Protistan-Bacterial Microbiota Exhibit Stronger Species Sorting and Greater Network Connectivity Offshore than Nearshore across a Coast-to-Basin Continuum.

Little is known regarding how community assembly and species association vary with habitat and depth. Here, we examined the assembly and association of protistan and bacterial communities across a coast-shelf-slope-basin gradient of the South China Sea using high-throughput sequencing of the V3 and V4 regions of the rRNA gene transcript. Our study revealed that homogenizing dispersal and drift exerted an influence on protistan communities comparable to that on bacterial communities. In contrast, selection and dispersal limitation exerted contrasting effects on the two microbial communities. Community assembly was governed to a greater degree by selection than by dispersal limitation in the bacterial community, and this was much lower in the protistan community. Moreover, this organismal assembly pattern was robust with habitat and depth. However, the relative importance of selection to dispersal limitation varied with habitat and depth in both communities, where horizontally it was higher offshore than nearshore and vertically it was lower in the bottom or deep chlorophyll maximum (DCM) than on the surface. The offshore possessed more microbial network complexity and more associations among microbial taxa than the nearshore, and vertically, the bottom possessed more complexity than the surface and the DCM. Moreover, temperature is strongly associated with the composition and co-occurrence of microbial communities, implying that temperature plays a dominant role in the selection of the protistan-bacterial microbiome across a coast-to-basin continuum. This study contributes to our understanding of the assembly mechanism and species association of protistan-bacterial microbiota across multiple habitats and depths.