Bacterial and Protistan Community Variation across the Changjiang Estuary to the Ocean with Multiple Environmental Gradients.

Plankton microorganisms play central roles in the marine food web and global biogeochemical cycles, while their distribution and abundance are affected by environmental variables. The determinants of microbial community composition and diversity in estuaries and surrounding waters with multiple environmental gradients at a fine scale remain largely unclear. Here, we investigated bacterial and protistan community assembly in surface waters from 27 stations across the Changjiang Estuary to the ocean, with salinity ranging from 0 to 32.1, using 16S rRNA and 18S rRNA gene amplicon sequencing. Statistical analyses revealed that salinity is the major factor structuring both bacterial and protistan communities. Salinity also acted as a significant environmental determinant influencing alpha-diversity patterns. Alpha diversity indices for bacterial and protistan communities revealed a species minimum in higher-salinity waters (22.1-32.1). Contrary to the protistan community, the highest bacterial diversity was identified in medium-salinity waters (2.8-18.8), contrasting Remane's Artenminimum concept. The distribution of major planktonic taxa followed the expected pattern, and the salinity boundary for Syndiniales was specifically identified. These findings revealed the significant effects of salinity on the microbial community across an estuary to ocean transect and the distinct response to salinity between bacterial and protistan communities.