The response of bacterial communities to organic matter in the surface sediment of the Pearl River estuary

Estuaries are considered to be among the most biodynamic locations on the globe, where materials are heavily processed by microbes, resulting in high elemental exchange. However, the precise contribution of bacteria in transforming estuarine organic matter has rarely been clarified. In this study, the temporal and spatial distributions of bacterial communities in the surface sediments were investigated at four sites in the Pearl River estuary over a 10-month period. The bacterial communities were affected significantly by salinity and shifted strongly from freshwater sediments (dominated by Alphaproteobacteria, Betaproteobacteria, Chloroflexi and Actinobacteria) to saline sediments (dominated by Deltaproteobacteria and Gammaproteobacteria). The abundance of bacteria was best explained by phototroph abundance in sediments compared to total organic carbon (TOC), carbon stable isotope ratios of TOC (delta C-13-TOC) and phototroph abundance in bottom water. Bacteria presented higher utilization efficiency for sedimental phototroph-derived organic matter in high-salinity (10.6-33.0 parts per thousand) sediments. Proteobacteria, the most abundant taxa, were considered the main consumer for phototroph-derived organic matter. In conclusion, bacterial communities perform the niche specificities and play a major role in carbon cycling by processing phototroph-derived organic matter in different habitats.