Flavobacteria in the Southern Ocean

The abundance, spatial distribution and diversity of class Flavobacteria, a bacterial group with a major role in marine secondary production, was investigated in the Southern Ocean euphotic zone of the ice pack off Eastern Antarctica and along a transect at 140-144 degrees East between latitudes 44.7 degrees South to 63.5 degrees South. Samples were comparatively analysed using 16S rRNA gene-based denaturing gradient gel electrophoresis (DGGE), fluorescent in situ hybridisation, real-time PCR and sequence analysis. The results were subsequently compared with direct cultivation approaches. Surface seawater samples were filter-fractionated into particulate and planktonic fractions and the abundance of particle-associated Flavobacteria, ascertained with real-time PCR and DGGE band analysis using Flavobacteriaspecific primers. Flavobacteria abundance was found to be significantly higher in Polar Front Zone (PFZ) and Antarctic Zone (AZ) water samples compared to warmer, nutrient limited Temperate Zone (TZ) and Sub-Antarctic Zone (SAZ) waters. Abundance of particle-associated Flavobacteria positively correlated with seawater chlorophyll a and nutrient concentrations. The abundance of planktonic Flavobacteria populations in the same samples remained relatively static, suggesting increased Flavobacteria abundance may relate to enhanced primary production in the PFZ and AZ. This was supported by comparisons of DGGE profiles that demonstrated significant differences occur in the total Flavobacteria community structure and 16S rRNA gene diversity between samples from the PFZ and AZ with samples from the TZ and SAZ. This suggests a shift to a different, more psychrophilic Flavobacteria community occurs across the Polar Front in the Southern Ocean. DGGE band sequences revealed a high diversity of class Flavobacteria within the Southern Ocean, with 24 genus-level lineages detected. Several of the phylotype clades detected were cosmopolitan in distribution, present in both polar and temperate oceans. Many of the phylotypes clustered in a large, so far uncultivated clade (previously termed DE cluster 2) widely distributed in seawater but apparently absent from sea-ice. Cosmopolitan phylotype clades occurred throughout the Southern Ocean, while several additional phylotype groups were found only in the colder waters of the PFZ and AZ. Examination of the cultivable diversity of Flavobacteria in Southern Ocean water samples, using a range of growth media, revealed a number of unique phylotypes including three novel genera, some grouping in clades for which only clones are currently available. Several other strains represented novel species belonging to the family Flavobacteriaceae, grouping in the genera Psychroserpens, Polaribacter and Tenacibaculum. A number of seawater microcosms were utilized to examine the colonization of bacteria-free diatom detritus by planktonic bacterial communities over a period of 30 days at 2 degrees Celsius. Flavobacteria phylotypes colonizing diatom detritus, determined by DGGE and real-time PCR analysis, were homologous with the dominant phylotypes in the particle-associated fraction of the samples from which they were taken. Colonisation occurred rapidly (less than 5 days) and comprised a succession of phylotypes, representing a diversity of Flavobacteria lineages. The increasing colonization corresponded to an increase in the dissolution of silicic acid compared with the corresponding control microcosms