Differing growth responses of major phylogenetic groups of marine bacteria to natural phytoplankton blooms in the western North Pacific Ocean.

Growth and productivity of phytoplankton substantially change organic matter characteristics, which affect bacterial abundance, productivity, and community structure in aquatic ecosystems. We analyzed bacterial community structures and measured activities inside and outside phytoplankton blooms in the western North Pacific Ocean by using bromodeoxyuridine immunocytochemistry and fluorescence in situ hybridization (BIC-FISH). Roseobacter/Rhodobacter, SAR11, Betaproteobacteria, Alteromonas, SAR86, and Bacteroidetes responded differently to changes in organic matter supply. Roseobacter/Rhodobacter bacteria remained widespread, active, and proliferating despite large fluctuations in organic matter and chlorophyll a (Chl-a) concentrations. The relative contribution of Bacteroidetes to total bacterial production was consistently high. Furthermore, we documented the unexpectedly large contribution of Alteromonas to total bacterial production in the bloom. Bacterial abundance, productivity, and growth potential (the proportion of growing cells in a population) were significantly correlated with Chl-a and particulate organic carbon concentrations. Canonical correspondence analysis showed that organic matter supply was critical for determining bacterial community structures. The growth potential of each bacterial group as a function of Chl-a concentration showed a bell-shaped distribution, indicating an optimal organic matter concentration to promote growth. The growth of Alteromonas and Betaproteobacteria was especially strongly correlated with organic matter supply. These data elucidate the distinctive ecological role of major bacterial taxa in organic matter cycling during open ocean phytoplankton blooms.