Zhen Yang

Dissolved carbon is a crucial component of freshwater ecosystems and plays an important role in the Earth’s carbon cycle. This paper delivers a groundbreaking exploration of dissolved carbon (DOC and DIC) variations spanning 12 years in a eutrophic lake where nutrient levels are gradually declining to reveal their spatial and temporal distribution patterns and the key drivers behind this variation. Our findings indicate that both DIC and DOC concentrations in Lake Chaohu exhibit a westward spatial gradient, with an overall upward trend in DIC levels from 2012 to 2023, contrasting with a downward trend in DOC. A similar variation trend in the inflowing rivers suggests that exogenous inputs predominantly dictate long-term carbon concentration changes in the lake. Additionally, a reduction of eutrophication levels in the lake may have also contributed to a decrease in endogenous DOC generated by algal metabolism. Seasonally, DIC and DOC concentrations display inverse trends throughout the year, with DOC levels closely tracking the seasonal patterns of the cyanobacterium Microcystis. Correlation and regression analyses reveal a significant positive relationship between Microcystis and DOC and a negative association with DIC, while Dolichospermum shows no correlation with either. This suggests that the impact of phytoplankton on DOC and DIC variations is species-specific. In Lake Chaohu, the annual succession of Microcystis may be a major driving factor for the intra-annual variations in DOC and DIC. Our findings deepen the comprehension of how phytoplankton modulates the intra-annual dynamics of DOC and DIC.

Harmful cyanobacterial blooms in eutrophic water bodies pose a major threat to the environment and to human health. The morphological functional traits of cyanobacteria play important roles in maintaining their competitive advantages. To explore the regulatory mechanisms of the morphological functional traits of different bloom-forming cyanobacteria, we performed a one-year-long phytoplankton survey from November 2016 to October 2017 in Lake Chaohu, China. The colony size and cell diameter of the dominant cyanobacteria were measured, and their relationships were analyzed. The results showed that Dolichospermum flos-aquae and Microcystis aeruginosa were the dominant cyanobacteria in the lake. Microcystis was the dominant species during the summer; the growth of Dolichospermum growth surpassed that of Microcystis, and Dolichospermum became the dominant species in the late autumn, winter and spring. From winter to spring, the colony size of Dolichospermum decreased from 222.25 μm to 10.51 μm, and the individual cell diameter increased from 6.5 μm to 7.4 μm. From summer to autumn, Dolichospermum showed enlarged colony sizes and reduced cell diameters. The Microcystis colony size increased from 83.71 μm in the spring to 196.71 μm in the summer and autumn, while cells diameter remained essentially at 3–4 μm from March to October in Lake Chaohu. The relationship between colony size and cell diameter in Dolichospermum was significantly positive, while that of Microcystis was not significant. These results suggest that Dolichospermum may maintain biomass through a trade-off between cell diameter and colony size, and that a flexible morphological regulatory mechanism exists. This study seeks to improve our understanding of how bloom-forming cyanobacteria maintain their dominance by regulating their morphological traits.

The shifts among bloom-forming cyanobacteria have attracted increasing attention due to the reductions in nitrogen and phosphorus during the eutrophication mitigation process. However, knowledge is limited regarding the pattern and drivers of the shifts among these cyanobacterial genera. In this study, we performed a 7-year long, monthly investigation in Lake Chaohu, to analyze the interannual and seasonal shifts between Microcystis and Dolichospermum. Our results showed that Microcystis was the dominant cyanobacterium in the western lake region in summer, whereas Dolichospermum was dominant in the other regions and seasons. The Microcystis biomass and ratio were driven primarily by total phosphorus and temperature. The sensitivity of Dolichospermum to nutrients and temperature was relatively weak compared to that of Microcystis. The shifts between Microcystis and Dolichospermum might be led by Microcystis. If the temperature and phosphorus level were relatively high, then Microcystis grew rapidly, and competitively excluded Dolichospermum. If the nutrient level, especially the phosphorus level, was low, then the exclusive power of Microcystis was weak, and Dolichospermum maintained its dominance, even in summer. The key temperature (~17 °C) determined the dominance of the two cyanobacteria. Microcystis never dominated, while Dolichospermum was always dominant below the key temperature. Microcystis and Dolichospermum had different means of responding to the interaction of temperature, nitrogen and phosphorus. The Dolichospermum biomass was sensitive to the variation in nitrogen level, and the sensitivity depended on temperature. While the Microcystis biomass was sensitive to the variation in phosphorus level, and the sensitivity depended on temperature and total nitrogen. The different ways might contribute to the succession of the two cyanobacteria. Our findings will be helpful for improving the understanding of the shift process between Microcystis and Dolichospermum.

