Colonization and community dynamics of class Flavobacteria on diatom detritus in experimental mesocosms based on Southern Ocean seawater.

In order to better understand the ecology of microorganisms responsible for secondary production in the Southern Ocean the activity of Flavobacteria communities on diatom detritus in seawater mesocosms was investigated. Seawater was collected from different parts of the Southern Ocean including the Polar Front Zone (PFZ), ice-edge area of the Antarctic Zone (AZ), and a site in the AZ ice pack. Detritus from the cosmopolitan marine diatom Nitzschia closterium Ehrenberg was resuspended in mesocosms containing seawater filtered to remove particulate organic matter, including particle-associated bacteria and most eukaryotes, but retaining native planktonic bacterial assemblages. Mesocosms were incubated at 2 degrees C and samples analysed for changes in community composition using denaturing gradient gel electrophoresis (DGGE), real-time PCR and fluorescent in-situ hybridization (FISH). DGGE banding patterns and FISH images demonstrated rapid bacterial colonization of the detritus, dominated by members of class gamma-Proteobacteria, alpha-Proteobacteria and Flavobacteria. Real-time PCR data indicated members of class Flavobacteria were involved in initial colonization of detrital aggregate, however relative abundance stayed at similar levels found for the original native particle-associated populations. 16S rRNA gene DGGE banding patterns and sequence analysis demonstrated significant variation in Flavobacteria community structure occurred in the first 20 days of the experiment before community stabilization occurred. The community structures between the three mesocosms also markedly differed and major colonizers were primarily derived from detectable members of the initial particle-associated Flavobacteria community, however the abundant uncultured Flavobacteria agg58 clone-related and DE cluster 2 clades, previously identified in Southern Ocean seawater were not observed to colonize the detritus.