an HAB forecast model to help scientists and local stakeholders with the present and future management of this peri-alpine lake. Our forecast strategy was based on pairing two machine learning models with a long-term database built over the past 34 years. We created HAB groups via a K-means model. Then, we introduced different lag times in the input of a random forest (RF) model, using a sliding window. Finally, we used a high-frequency dataset to compare the natural mechanisms with numerical interaction using individual conditional expectation plots. We demonstrate that some HAB events can be forecasted over a year scale. The information contained in the concentration data of the cyanobacteria was synthesized in the form of four intensity groups that directly depend on the P. rubescens concentration. The categorical transformation of these data allowed us to obtain a forecast with correlation coefficients that stayed above a threshold of 0.5 until one year for the counting cells and two years for the biovolume data. Moreover, we found that the RF model predicted the best P. rubescens abundance for water temperatures around 14°C. This result is consistent with the biological processes of the toxic cyanobacterium. In this study, we found that the coupling between K-means and RF models could help in forecasting the development of the bloom-forming P. rubescens in Lake Geneva. This methodology could create a numerical decision support tool, which should be a significant advantage for lake managers.

Multimedia distributions, bioaccumulation, and trophic transfer of microcystins in the Geum River Estuary, Korea: Application of compound-specific isotope analysis of amino acids

To determine distributions, bioaccumulation, and trophic transfer of freshwater cyanobacterial toxins such as microcystins (MCs), surface water, suspended solids, sediments, and coastal organisms were collected from seven stations in inner and outer regions of the estuary dam in the Geum River Estuary in June and July 2017. Concentrations of MC variants (MC-LR, -RR, and -YR) in the multimedia samples were analyzed using a HPLC-MS/MS. Trophic position (TP) of organisms (fish, bivalve, gastropod, decapod, and polychaete) was determined by nitrogen stable isotope analyses of both bulk tissues and amino acids. From July to August 2017, great concentrations of MCs were detected in discharged freshwater ranging from 0.4 to 75 μg L^-1. Considerable amounts of MCs are delivered to the Geum River Estuary in summer season. MCs spread far away as dissolved phases (18.7-49.5 ng L^-1) in July when large amount of freshwater was discharged during the rainy season. Concentrations of MCs in marine organisms varied among species, ranging from 40 to 870 ng g^-1 dw. Bioaccumulated MCs tend to decrease with increasing TP of organisms, suggesting that MCs are biodiluted through the marine food web. Compound-specific isotope analysis (nitrogen of amino acids) provides more reliable TPs compared with those by bulk isotope analysis in a closed estuary (such as the Geum River Estuary) with large fluctuations in the isotope ratio of primary producers.

Role of potentially toxic cyanobacteria in crustacean zooplankton diet in a eutrophic lake

The coexistence of potentially toxic bloom-forming cyanobacteria (CY) and generally smaller-sized grazer communities has raised the question of zooplankton (ZP) ability to control harmful cyanobacterial blooms and highlighted the need for species-specific research on ZP-CY trophic interactions in naturally occurring communities. A combination of HPLC, molecular and stable isotope analyses was used to assess in situ the importance of CY as a food source for dominant crustacean ZP species and to quantify the grazing on potentially toxic strains of Microcystis during bloom formation in large eutrophic Lake Peipsi (Estonia). Aphanizomenon, Dolichospermum, Gloeotrichia and Microcystis dominated bloom-forming CY, while Microcystis was the major genus producing cyanotoxins all over the lake. Grazing studies showed that CY, and especially colonial CY, formed a significant, and also preferred component of algae ingested by the cladocerans Bosmina spp. and Daphnia spp. while this was not the case for the more selective calanoid copepod Eudiaptomus gracilis. Molecular analyses confirmed the presence of CY, including Microcystis, in ZP guts. Further analyses using qPCR targeting cyanobacterial genus-specific mcyE synthase genes indicated that potentially toxic strains of Microcystis can be ingested directly or indirectly by all the dominant crustacean grazers. However, stable isotope analyses indicated that little, if any, assimilation from ingested bloom-forming CY occurred. The study suggests that CY, and particularly Microcystis with both potentially toxic and non-toxic strains, can be widely ingested by cladoceran grazers during a bloom event with implications for control of CY abundance and for transfer of CY toxins through the food web.

