State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China; The Fuxianhu Station of Plateau Deep Lake Research, Chinese Academy of Sciences, Yuxi 653100, China.
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China; Department of Postgraduate Administration, Nanjing University of Information Science and Technology, Nanjing 210044, China.
Water Res. 2024 Jan 1;248:120848. doi: 10.1016/j.watres.2023.120848. Epub 2023 Nov 8.
Biological nitrogen fixation plays a critical role in maintaining primary production, particularly in systematic nitrogen deficiency. However, little is known about the dynamics within diazotrophic community facing ongoing nutrient enrichment in freshwater lakes. Here, a consecutive five-year investigation on diazotrophic community was conducted in Lake Fuxian, an oligotrophic deep alpine lake on the trajectory to eutrophic state. Results showed a regime shift from heterotrophic to autotrophic diazotrophs induced by total phosphorus (TP) enrichment. Specifically, heterotrophic diazotrophs dominated the diazotrophic community when TP was lower than 21.8 μg/L, whereas heterotrophic diazotrophs or diazotrophic Cyanobacteria randomly dominated when TP ranged between 21.8 μg/L and 28.8 μg/L. When TP was higher than 28.8 μg/L, diazotrophic Cyanobacteria accounted for 60.4%-97.7% of the total N-fixers, indicating diazotrophic biodiversity significantly declined under TP enrichment scenario. Moreover, the dominance of diazotrophic Cyanobacteria further facilitated phytoplankton growth, which strengthened positive feedback between phytoplankton and phosphorus under nitrogen deficiency conditions. This is the first report on the threshold-like state responses of freshwater diazotrophs to environmental drivers. Our study expands the knowledge of the diazotrophic dynamics in freshwater ecosystems and contributes quantitative evidence of ecological thresholds for future environmental policymaking.
生物固氮在维持初级生产力方面起着关键作用，特别是在系统氮缺乏的情况下。然而，对于在淡水湖中持续营养丰富的情况下固氮生物群落的动态变化，人们知之甚少。本研究以正在向富营养化状态转变的贫营养深水高山湖泊抚仙湖为对象，连续五年对固氮生物群落进行了调查。结果表明，总磷（TP）富化引起了从异养固氮生物向自养固氮生物的转变。具体而言，当 TP 低于 21.8μg/L 时，异养固氮生物主导固氮生物群落，而当 TP 在 21.8μg/L 至 28.8μg/L 之间时，异养固氮生物或固氮蓝藻随机主导。当 TP 高于 28.8μg/L 时，固氮蓝藻占总氮固定者的 60.4%-97.7%，表明在 TP 富化情景下，固氮生物多样性显著下降。此外，固氮蓝藻的优势进一步促进了浮游植物的生长，在氮缺乏条件下加强了浮游植物和磷之间的正反馈。这是首次报道淡水固氮生物对环境驱动因素的类似阈值的状态响应。本研究扩展了对淡水生态系统固氮动态的认识，并为未来环境政策制定提供了定量的生态阈值证据。
