Dynamics of eukaryotic and bacterial communities in coastal ecosystem

Sagami Bay, Japan is influenced by both warm Kuroshio current and cold Oyashio current, and rich nutrients are supplied from multiple river sources and the deep-sea, forming a dynamic ecosystem. The aim of the present study was to investigate eukaryotic and bacterial communities in the coastal waters of Sagami Bay, using 16S rRNA and 18S rRNA sequencing, and to assess the seasonal and vertical dynamics in relation to physicochemical and biological environments. We found 1,660 operational taxonomic units (OTUs; 97% identity) of the eukaryotic and 4,477 OTUs of bacterial communities. Vertical characteristics of community diversity were different between the eukaryotic and bacterial communities; the lowest diversity were observed in the surface water in both communities, whereas the highest diversity was observed in intermediate depth layers (10-50m) for eukaryotic community and in bottom depth layer (100m) for bacterial community. Both communities were clustered into several groups characterized by season and depth, and redundancy analysis suggested that these communities were regulated by common environmental variables such as mixed layer depth, euphotic layer depth, water temperature, nitrate+nitrite concentration and chlorophyll a concentration. The annual dynamic of the coastal microbial communities is driven not only by the seasonal changes of abiotic and biotic factors, and short-term rapid changes by river water inflow and phytoplankton blooms, but also largely influenced by deep-seawater upwellings due to the unique seafloor topography. Eukaryotic and bacterial communities showed synchronous seasonal and vertical changes with environmental variability.