Meteorological and Nutrient Conditions Influence Microcystin Congeners in Freshwaters

Cyanobacterial blooms increasingly impair inland waters, with the potential for a concurrent increase in cyanotoxins that have been linked to animal and human mortalities. Microcystins (MCs) are among the most commonly detected cyanotoxins, but little is known about the distribution of different MC congeners despite large differences in their biomagnification, persistence, and toxicity. Using raw-water intake data from sites around the Great Lakes basin, we applied multivariate canonical analyses and regression tree analyses to identify how different congeners (MC-LA, -LR, -RR, and -YR) varied with changes in meteorological and nutrient conditions over time (10 years) and space (longitude range: 77°2'60 to 94°29'23 W). We found that MC-LR was associated with strong winds, warm temperatures, and nutrient-rich conditions, whereas the equally toxic yet less commonly studied MC-LA tended to dominate under intermediate winds, wetter, and nutrient-poor conditions. A global synthesis of lake data in the peer-reviewed literature showed that the composition of MC congeners differs among regions, with MC-LA more commonly reported in North America than Europe. Global patterns of MC congeners tended to vary with lake nutrient conditions and lake morphometry. Ultimately, knowledge of the environmental factors leading to the formation of different MC congeners in freshwaters is necessary to assess the duration and degree of toxin exposure under future global change.

Specificity of the metabolic signatures of fish from cyanobacteria rich lakes

With the increasing impact of the global warming, occurrences of cyanobacterial blooms in aquatic ecosystems are becoming a main worldwide ecological concern. Due to their capacity to produce potential toxic metabolites, interactions between the cyanobacteria, their cyanotoxins and the surrounding freshwater organisms have been investigated during the last past years. Non-targeted metabolomic analyses have the powerful capacity to study simultaneously a high number of metabolites and thus to investigate in depth the molecular signatures between various organisms encountering different environmental scenario, and potentially facing cyanobacterial blooms. In this way, the liver metabolomes of two fish species (Perca fluviatilis and Lepomis gibbosus) colonizing various peri-urban lakes of the Île-de-France region displaying high biomass of cyanobacteria, or not, were investigated. The fish metabolome hydrophilic fraction was analyzed by ¹H NMR analysis coupled with Batman peak treatment for the quantification and the annotation attempt of the metabolites. The results suggest that similar metabolome profiles occur in both fish species, for individuals collected from cyanobacterial blooming lakes compared to organism from non-cyanobacterial dominant environments. Overall, such environmental metabolomic pilot study provides new research perspectives in ecology and ecotoxicology fields, and may notably provide new information concerning the cyanobacteria/fish ecotoxicological interactions.

Bioaccumulation of microcystins in seston, zooplankton and fish: A case study in Lake Zumpango, Mexico

Cyanotoxins from toxic blooms in lakes or eutrophic reservoirs are harmful to several organisms including zooplankton, which often act as vectors of these secondary metabolites, because they consume cyanobacteria, bioaccumulate the cyanotoxins and pass them on along the food chain. Microcystins are among the most commonly found cyanotoxins and often cause zooplankton mortality. Although cyanobacterial blooms are common and persistent in Mexican water bodies, information on the bioaccumulation of cyanotoxins is scarce. In this study we present data on the bioaccumulation of cyanotoxins from Planktothrix agardhii, Microcystis sp., Cylindrospermopsis raciborskii and Dolichospermum planctonicum blooms in the seston (suspended particulate matter more than 1.2 μm) by zooplankton and fish (tilapia (Oreochromis niloticus) and mesa silverside (Chirostoma jordani) samples from Lake Zumpango (Mexico City). The cyanotoxins were extracted from the seston, zooplankton and fish tissue by disintegration using mechanical homogenization and 75% methanol. After extraction, microcystins were measured using an ELISA kit (Envirologix). Concentration of microcystins expressed as equivalents, reached a maximum value of 117 μg g^-1 on sestonic samples; in zooplankton they were in the range of 0.0070-0.29 μg g^-1. The dominant zooplankton taxa included Acanthocyclops americanus copepodites, Daphnia laevis and Bosmina longirostris. Our results indicate twice the permissible limits of microcystins (0.04 μg kg^-1 d^-1) for consumption of cyanobacterial products in whole fish tissue of Chirostoma jordani. The data have been discussed with emphasis on the importance of regular monitoring of water bodies in Mexico to test the ecotoxicological impacts of cyanobacterial blooms and the risk that consumption of products with microcystins could promote.

