Integrated physiological and metabolomic analysis reveals new insights into toxicity pathways of paraquat to Microcystis aeruginosa.

Chemical pollutants, such as herbicides, released into the aquatic environment adversely affect the phytoplankton community structure. While majority of herbicides are specifically designed to target photosynthetic processes, they also can be toxic to phytoplankton; however, despite the photosynthetic toxicity, some herbicides can target multiple physiological processes. Therefore, a full picture of toxicity pathway of herbicide to phytoplankton is necessary. In the present study, the cyanobacterium Microcystis aeruginosa was exposed to two levels (17 µg L-1 (EC10) and 65 µg L-1 (EC50)) of paraquat for 72 h. The physiological and metabolic responses were analyzed to elucidate the toxicity pathway and establish the adverse outcome pathway of paraquat to M. aeruginosa. The results revealed that enhanced glycolysis (upregulation of pyruvic acid level) and tricarboxylic acid cycle (upregulation of the levels of malic acid, isocitric acid and citric acid) exposed to EC10 level of paraquat, which probably acted as a temporary strategy to maintain a healthy energy status in M. aeruginosa cells. Meanwhile, the expressions of glutathione and benzoic acid were enhanced to scavenge the excessive reactive oxygen species (ROS). Additionally, the accumulation of pigments (chlorophyll a and carotenoid) might play a supplementary role in the acclimation to EC10 level paraquat treatment. In cells exposed to paraquat by EC50 level, the levels of SOD, CAT, glutathione and benzoic acid increased significantly; however, the ROS exceeded the tolerance level of antioxidant system in M. aeruginosa. The adverse effects were revealed by inhibition of chlorophyll a fluorescence, the decreases in several carbohydrates (e.g., glucose 1-phosphate, fructose and galactose) and total protein content. Consequently, paraquat-induced oxidative stress caused the growth inhibition of M. aeruginosa. These findings provide new insights into the mode of action of paraquat in M. aeruginosa.

Microcystin pollution in lakes and reservoirs: A nationwide meta-analysis and assessment in China.

The frequent occurrence of microcystins (MCs) has caused a series of water security issues worldwide. Although MC pollution in natural waters of China has been reported, a systematic analysis of the risk of MCs in Chinese lakes and reservoirs is still lacking. In this study, the distribution, trend, and risk of MCs in Chinese lakes and reservoirs were comprehensively revealed through meta-analysis for the first time. The results showed that MC pollution occurrence in numerous lakes and reservoirs have been reported, with MC pollution being distributed in the waters of 15 provinces in China. For lakes, the maximum mean total MC (TMC) and dissolved MC (DMC) concentrations occurred in Lake Dianchi (23.06 µg/L) and Lake Taihu (1.00 µg/L), respectively. For reservoirs, the maximum mean TMC and DMC concentrations were detected in Guanting (4.31 µg/L) and Yanghe reservoirs (0.98 µg/L), respectively. The TMC concentrations in lakes were significantly higher than those in the reservoirs (p < 0.05), but no difference was observed in the DMC between the two water bodies (p > 0.05). Correlation analysis showed that the total phosphorus concentrations, pH, transparency, chlorophyll a, and dissolved oxygen were significantly related to the DMC in lakes and reservoirs. The ecological risks of DMC in Chinese lakes and reservoirs were generally at low levels, but high or moderate ecological risks of TMC had occurred in several waters, which were not negligible. Direct drinking water and consumption of aquatic products in several MC-polluted lakes and reservoirs may pose human health risks. This study systematically analyzed the pollution and risk of MCs in lakes and reservoirs nationwide in China and pointed out the need for further MC research and management in waters.

Analysis of environmental drivers influencing interspecific variations and associations among bloom-forming cyanobacteria in large, shallow eutrophic lakes.

Non-diazotrophic Microcystis and filamentous N2-fixing Aphanizomenon and Dolichospermum (formerly Anabaena) co-occur or successively dominate freshwaters globally. Previous studies indicate that dual nitrogen (N) and phosphorus (P) reduction is needed to control cyanobacterial blooms; however, N limitation may cause replacement of non-N2-fixing by N2-fixing taxa. To evaluate potentially counterproductive scenarios, the effects of temperature, nutrients, and zooplankton on the spatio-temporal variations of cyanobacteria were investigated in three large, shallow eutrophic lakes in China. The results illustrate that the community composition of cyanobacteria is primarily driven by physical factors and the zooplankton community, and their interactions. Niche differentiation between Microcystis and two N2-fixing taxa in Lake Taihu and Lake Chaohu was observed, whereas small temperature fluctuations in Lake Dianchi supported co-dominance. Through structural equation modelling, predictor variables were aggregated into 'composites' representing their combined effects on species-specific biomass. The model results showed that Microcystis biomass was affected by water temperature and P concentrations across the studied lakes. The biomass of two filamentous taxa, by contrast, exhibited lake-specific responses. Understanding of driving forces of the succession and competition among bloom-forming cyanobacteria will help to guide lake restoration in the context of climate warming and N:P stoichiometry imbalances.

Toxicological and ecotoxicological evaluation of the water quality in a large and eutrophic freshwater lake of China.

Effect-based methods (EBMs) are recommended as holistic approach for diagnosis and monitoring of water quality; however, the application of EBMs is still scare in China. In the present study, water quality of the freshwater lake Taihu (China) was investigated by EBMs. Different types of water samples were collected from three bays of the lake during 2015, 2016 and 2017. A battery of seven effect-based bioassays, including both specific and non-specific toxicity assays, was used. The bioassay battery was recently suggested based on joint activities of the EU project SOLUTIONS and the NORMAN network on emerging pollutants and is also under discussion for being implemented into monitoring activities in the context of the European Water Framework Directive (WFD). Adverse effects were observed towards the primary producer, primary consumer and fish, indicating the potential ecotoxicity of water in Taihu Lake. Mutagenic and estrogenic effects were found in the Ames fluctuation assay and ERα CALUX (Chemically Activated Luciferase Gene-eXpression) assay, respectively, highlighting the potential risks on human health. Algal growth inhibition and mutagenic effects can be observed during each of the three years. Acute toxicity towards Daphnia magna and estrogen receptor agonistic effects were found in at least one of the samples collected in 2016 and 2017, but not in 2015. The endpoints for fish toxicity in the Danio rerio fish embryo test included both lethal and additionally several sublethal effects (only for samples from 2017) and were not compared between years. Algal growth inhibition, fish embryo toxicity, mutagenic effect and estrogenicity were observed in each of the three bays, while Daphnia acute toxicity was only found in Zhushan Bay. Taking together, this study provides a big picture on the water quality of Taihu Lake. The battery of effect-based tools is promising to be a routine for water quality monitoring in China.