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

Crustacean Zooplankton Ingestion of Potentially Toxic Microcystis: In Situ Estimation Using mcyE Gene Gut Content Detection in a Large Temperate Eutrophic Lake

Grazing by zooplankton can regulate bloom-forming cyanobacteria but can also transfer toxin-producing cells, as well as toxic metabolites, to the food web. While laboratory investigations have provided extensive knowledge on zooplankton and toxic cyanobacteria interactions, information on zooplankton feeding on toxin-producing cyanobacteria in natural water bodies remains scarce. In this study, we quantified Microcystis-specific mcyE synthase genes from the gut contents of various cladoceran and copepod taxa to assess the in situ crustacean community and taxon-specific ingestion of potentially toxic Microcystis in Lake Peipsi, a large eutrophic lake in Estonia, Northern Europe. Microcystis cells with mcyE genes were found in all crustaceans examined. However, some species, such as the cyclopoid copepod Mesocyclops leuckarti, were more efficient in ingesting potentially toxic Microcystis than other co-occurring cladocerans (Daphnia spp., Bosmina spp., Chydorus sphaericus) and copepods (Eudiaptomus gracilis). The amount of toxigenic Microcystis cells grazed by crustacean population changed temporarily, and copepods were the predominant consumers of toxigenic Microcystis during several months of the 5-month study period. Crustacean ingestion of toxigenic Microcystis was not related to Microcystis biomass or mcyE gene copy numbers in the environment but was instead related to the abundance of major crustacean grazers. Our findings emphasize the close interaction between crustacean zooplankton and toxigenic Microcystis, indicating that some species may play a more significant role in linking toxic cells within the food web than others.

Biotechnological approaches for suppressing Microcystis blooms: insights and challenges

Cyanobacterial harmful algal blooms, particularly those dominated by Microcystis, pose significant ecological and health risks worldwide. This review provides an overview of the latest advances in biotechnological approaches for mitigating Microcystis blooms, focusing on cyanobactericidal bacteria, fungi, eukaryotic microalgae, zooplankton, aquatic plants, and cyanophages. Recently, promising results have been obtained using cyanobactericidal bacteria: not through the inoculation of cultured bacteria, but rather by nurturing those already present in the periphyton or biofilms of aquatic plants. Fungi and eukaryotic microalgae also exhibit algicidal properties; however, their practical applications still face challenges. Zooplankton grazing on Microcystis can improve water quality, but hurdles exist because of the colonial form and toxin production of Microcystis. Aquatic plants control blooms through allelopathy and nutrient absorption. Although cyanophages hold promise for Microcystis control, their strain-specificity hinders widespread use. Despite successful laboratory validation, field applications of biological methods are limited. Future research should leverage advanced molecular and bioinformatic techniques to understand microbial interactions during blooms and offer insights into innovative control strategies. Despite progress, the efficacy of biological methods under field conditions requires further verification, emphasizing the importance of integrating advanced multi-meta-omics techniques with practical applications to address the challenges posed by Microcystis blooms. KEY POINTS: • A diverse range of biotechnological methods is presented for suppressing Microcystis blooms. • Efficacy in laboratory experiments needs to be proved further in field applications. • Multi-meta-omics techniques offer novel insights into Microcystis dynamics and interactions.

Seasonal dynamics of toxigenic Microcystis in a large, shallow Lake Peipsi (Estonia) using microcystin mcyE gene abundance

Large and temperate Lake Peipsi is the fourth largest lake in Europe, where the massive cyanobacterial blooms are composed mostly of Microcystis spp., which have been common for several decades now. The seasonal dynamics of potentially toxic Microcystis were studied using microscopy and quantitative polymerase chain reaction (qPCR) by assessing the microcystin-encoding microcystin synthetase gene E (mcyE) abundances. Water samples were analyzed over the lake areas, varying in depth, trophic level, and cyanobacterial composition during the growing period of 2021. The Microcystis mcyE genes were detected through the growing period (May-October), forming peak abundances in September with decreasing temperatures (8.9-11.1 °C). Total phosphorus (TP) and nitrate (NO3-) were the most relevant environmental variables influencing the Microcystis biomass as well as mcyE abundances. Comparison with previous years (2011, 2012) indicated that the abundance and seasonal dynamics of toxigenic Microcystis can be highly variable between the years and lake areas, varying also in dominant Microcystis species. Contrary to expectations, based on mcyE abundances, the increased risk of toxin-producing Microcystis can occur in Peipsi through the growing period, independently of the water temperature and biomasses of Microcystis.

