Cultivation and biochemical characterization of heterotrophic bacteria associated with phytoplankton bloom in the Amundsen sea polynya, Antarctica

Polynyas are a key ecosystem for carbon cycling in the Antarctic Ocean due to the intensive primary production. Most of the knowledge regarding the bacterioplankton community in the Antarctic Ocean that is responsible for re-mineralization of fixed carbon comes from metagenomic analyses. Here, the extinction-dilution method was used to obtain representative heterotrophs from a polynya in the Amundsen Sea, Antarctica, and their biochemical potential for carbon re-mineralization were assessed. All 23 strains have close relatives belonging to type strains within the following genera (number of strains; % 16S rRNA gene sequence similarity): Bizionia (4; >97.8%), Leeuwenhoekiella (1; 96.2%), Pseudoalteromonas (14; >98.5%), Pseudomonas (1; 99.4%) and Sulfitobacter (3; 100%), which were also observed in 454 pyrosequencing-based analysis of 16S rRNA gene sequences of the polynya. Although sequence reads related to Polaribacter were the most common, Polaribacter strains could only be obtained from colonies cultured on agar plates. The strain of Leeuwenhoekiella showed a prominent ability in hydrolyzing diverse esters, amides, and glycosides while the strains of Pseudoalteromonas, Polaribacter, and Bizionia showed extracellular enzyme activities only on a narrow range of amides. The strains of Leeuwenhoekiella, Pseudoalteromonas, and Sulfitobacter utilized various labile carbon sources: carbohydrates, organic acids, amino acids, and peptides. The most frequent isolates, strains of Pseudoaltermonas, showed marked differences in terms of their potential to utilize different types of labile carbon sources, which may reflect high genomic diversity. The strains of Bizionia and Pseudomonas did not utilize carbohydrates. Unique biochemical properties associated with extracellular hydrolase activities and labile carbon utilization were revealed for dominant culturable heterotrophs which gives insights into their roles in active re-mineralization of fixed carbons in polynya.