Seasonal Dynamics of Bacterial Community Composition in Coastal Seawater at Sendai Bay, Japan

Abstract

Metagenome analysis could be a useful tool to monitor biological diversities in marine environments. However, environmental conditions fluctuate widely in coastal areas. We examined bacterial community compositions in coastal seawater from Sendai Bay for 2 years to clarify their temporal variation in relation to physical and biological environmental factors. Sequencing of 16S rRNA gene amplicons revealed obvious annual variation and the significant associations of temperature, salinity, and chlorophyll a concentration with the community composition change. The most dominant operational taxonomic unit (OTU) was assigned to the oceanic bacterium, SAR11. An OTU assigned to Rhodobacteraceae was also dominant during phytoplankton blooms. OTUs assigned to cyanobacteria or Alteromonadaceae were increased in the warm water period after phytoplankton blooming, from June to September. Of the top 20 OTUs, 12 OTUs had a significant correlation with water temperature, indicating seasonal fluctuation. Three OTUs significantly correlated with salinity, indicating that freshwater discharge influenced the bacterial community in the bay. Five OTUs that represented 38% of the total did not show any correlation with environmental factors. The dominant SAR11 OTU was included in this category, implying that hydrographical conditions are very important factors determining microbial community composition in coastal seawater.