Different community structure and temperature optima of heterotrophic picoplankton in various regions of the Southern Ocean

The temperature control of the growth of heterotrophic picoplankton was stu died in the Southern Ocean along a transect between the Polar Front (49 deg rees S) and the shelf ice edge at 70 degrees S in the austral summer. Growt h was measured by thymidine and leucine incorporation applying the dual-lab el approach. Psychrotolerant or mesophilic communities with a growth optimu m 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 t he shelf ice edge, psychrophilic communities with growth optima of 11 and 4 to 8 degrees C, respectively, were present. Growth at the ambient temperat ure of ca 0 degrees C or slightly below was reduced. By in situ hybridizati on with fluorescent rRNA-targeted oligonucleotide probes, between 63 and 96 % of all DAPI-stainable picoplankton cells were identified as Bacteria wher eas Archaea were never detected. Cells belonging to the Cytophaga/Flavobact erium cluster were most abundant from all bacterial groups tested (alpha-, beta-, and gamma-subclass of Proteobacteria, Cytophaga/Flavobacterium clust er) and comprised 20 % of DAPI counts at the Polar Front and 40 % in the AC C. In the marginal ice zone during a bloom of Phaeocystis sp. they even com prised 72%. gamma-subclass proteobacteria accounted for <1% of DAPI counts at the Polar Front and for 7 to 9% further south. alpha-subclass proteobact eria never exceeded 1% and beta-subclass proteobacteria were not detected a t all. The results indicate that different communities of heterotrophic pic oplankton established along the transect from the Polar Front south to the marginal ice zone as shown by the reduction of the temperature optima and c oinciding with a change in the community structure and a pronounced dominan ce of bacteria of the Cytophaga/Flavobacterium cluster in the psychrophilic communities.