Bacterioplankton communities in the Southern Ocean: composition and growth response to various substrate regimes

To examine the adaptation of bacterioplankton communities in the Southern Ocean to various biopolymeric substrates, we carried out experiments at the Polar Front and in the Antarctic Coastal Current (CC) in which samples were enriched with agarose, starch, peptone and extracts of the green alga Scenedesmus acutus and diatoms from the Polar Front. Growth and metabolic activity were assessed based on leucine incorporation and turnover rates of glucose, dissolved free amino acids and protein. In both experiments, growth was highest on peptone and the diatom extract and lowest on agarose. There was a general trend of decreasing proportions of Alphaproteobacteria and increasing proportions of Flavobacteria/Sphingobacteria and Gamma proteobacteria. The growth response and shift to Flavobacteria/Sphingobacteria was more pronounced in the CC experiment. A cluster analysis of denaturing gradient gel electrophoresis (DGGE) banding patterns showed that bacterial communities in each treatment of both experiments differed and that those in the treatments with peptone and the Scenedesmus and diatom extracts clustered together. Sequencing of excised bands revealed that Gammaproteobacteria comprised members of Pseudoalteromonadaceae, the Colwellia cluster of Alteromonadaceae and 1 other cluster. Alphaproteobacteria comprised only members of the Roseobacter clade and the Bacteroidetes phylum only of Flavobacteria and Cytophagales. The results show that the bacterio plankton communities in the Southern Ocean are able to adapt to various biopolymeric substrates. The most pronounced response to the additions of peptone and the diatom extract rich in protein is in line with previous observations that proteins are preferred bacterial substrates in this oceanic region.