Transcriptomic Analysis Reveals the Pathways Associated with Resisting and Degrading Microcystin in Ochromonas

Toxic Microcystis bloom is a tough environment problem worldwide. Microcystin is highly toxic and is an easily accumulated secondary metabolite of toxic Microcystis that threatens water safety. Biodegradation of microcystin by protozoan grazing is a promising and efficient biological method, but the mechanism in this process is still unclear. The present study aimed to identify potential pathways involved in resisting and degrading microcystin in flagellates through transcriptomic analyses. A total of 999 unigenes were significantly differentially expressed between treatments with flagellates Ochromonas fed on microcystin-producing Microcystis and microcystin-free Microcystis. These dysregulated genes were strongly associated with translation, carbohydrate metabolism, phagosome, and energy metabolism. Upregulated genes encoding peroxiredoxin, serine/threonine-protein phosphatase, glutathione S-transferase (GST), HSP70, and O-GlcNAc transferase were involved in resisting microcystin. In addition, genes encoding cathepsin and GST and genes related to inducing reactive oxygen species (ROS) were all upregulated, which highly probably linked with degrading microcystin in flagellates. The results of this study provided a better understanding of transcriptomic responses of flagellates to toxic Microcystis as well as highlighted a potential mechanism of biodegrading microcystin by flagellate Ochromonas, which served as a strong theoretical support for control of toxic microalgae by protozoans.

Global change-driven effects on dissolved organic matter composition: Implications for food webs of northern lakes

Northern ecosystems are experiencing some of the most dramatic impacts of global change on Earth. Rising temperatures, hydrological intensification, changes in atmospheric acid deposition and associated acidification recovery, and changes in vegetative cover are resulting in fundamental changes in terrestrial-aquatic biogeochemical linkages. The effects of global change are readily observed in alterations in the supply of dissolved organic matter (DOM)-the messenger between terrestrial and lake ecosystems-with potentially profound effects on the structure and function of lakes. Northern terrestrial ecosystems contain substantial stores of organic matter and filter or funnel DOM, affecting the timing and magnitude of DOM delivery to surface waters. This terrestrial DOM is processed in streams, rivers, and lakes, ultimately shifting its composition, stoichiometry, and bioavailability. Here, we explore the potential consequences of these global change-driven effects for lake food webs at northern latitudes. Notably, we provide evidence that increased allochthonous DOM supply to lakes is overwhelming increased autochthonous DOM supply that potentially results from earlier ice-out and a longer growing season. Furthermore, we assess the potential implications of this shift for the nutritional quality of autotrophs in terms of their stoichiometry, fatty acid composition, toxin production, and methylmercury concentration, and therefore, contaminant transfer through the food web. We conclude that global change in northern regions leads not only to reduced primary productivity but also to nutritionally poorer lake food webs, with discernible consequences for the trophic web to fish and humans.

Microcystin Content in Phytoplankton and in Small Fish from Eutrophic Nyanza Gulf, Lake Victoria, Kenya

The human health risks posed by exposure to cyanobacterial toxins such as microcystin (MC) through water and fish consumption remain poorly described. During the last two decades, coastal regions of Lake Victoria such as Nyanza Gulf (Kisumu Bay) have shown severe signs of eutrophication with blooms formed by Microcystis producing MC. In this study, the spatial variability in MC concentration in Kisumu Bay was investigated which was mostly caused by Microcystis buoyancy and wind drifting. Small fish (<6 cm) mainly composed of Rastrineobola argentea were examined for MC content by means of biological methods such as ELISA and protein phosphatase inhibition assay (PPIA) and partly by chemical-analytical methods such as LC-MS/MS. Overall, the MC content in small fish was related to the MC content observed in the seston. When comparing the MC content in the seston in relation to dry weight with the MC content in small fish the latter was found three orders of magnitude decreased. On average, the ELISA-determined MC contents exceeded the PPIA-determined MC contents by a factor of 8.2 ± 0.5 (SE) while the MC contents as determined by LC-MS/MS were close to the detection limit. Using PPIA, the MC content varied from 25–109 (mean 62 ± 7) ng/g fish dry weight in Kisumu Bay vs. 14 ± 0.8 ng MC/g in the more open water of L. Victoria at Rusinga channel. Drying the fish under the sun showed little effect on MC content, although increased humidity might indirectly favor photocatalyzed MC degradation.

