Effect of phosphorus fraction in shaping bacterial and archaeal community succession in the largest hydrologically connected lake of Northeast Asia.

Microbial beta diversity and its components are key ecological indicators for understanding community assembly in lake sediments, yet their coupling with phosphorus (P) fractions remains poorly understood in hydrologically connected lake systems. In this study, sediment cores were collected from Xingkai Lake, the largest freshwater lake in Northeast Asia with a unique twin lake structure, and were analyzed using metagenomic sequencing combined with sequential P fractionation. Results showed that total beta diversity and species turnover for bacteria and archaea increased significantly with sediment depth in both lakes, with faster turnover rates in Daxingkai Lake. Nestedness was generally not significant in Daxingkai Lake but showed a significant positive trend with depth for archaea in Xiaoxingkai Lake. The dominant P fraction in the Daxingkai Lake sediments were HCl-Pi and residual P, while NaOH-Pi dominated in Xiaoxingkai lake sediments. Organic P explained the largest proportion of bacterial beta diversity variation in Daxingkai Lake, while inorganic P was the primary driver in Xiaoxingkai Lake. Conversely, inorganic P dominated the archaeal beta diversity variation in Daxingkai, whereas organic P dominated in Xiaoxingkai. These findings demonstrate that species turnover is the dominant component of beta diversity along the sediment depth gradient. The contrasting roles of organic P and inorganic P in shaping microbial beta diversity highlight the importance of P resource partitioning in driving microbial community succession and provide a basis for developing microbial beta diversity indicators to support eutrophication assessment and sediment management in hydrologically connected lake systems.