Distinct structure, assembly, and gene expression of microplankton in two Arctic estuaries with varied terrestrial inputs

The Laptev Sea is a major Marginal Sea in the Western Arctic Ocean. The Arctic amplification brought by global warming influences the hydrological properties of rivers passing through the permafrost zone, which would alter the biological community structure at continental margin. In this study, the structure, assembly, and gene expression of planktonic microbial communities in two estuaries (Protoka Ularovskaya River Estuary, PURE; Lena River Estuary, LRE) of Laptev Sea were examined to investigate the environmental effects of polar rivers. PURE and LRE exhibited distinct environmental characteristics: low temperature and high salinity for PURE, and high temperature and low salinity for LRE, influenced by runoff size. Salinity more closely influenced microbial communities in LRE, with freshwater species playing a significant role in community composition. The findings revealed differences between two estuaries in community composition and diversity. Prokaryotes and microeukaryotes had shown different assembly patterns in response to habitat changes caused by terrestrial freshwater input. Furthermore, compared with the PURE, the co-occurrence and inter-domain network of the LRE, which was more affected by terrestrial input, was more complex and stable. Functional gene prediction revealed a higher gene expression of methane metabolism in LRE than in PURE, particularly those related to methane oxidation, and this conclusion could help better explore the impact of global warming on the methane cycle in the Arctic Marginal Seas. This study explored the increased freshwater runoffs under the background of global warming dramatically affect Arctic microplankton communities from community structure, assembly and gene expression aspects.