An overview of the accumulation of microcystins in aquatic ecosystems

Cyanotoxins produced by toxic cyanobacteria pose a major, worldwide environmental threat to freshwater ecosystems. Microcystins (MCs) are considered to be the most hazardous groups. Indeed, some of the largest aquatic ecosystems on the earth are being contaminated with MCs. Questions have arisen regarding their transfer and bioaccumulation in natural environment. This review summarizes the present state of knowledge regarding toxic cyanobacteria and MCs, with a specific focus on their distribution in different components of aquatic ecosystems. Their accumulation in water columns, aquatic animals, plants, and sediments is summarized. MCs have been contaminating all areas of the aquatic ecosystems. Of these, the water column was the most contaminated with MCs and served as an intermediate transmission substation. Via this route, MCs could enter to other stations such as sediment, animals, aquatic and terrestrial plants. Therefore, the use of water contaminated with MCs may induce food chain contaminations with considerable health risks.

Heavy metal migration and risk transference associated with cyanobacterial blooms in eutrophic freshwater

The distribution of metals in cyanobloom-forming lakes, and potential risks of these metals during irrigation with water derived from the bloom were evaluated in this study. Seven metals were monitored throughout a cyanobacterial bloom season in Lake Taihu. Cyanobloom bio-dilution of the targeted metals could be explained by the negative relationships between total phytoplankton metal contents (Cu, Fe, Zn, Pb and Cr) and Chl a concentrations (p<0.05). Meanwhile, the ratios of extracellular bound to total cellular bound metals (Cu, Zn, Pb, Cr and Cd) were positively correlated with the ratios of cyanophyta to total phytoplankton (p<0.01), indicating the enhanced extracellular bound of these metals during cyanobloom period. Secondly, Cd, Pb and Cr were detected in several local vegetables. In comparison to reference vegetables, vegetables (e.g., radish, soybean, and cowpea), which were irrigated with cyanobloom broth collected from Lake Taihu, presented high health risk index (HRI) and were not safe for human consumption. Collectively, the frequent dominant colonial Microcystis blooms which performed high metal affinity might mediate the distribution of heavy metals in lake and potentially transferred these pollutants into terrestrial system.

Monitoring and research of microcystins and environmental factors in a typical artificial freshwater aquaculture pond

Freshwater aquaculture ponds are important artificially regulated aquatic ecosystems which provide a large number of freshwater fish products in China. The cyanobacteria bloom and microcystin (MC) pollution caused by anthropogenic eutrophication have attracted much attention due to their toxic effects. To provide an insight into the cyanobacterial problem in the ponds, the environmental parameters and MCs of a typical artificial pond in the Yangtze River Delta region of China were monitored and studied from May to December 2015. During the monitoring period, the ponds were in serious eutrophication with total phosphorus (TP) concentrations between 0.95 and 1.80 μg/L, and total nitrogen (TN) concentrations between 1.1 and 4.86 μg/L. High feed coefficient and high fish stock were the main reasons for the eutrophication. The results showed that the water temperature was the key factor that affected the cyanobacteria blooming in the pond. The chlorophyll a concentration was significantly positively correlated with the cyanobacteria density during the blooming season. MC-LR and MC-RR existed simultaneously and showed a significant positive correlation. The peak concentrations of dissolved MC-LR and MC-RR in the pond water were 40.6 and 4.7 μg/L, respectively, which is considered highly toxic. Free MC-LR and MC-RR were also found in the aquaculture products. MC-LR concentrations in the bighead carp (Aristichthys nobilis) liver and shrimp (Macrobrachium nipponense) muscle were up to 2.64 and 4.17 μg/kg, respectively. MC-RR concentration was up to 1.89 μg/kg in the black carp (Mylopharyngodon piceus) liver. The results implied the potential health risks for citizens and pets caused by current artificial freshwater aquaculture pond systems.