Effects of Algicidal Macrophyte Metabolites on Cyanobacteria, Microcystins, Other Plankton, and Fish in Microcosms

To control harmful algae blooms (HABs), methods based on natural mechanisms are now required. We investigated the effects of an algicide derived from macrophyte metabolites, namely mixtures of gallic, tetradecanoic, heptanoic, and octanoic acids (1:1:1:1 mass ratio, a total concentration of 14 mg/L), on the biomass of cyanobacteria and other plankton and the production of microcystins under experimental conditions. Two types of microcosms have been created: simple (microalgae, cyanobacteria, and zooplankton) and complex (microalgae, cyanobacteria, zooplankton, and planktivorous fish). We observed the dynamics of the phytoplankton structure, the concentrations of microcystins and chlorophyll-a, hydrochemistry, and the status of zooplankton and fish in both types of microcosms with and without algicide for one month (from 19 July to 19 August 2021). The introduction of algicide caused changes in phytoplankton structure, a drop in cyanobacterial biomass, and a decrease in the total concentration of microcystins. Surprisingly, the contributions of the most toxic microcystins (LR form) were higher in both types of microcosms exposed to algicide than in microcosms without algicide. The inhibitory effect on the cyanobacterial biomass was most significant in complex ecosystems (containing fish), while it was only observed at the end of the exposure in simple ecosystems. Not only algicide but also phytoplankton consumed by fish and zooplankton, as well as nutrient excretory activity by both consumers, seem to have impact on cyanobacterial biomass. This study found that the using chemical substances similar to macrophyte metabolites can help regulate HABs and cyanotoxins. However, the results differ depending on ecosystem type.

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.

Intensified sensitivity and adaptability of zooplankton Bosminidae in subtropical shallow freshwater lakes with increasing trophic level

The deterioration in lake water environments, especially increasing lake eutrophication, is prevalent all over the world, which has seriously affected the balance and stability of the internal ecosystem of lakes. In this study, modern water and sediment samples were collected from three subtropical freshwater lakes with significant differences in nutrient levels to analyze the concentration of the zooplankton Cladocera Bosminidae and its relationship with lakes’ ecological changes. The results show that the deterioration in lake water environments caused by increasing eutrophication limits the survival of most zooplankton. However, the Bosminidae shows a positive adaptability to eutrophication and high sensitivity to the changes in the lake environment. In addition, the lake eutrophication process caused by the intensification of human activities enhances the survival advantage of Bosminidae with more food sources, which is more conducive to its rapid reproduction.

Integration of Transcriptomics and Microbiomics Reveals the Responses of Bellamya aeruginosa to Toxic Cyanobacteria

Frequent outbreaks of harmful cyanobacterial blooms and the cyanotoxins they produce not only seriously jeopardize the health of freshwater ecosystems but also directly affect the survival of aquatic organisms. In this study, the dynamic characteristics and response patterns of transcriptomes and gut microbiomes in gastropod Bellamya aeruginosa were investigated to explore the underlying response mechanisms to toxic cyanobacterial exposure. The results showed that toxic cyanobacteria exposure induced overall hepatopancreatic transcriptome changes. A total of 2128 differentially expressed genes were identified at different exposure stages, which were mainly related to antioxidation, immunity, and metabolism of energy substances. In the early phase (the first 7 days of exposure), the immune system may notably be the primary means of resistance to toxin stress, and it performs apoptosis to kill damaged cells. In the later phase (the last 7 days of exposure), oxidative stress and the degradation activities of exogenous substances play a dominant role, and nutrient substance metabolism provides energy to the body throughout the process. Microbiomic analysis showed that toxic cyanobacteria increased the diversity of gut microbiota, enhanced interactions between gut microbiota, and altered microbiota function. In addition, the changes in gut microbiota were correlated with the expression levels of antioxidant-, immune-, metabolic-related differentially expressed genes. These results provide a comprehensive understanding of gastropods and intestinal microbiota response to toxic cyanobacterial stress.

