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The temperature control of the growth of heterotrophic picoplankton was studied in the Southern Ocean along a transect between the Polar Front (49 degrees S) and the shelf ice edge at 70 degrees S in the austral summer. Growth was measured by thymidine and leucine incorporation applying the dual-label approach. Psychrotolerant or mesophilic communities with a growth optimum of >18 degrees C were present in surface waters at the Polar Front at an in situ temperature of 4 to 5 degrees C. Further south in surface waters of the Antarctic circumpolar current (ACC), in the marginal ice zone and at the shelf ice edge, psychrophilic communities with growth optima of 11 and 4 to 8 degrees C, respectively, were present. Growth at the ambient temperature of ca 0 degrees C or slightly below was reduced. By in situ hybridization with fluorescent rRNA-targeted oligonucleotide probes, between 63 and 96% of all DAPI-stainable picoplankton cells were identified as Bacteria whereas Archaea were never detected. Cells belonging to the Cytophaga/Flavobacterium cluster were most abundant from all bacterial groups tested (alpha-, beta-, and gamma-subclass of Proteobacteria, Cytophaga/Flavobacterium cluster) and comprised 20 % of DAPI counts at the Polar Front and 40 % in the ACC. In the marginal ice zone during a bloom of Phaeocystis sp. they even comprised 72%. gamma-subclass proteobacteria accounted for <1% of DAPI counts at the Polar Front and for 7 to 9% further south. alpha-subclass proteobacteria never exceeded 1% and beta-subclass proteobacteria were not detected at all. The results indicate that different communities of heterotrophic picoplankton established along the transect from the Polar Front south to the marginal ice zone as shown by the reduction of the temperature optima and coinciding with a change in the community structure and a pronounced dominance of bacteria of the Cytophaga/Flavobacterium cluster in the psychrophilic communities.