The synergetic effects of turbulence and turbidity on the zooplankton community structure in large, shallow Lake Taihu

Climate change is predicted to influence the heat budget of aquatic ecosystems and, in turn, affect the stability of the water column leading to increased turbulence coupled with enhanced turbidity. However, the synergetic effects of turbulence and turbidity on zooplankton community structure remain to be understood in large, shallow lakes. To determine the possible synergetic effects of these factors on zooplankton communities, a 15-day mesocosm experiment was carried out and tested under four turbulence and turbidity regimes namely control (ɛ = 0, 7.6 ± 4.2 NTU), low (ɛ = 6.01 × 10^-8 m² s^-3, 19.4 ± 8.6 NTU), medium (ɛ = 2.95 × 10^-5 m² s^-3, 55.2 ± 14.4 NTU), and high (ɛ = 2.39 × 10^-4 m² s^-3, 741.6 ± 105.2 NTU) conditions, which were comparable to the natural conditions in Lake Taihu. Results clearly showed the negative effects of turbulence and turbidity on zooplankton survival, which also differed among taxa. Specifically, increased turbulence and turbidity levels influenced the competition among zooplankton species, which resulted to the shift from being large body crustacean-dominated (copepods and cladocerans) to rotifer-dominated community after 3 days. The shift could be associated with the decrease in vulnerability of crustaceans in such environments. Our findings suggested that changes in the level of both turbidity and turbulence in natural aquatic systems would have significant repercussions on the zooplankton communities, which could contribute to the better understanding of community and food web dynamics in lake ecosystems exposed to natural mixing/disturbances.

Analysis of individual and total microcystins in surface water by on-line preconcentration and desalting coupled to liquid chromatography tandem mass spectrometry

A fast and high-throughput method is proposed for the determination of total microcystins (ΣMC) in environmental surface waters. After a 1-h Lemieux-von Rudloff oxidation step to yield the 2-methyl-3-methoxy-4-phenylbutyric acid (MMPB) moiety, samples were quenched, filtered, and directly analyzed. This was achieved via solid phase extraction (SPE) coupled on-line to ultra-high performance liquid chromatography electrospray ionization triple stage quadrupole mass spectrometry. The choice of on-line SPE settings was conducted using experimental designs. Given the matrix complexity of oxidation extracts, the on-line desalting step was found to be a critical parameter to ensure suitable method robustness. The on-line sample loading volume was 5mL, and the wash volume applied for on-line desalting was 3mL. Instrumental analysis was performed in just 8min. The method limit of quantification was 0.5ngL^-1 ΣMC (i.e. 2000 times lower than the current World Health Organization - WHO drinking water guideline). Excellent determination coefficients were observed for matrix-free and matrix-based calibration curves alike, and the linearity range tested spanned∼4 orders of magnitude. Accuracy and intermediate precision did not depend on the spike level and proved satisfactory (in the range of 93-110% and 3-6%, respectively). A thorough assessment of instrumental matrix effects was conducted by comparing standard additions curves in several lake and river oxidation extracts with the matrix-free reference. Regardless of the internal standard used (4-PB or D3-MMPB), instrumental matrix effects were efficiently compensated. The matrix effect that may occur at the earlier sample preparation stage was evaluated separately. While the oxidation step was generally not complete (yield ∼65%), the conversion rates of MCs into MMPB remained within a consistent range of values regardless of matrix type. No significant back-pressure was observed upon consecutive injections of oxidation-based samples, while the instrumental sensitivity remained unaffected. The herein described method could therefore be eligible for future large-scale monitoring surveys. The method was applied to a selection of surface water samples (n=30) collected across the province of Québec, Canada, and the results were compared to those achieved by an individual variant analysis of 8 MC congeners and a commercial ELISA kit.

A Systematic Investigation into the Environmental Fate of Microcystins and The Potential Risk: Study in Lake Taihu

A systematic investigation was conducted in Lake Taihu in autumn of 2013 and 2014, in order to understand the environmental fate of microcystins (MCs) and evaluate the health risk from MCs. Samples of water, algal cells, macrophytes, shrimps and fish were taken to detect MCs by HPLC-MS/MS after solid phase extraction. Widespread MC contamination in water, algal cells, macrophytes, shrimps and fish was found in Lake Taihu. The ubiquitous presence of MCs in water, algal cells and biota was found in 100% of samples. MC accumulation was in the order of primary producer > tertiary consumer > secondary consumer > primary consumer. The highest levels of MCs in macrophytes, shrimps and fish tissue were found in Potamogeton maackianus, Exopalaemon modestus, and Hyporhamphus intermedius, respectively. The MCs level in shrimps and the tissues of three fish species, Neosalanx tangkahkeii taihuensis, Coilia ectenes and silver carp, was closely linked to their dietary exposure. Ceratophyllum demersum L. was an ideal plant for introduction into lakes to protect against Microcystis blooms and MCs, due to its ability to absorb nutrients, accumulate large amounts of MCs and tolerate these toxins compared to other macrophytes. The average daily intakes (ADIs) of MCs for Exopalaemon modestus and three fish species, Coilia ectenes, Hyporhamphus intermedius and Carassius carassius, were all above the tolerable daily intakes (TDI) set by the World Health Organization (WHO), implying there existed potential threats to human health.

