Seasonal succession in marine bacterioplankton

Here we examine the seasonal distribution of marine bacterioplankton species in the northern Baltic Sea. The population density of different bacteria was determined by whole-genome DNA hybridization to community DNA. During spring, concomitant with the phytoplankton spring bloom and its decay, the bacterial community was dominated by 5 bacterial species belonging to the Flexibacter-Cytophaga-Bacteroides phylum. Together they accounted for up to 43 % of the intact cells as indicated by the number of nucleoid-containing cells at this time of year (20 % of the total bacterial counts). In late June a number of new bacteria proliferated to form a summer community. The dominant bacteria during summer were largely members of the alpha-Proteobacteria, with a significant contribution of Sphingomonas and Caulobacter species. The capacity of these species to cope with oligotrophic growth conditions may explain their success at this time of year, when low PO4 concentrations Limit bacterial growth. The different communities were reflected in the correlation between bacterial production and temperature. During summer significantly lower levels: of production at corresponding temperatures were found compared to spring and autumn. We suggest this to be a result of different physiological predisposition of the dominant bacteria during the respective seasons. A compilation of current research demonstrated the consistent finding of a low or moderate diversity in the marine environment. Thus, an understanding of how variability in physiological capacities among dominant bacterial groups results in niche differentiation is conceivable.