Growth and distribution patterns of Roseobacter/Rhodobacter, SAR11, and Bacteroidetes lineages in the Southern Ocean

Roseobacter/Rhodobacter and SAR11, affiliated with Alphaproteobacteria, and the phylum Bacteroidetes constitute a large proportion of marine planktonic bacteria, but information about their growth and distribution patterns in the Southern Ocean is scarce. The aim of the present study is to determine patterns in the biomass and productivity of Roseobacter/Rhodobacter, SAR11, and Bacteroidetes groups along the steep temperature, salinity, and organic matter gradients in the Southern Ocean by using catalyzed reporter deposition-fluorescence in situ hybridization and bromodeoxyuridine (BrdU) immunocytochemistry FISH. We found that Roseobacter/Rhodobacter, SAR11, and Bacteroidetes are prominent contributors to total bacterial biomass and production. SAR11 bacteria were the predominant lineage, but their biomass was low in the coldest regions. In contrast, the biomasses of Roseobacter/Rhodobacter and Bacteroidetes lineages were positively correlated with organic matter concentrations. The Roseobacter/Rhodobacter had the highest proportion of BrdU-positive (i.e., actively growing) cells among the three phylotypes at all stations, despite their low abundance. The relative contribution of Bacteroidetes to the total bacterial productivity (number of active cells) was negatively correlated with temperature. These results suggest that the growth and distribution patterns of Roseobacter/Rhodobacter, SAR11, and Bacteroidetes were determined by different environmental gradients (e.g., organic matter concentrations or temperature) in the Southern Ocean.