Occurrence of Cyanobacteria and microcystins in hydroelectric reservoirs used for fish farming

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.

1. 蓝细菌（Cyanobacteria）因全球范围内加速的富营养化与气候变化而备受关注。水产养殖是淡水富营养化的诱因之一。蓝细菌可产生具有多样化学结构与毒理学特性的毒素，其中最广为人知的藻毒素类群为肝毒性微囊藻毒素（microcystins），该类毒素可诱发肝脏与皮肤肿瘤。
2. 本研究针对巴西水力发电水库中水产养殖系统的蓝细菌、微囊藻毒素与养殖水体微生物组（microbiome）之间的关联展开评估，采样覆盖6个养鱼场的水体及其上下游水体。此外，研究通过检测微囊藻毒素合成酶基因E（mcyE）并定量微囊藻毒素，对毒素产生情况进行了评估。
3. 本研究采用高效液相色谱-光电二极管阵列检测器（HPLC-PDA）对微囊藻毒素LR、YR与RR的浓度进行了测定，结果在5个养鱼场的生物量样本以及所有上下游水体样本中均检出上述毒素。在3个采样点位，养鱼场内的微囊藻毒素浓度低于上游水体，但高于下游水体。各微囊藻毒素变体的平均浓度范围为0.036 μg/L至0.931 μg/L，实测总浓度最高值为1.59 μg/L。
4. 对16S rRNA基因的扩增子测序结果显示，变形菌门（Proteobacteria）、拟杆菌门（Bacteroidetes）、蓝细菌门与放线菌门（Actinobacteria）是丰度最高的细菌门类。所有样本中均检出聚球藻属（Synechococcus）与微囊藻属（Microcystis）这两个蓝细菌属，且它们是微生物种群中的优势类群。蓝细菌门的最高相对丰度出现在养鱼场4与5的下游区域。仅在养鱼场3与4的场内水体中，观察到蓝细菌门相对丰度出现上升。
5. 潜在微囊藻毒素产生菌念珠藻属（Nostoc）与浮丝藻属（Planktotrix）与微囊藻毒素浓度呈现中等到极强的正相关，而微囊藻属与微囊藻毒素丰度仅呈现弱到中等程度的相关。聚球藻属与检出的微囊藻毒素之间则呈现极强的负相关。
6. 与其他采样点位相比，点位3与4的微囊藻毒素浓度更高，且蓝细菌的物种多样性更为丰富。通过定量聚合酶链式反应（qPCR）在所有采样点位均检出了mcyE基因，验证了所有点位均存在微囊藻毒素的检测结果。