Lake Chaohu is a eutrophic lake that experiences massive cyanobacterial blooms. The high concentrations of microcystin observed in this lake are the result of the bloom’s high proportion of toxic cyanobacteria strains. Groundwater is the important source of water for drinking, washing, and irrigation in the watershed of Lake Chaohu. This study examines the space-time distribution of microcystins and related environmental factors in wells near Lake Chaohu. All collected groundwater samples from the Lake Chaohu region had detectable concentrations of microcystins. The highest concentration of microcystins, 1.07 μg L⁻¹, occurred in a well hundreds of meters from the western coast of the lake in September. The distance from the lake shore to the well was significantly and positively correlated with the microcystin concentration in the groundwater. Moreover, a correlation analysis shows that the microcystin concentration in the groundwater was positively correlated with the total dissolved phosphorus (TDP) and microcystin concentration of the nearby lake water. Therefore, the microcystin in the groundwater likely originates from penetration by nearby lake water. Our results suggest that the groundwater near Lake Chaohu poses a significant health risk for the local residents when used for drinking water.

Microcystins (MCs) are cyclic hepatotoxic heptapeptides produced by cyanobacteria that can be toxic to aquatic and terrestrial organisms. MC synthesis and degradation are thought to be influenced by several different physical and environmental parameters. In this study, the effects of different intensities of UV-B radiation on MC biosynthesis in Microcystis cells and on its extracellular degradation were investigated by mRNA analysis and degradation experiments. Exposure to UV-B at intensities of 1.02 and 1.45 W/m² not only remarkably inhibited the growth of Microcystis, but also led to a decrease in the MC concentration. In addition, mcyD transcription was decreased under the same UV-B intensities. These results demonstrated that the effects of UV-B exposure on the biosynthesis of MCs in Microcystis cells could be attributed to the regulation of mcy gene transcription. Moreover, the MC concentration was decreased significantly after exposure to different intensities of UV-B radiation. Of the three MC variants (MC-LR, -RR and -YR, L, R and Y are abbreviations of leucine, arginine and tyrosine), MC-LR and MC-YR were sensitive to UV-B radiation, whereas MC-RR was not. In summary, our results showed that UV-B radiation had a negative effect on MC production in Microcystis cells and MC persistence in the extracellular space.

Lake Chaohu, which is a large, shallow, hypertrophic freshwater lake in southeastern China, has been experiencing lake-wide toxic Microcystis blooms in recent decades. To illuminate the relationships between microcystin (MC) production, the genotypic composition of the Microcystis community and environmental factors, water samples and associated environmental data were collected from June to October 2012 within Lake Chaohu. The Microcystis genotypes and MC concentrations were quantified using quantitative real-time PCR (qPCR) and HPLC, respectively. The results showed that the abundances of Microcystis genotypes and MC concentrations varied on spatial and temporal scales. Microcystis exists as a mixed population of toxic and non-toxic genotypes, and the proportion of toxic Microcystis genotypes ranged from 9.43% to 87.98%. Both Pearson correlation and stepwise multiple regressions demonstrated that throughout the entire lake, the abundances of total and toxic Microcystis and MC concentrations showed significant positive correlation with the total phosphorus and water temperature, suggesting that increases in temperature together with the phosphorus concentrations may promote more frequent toxic Microcystis blooms and higher concentrations of MC. Whereas, dissolved inorganic carbon (DIC) was negatively correlated with the abundances of total and toxic Microcystis and MC concentrations, indicating that rising DIC concentrations may suppress toxic Microcystis abundance and reduce the MC concentrations in the future. Therefore, our results highlight the fact that future eutrophication and global climate change can affect the dynamics of toxic Microcystis blooms and hence change the MC levels in freshwater.