Spatial variation of bacterial community composition at the expiry of spring phytoplankton bloom in Sendai Bay, Japan

In order to characterize how bacterial communities are propagated over spatial scales in a coastal area, the bacterial community composition was examined along with a transect line set in a bay at an expiry of spring phytoplankton bloom. Four distinctive bacterial communities were found within the bay by a fingerprinting method of 16S rRNA gene amplicons. The most widely distributed one was distributed in the surface and middle layers at whole area of the bay. The water was characterized by low inorganic nutrients concentration and high bacterial abundance, suggesting that the bacterial community had been developed in the bloom. Pyrosequencing analyses of the gene amplicons indicated that Rhodobacteriaceae and Flavobacteriaceae were abundant in the bacterial community, though the most abundant bacterial taxon was SAR11. The second group was distributed in the bottom water at the coastal side of the bay where considerably high Chl. a concentration was observed, probably because of the sedimentation of phytoplankton bloom. The community diversity was high and Alteromonadaceae, Saprospiraceae, and some families of Actinobacter existed more in this community than the others. The third group was distributed in the deep water near the border with the outside of the bay. The ratio of SAR11 was the highest in this community; besides, Burkholderianceae and Rhodospilliraceae existed in relatively high abundances. Another bacterial community having intermediate characters was observed in the middle to bottom layers around a central part of the bay where vertical water mixing was observed. These findings suggest that spatially different bacterial communities were formed under the influences of phytoplankton bloom and/or hydrographic events such as oceanic seawater intrusion of the bay.