Methane-Derived Carbon as a Driver for Cyanobacterial Growth

Methane, a potent greenhouse gas produced in freshwater ecosystems, can be used by methane-oxidizing bacteria (MOB) and can therefore subsidize the pelagic food web with energy and carbon. Consortia of MOB and photoautotrophs have been described in aquatic ecosystems and MOB can benefit from photoautotrophs which produce oxygen, thereby enhancing CH₄ oxidation. Methane oxidation can account for accumulation of inorganic carbon (i.e., CO₂) and the release of exometabolites that may both be important factors influencing the structure of phytoplankton communities. The consortium of MOB and phototroph has been mainly studied for methane-removing biotechnologies, but there is still little information on the role of these interactions in freshwater ecosystems especially in the context of cyanobacterial growth and bloom development. We hypothesized that MOB could be an alternative C source to support cyanobacterial growth in freshwater systems. We detected low δ¹³C values in cyanobacterial blooms (the lowest detected value −59.97‰ for Planktothrix rubescens) what could be the result of the use of methane-derived carbon by cyanobacteria and/or MOB attached to their cells. We further proved the presence of metabolically active MOB on cyanobacterial filaments using the fluorescein isothiocyanate (FITC) based activity assay. The PCR results also proved the presence of the pmoA gene in several non-axenic cultures of cyanobacteria. Finally, experiments comprising the co-culture of the cyanobacterium Aphanizomenon gracile with the methanotroph Methylosinus sporium proved that cyanobacterial growth was significantly improved in the presence of MOB, presumably through utilizing CO₂ released by MOB. On the other hand, ¹³C-CH₄ labeled incubations showed the uptake and assimilation of MOB-derived metabolites by the cyanobacterium. We also observed a higher growth of MOB in the presence of cyanobacteria under a higher irradiance regime, then when grown alone, underpinning the bidirectional influence with as of yet unknown environmental consequences.

Whole-genome characterization and comparative genomics of a novel freshwater cyanobacteria species: Pseudanabaena punensis

Cyanobacteria are emerging as a potential source of novel, beneficial bioactive compounds. However, some cyanobacteria species can harm water quality and public health through the production of toxins. Therefore, surveying the occurrence and generating genomic resources of cyanobacteria producing harmful compounds could help develop the control methods necessary to manage their growth and limit the release contaminants into the water bodies. Here, we describe a novel strain, Pseudanabaena punensis isolated from the open ends of pipelines supplying freshwater. This isolate was characterized morphologically, biochemically and by whole-genome sequence analysis. We also provide genomic information for P. punensis to help understand and highlight the features unique to this isolate. Morphological and genetic (analysis using 16S rRNA and rbcL genes) data were used to assign this novel strain to phylogenetic and taxonomic groups. The isolate was identified as a filamentous and non-heterocystous cyanobacteria. Based on morphological and 16S rRNA phylogeny, this isolate shares characteristics with the Pseudanabaenaceae family, but remains distinct from well-characterized species suggesting its polyphyletic assemblage. The whole-genome sequence analysis suggests greater genomic and phenotypic plasticity. Genome-wide sequence and comparative genomic analyses, comparing against several closely related species, revealed diverse and important genes associated with synthesizing bioactive compounds, multi-drug resistance pathway, heavy metal resistance, and virulence factors. This isolate also produces several important fatty acids with potential industrial applications. The observations described in this study emphasize both industrial applications and risks associated with the freshwater contamination, and therefore genomic resources provided in this study offer an opportunity for further investigations.