Bacterial community assembly and the composition of dissolved and bulk organic matter varied along the salinity gradient in the Yangtze river estuary sediments

Revealing biogeochemical processes in estuarine sediments is important to understand estuarine ecosystems. In this study, 23 sites were selected along the north channel of the Yangtze Estuary deep-water channel, and surface sediments were collected. Bacterial communities were analyzed using high-throughput sequencing, along with measurements of sediment-derived environmental factors, including dissolved organic matter (DOM) spectra, sediment organic matter (SOM) composition, and chemical properties. Key environmental factors shaping community structure were identified through further analysis. Spatial variation in bacterial community diversity divided the sampling sites into three groups including riverine, transitional, and coastal regions. Pseudomonadota was the dominant bacterial phylum in the sediments. The proportion of Pseudomonadota increased with increasing salinity in the riverine region and was higher there than in the other regions. Both the Chao1 and Shannon indices decreased with increasing salinity in the riverine region but increased with increasing salinity in the coastal region. Null model analysis showed that bacterial community assembly in both the riverine and coastal regions was dominated by deterministic processes. Structural equation model indicated that salinity was the primary factor affecting sediment bacterial community diversity, while ammonia nitrogen, pH and available phosphorus were also important. The protein-like substances of DOM decreased with increasing salinity in the riverine region but increased in the coastal region. The proportion of nitrogen-containing organic matter in SOM increased with increasing salinity. Both the composition of SOM and the spectral characteristics of DOM had non-negligible explanatory rate to sediment bacterial communities. These findings shed light on the complex interplay between environmental factors and microbial community assembly in estuarine sediments, offering valuable implications for estuarine ecosystem management.