Occurrence of Cyanobacteria and microcystins in hydroelectric reservoirs used for fish farming. Cyanobacterial blooms Brazil fish farms

1. Cyanobacteria have become a concern due to a globally accelerated eutrophication and climate changes. Aquaculture is one of the contributing factors for eutrophication of freshwater. Cyanobacteria can produce toxins with diverse chemical structures and toxicological properties. The most well-known group of cyanotoxins are the hepatotoxic microcystins that can induce liver and skin tumors.2. Relationships between cyanobacteria, microcystins and microbial community (microbiome) of aquaculture in Brazilian hydroelectric reservoirs was evaluated in water from six fish farms and in downstream and upstream water. In addition, the toxin production was evaluated through detection of the microcystin synthetase gene E (mcyE) and by quantification of microcystins. 3. Concentrations of microcystin LR, YR and RR were evaluated by HPLC-PDA and detected in biomass in 5 fish farms and in all upstream and downstream waters. At three locations, the microcystin concentrations were lower inside the farm than upstream and higher than downstream. Mean concentrations of individual microcystin variants ranged from 0.036 μg/L to 0.931 μg/L and the highest total measured concentration was 1.59 µg/L. 4. Amplicon sequencing of the 16S rRNA gene showed that Proteobacteria, Bacteroidetes, Cyanobacteria and Actinobacteria were the most abundantly observed bacterial phyla. The two cyanobacterial genera Synechococcus and Microcystis were observed in all samples and were abundant organisms in microbial populations. The highest relative abundance of Cyanobacteria was found downstream of fish farm 4 and 5. An increase in relative abundance of Cyanobacteria inside fish farms was only observed in the fish farms 3 and 4. 5. Potential microcystin producers Nostoc and Planktotrix showed moderate to strong positive correlations with the microcystin concentrations, while Microcystis had a weak to moderate correlation with abundance of the microcystins. A strong negative correlation was observed between Synechococcus and the detected microcystins. 6. Locations 3 and 4 had higher microcystin concentrations and a more diverse representation of Cyanobacteria, compared to the other sampled locations. The mcyE gene was found in all locations by qPCR, supporting the observed presence of microcystin in all locations.