DIVERSITY AND ASSOCIATION OF PSYCHROPHILIC BACTERIA IN ANTARCTIC SEA-ICE

The bacterial populations associated with sea ice sampled from Antarct ic coastal areas were investigated by use of a phenotypic approach and a phylogenetic approach based on genes encoding 16S rRNA (16S rDNA). The diversity of bacteria associated with sea ice eras also compared w ith the bacterial diversity of seawater underlying sea ice. Psychrophi lic (optimal growth temperature, less than or equal to 15 degrees C; n o growth occurring at 20 degrees C) bacterial diversity was found to b e significantly enriched in sea ice samples possessing platelet and bo ttom ice diatom assemblages, with 2 to 9 distinct (average, 5.6 +/- 1. 8) psychrophilic taxa isolated per sample, Substantially fewer psychro philic isolates mess recovered from ice cores with a low or negligible population of ice diatoms or from under-ice seawater samples (less th an one distinct taxon isolated per sample). In addition, psychrophilic taxa that were isolated from under-ice seawater samples were in gener al phylogenetically distinct from psychrophilic taxa isolated from sea ice cores. The taxonomic distributions of psychrotrophic bacterial is olates (optimal growth temperature, >20 degrees C; growth can occur at similar to 4 degrees C) isolated from sea ice cores and under-ice sea water were quite similar. Overall, bacterial isolates from Antarctic s ea ice were found to belong to four phylogenetic groups, the alpha and gamma subdivisions of the Proteobacteria, the gram-positive branch, a nd the Flexibacter-Bacteroides-Cytophaga phylum. Most of the sea ice s trains examined appeared to be novel tars based on phylogenetic compar isons, with 45% of the strains being psychrophilic. 16S rDNA sequence analysis revealed that psychrophilic strains belonged to the genera Co lwellia, Shewanella, Marinobacter, Planococcus, and novel phylogenetic lineages adjacent to Colwellia and Alteromonas and within the Flexiba cter-Bacteroides-Cytophaga phylum. Psychrotrophic strains were found t o be members of the genera Pseudoalteromonas, Psychrobacter, Halomonas , Pseudomonas, Hyphomonas, Sphingomonas, Arthrobacter Planococcus, and Halobacillus. From this survey, it is proposed. that ice diatom assem blages provide niches conducive to thr proliferation of a diverse arra y of psychrophilic bacterial species.