Effects of flow speed and circulation interval on water quality and zooplankton in a pond-ditch circulation system

A pond-ditch circulation system (PDCS) shows great promises for ecological restoration of rural contaminated water in southern China. In this study, the optimal flow speed, circulation interval, and their combination for the system were investigated for higher pollutant removal efficiency and lower costs in three separate experiments: I, II, and III, respectively. In each experiment, there are three PDCSs (S1, S2, and S3) with different water circulation speeds or circulation intervals, respectively. The results demonstrated that in experiment I, total nitrogen (TN) removal rates, species numbers, and diversity indexes of zooplankton in S1 with a flow speed of 3.6 L/h were significantly higher than those in S2 (7.2 L/h) and S3 (10.2 L/h), respectively. Similarly, in experiment II, S3 circulating every other 4 h had significantly higher TN reduction rates, species numbers, and diversity indexes than S1 and S2 circulating every other 1 and 2 h, respectively. In experiment III, water qualities in S1 (circulation of 3.6 L/h + interval of 4 h) were better than those in S2 (7.2 L/h + 4 h) and S3 (10.2 L/h + 6 h), respectively. Together, circulation at every other 4 h (3.6 L/h) is probably the optimal operating condition for the PDCS in remediating rural contaminated water.

Nitrogen fixation by cyanobacteria stimulates production in Baltic food webs

Filamentous, nitrogen-fixing cyanobacteria form extensive summer blooms in the Baltic Sea. Their ability to fix dissolved N2 allows cyanobacteria to circumvent the general summer nitrogen limitation, while also generating a supply of novel bioavailable nitrogen for the food web. However, the fate of the nitrogen fixed by cyanobacteria remains unresolved, as does its importance for secondary production in the Baltic Sea. Here, we synthesize recent experimental and field studies providing strong empirical evidence that cyanobacterial nitrogen is efficiently assimilated and transferred in Baltic food webs via two major pathways: directly by grazing on fresh or decaying cyanobacteria and indirectly through the uptake by other phytoplankton and microbes of bioavailable nitrogen exuded from cyanobacterial cells. This information is an essential step toward guiding nutrient management to minimize noxious blooms without overly reducing secondary production, and ultimately most probably fish production in the Baltic Sea.

Experimental models of microcystin accumulation in Daphnia magna grazing on Planktothrix rubescens: implications for water management

In this study, we investigated the kinetic aspects of the microcystin (MC) transfer from Planktothrix rubescens to Daphnia magna by carrying out exposure experiments in small simple mesocosms. We hypothesized that higher fractions of toxic cyanobacteria in the diet of grazers would shift the balance towards a greater than linear, i.e. non-linear accumulation of MC in D. magna. This hypothesis was tested by exposing D. magna to varying initial densities of MC-producing P. rubescens. The evolving models of MC accumulation differed largely as a result of the duration of exposure and initial MC concentrations used. Within the first 24h of exposure, MC accumulation in D. magna was linear, irrespective of the initial densities of toxic P. rubescens and thus MC concentrations. After 48 h of exposure, MC accumulation in D. magna showed an exponential pattern, possibly due to a delayed digestion of P. rubescens and/or decreased MC detoxification capabilities when compared with higher ambient concentrations of MC. After 72 h toxin concentrations in Daphnia drop in all experiments as a consequence of the reduced cyanobacterial cells in the medium and the detoxification of MC within Daphnia. The results obtained suggest that in lakes with higher MC content and longer cyanobacterial bloom period MC accumulation in D. magna should be more pronounced than in mesotrophic lakes with lower MC content. The latter interpretation, however, should be verified investigating accumulation of MC both in larger mesocosms and in situ, in lakes of different trophic status.