Predominance of β-proteobacteria in summer melt pools on Arctic pack ice

The diversity and community structure of bacteria in melt pools on Arctic pack ice floes were dominated by beta-proteobacteria. Thirty-five percent of the pure cultures isolated in 1997 from pack ice floes north of Svalbard and in the Fram Strait were from the beta-proteobacteria group. Within this group, there were only two phylotypes clustering within the widespread Beta I cluster, also known as the Comamonadaceae clade. One phylotype, most closely related to Aguaspirilltan arcticum (96.0-97.3% identical), was frequent among cultures isolated from 10 melt pools. A 16S ribosomal RNA (rRNA) gene clone library, constructed from a melt pool that was sampled 2 yr later in the Fram Strait. was also dominated by beta-proteobacteria, in particular the same recurrent isolate phylotype designated MP-BetaI. Fluorescence in situ hybridization of 20 melt pools corroborated the cultivation and cloning data. beta-Proteobacteria were the most abundant bacterial group, constituting similar to49% of the bacteria that were stained by 4'6-diamidino-2-phenylindole (DAPI). alpha- and gamma-proteobacteria accounted for only 2% each, the Cytophaga-Flavobacteritan group accounted for 9%, and the Actinobacteria spp. accounted for 9%. Approximately 63% of the beta-proteobacterial fraction that was found in the melt pools was determined with a newly developed probe to be the recurrent beta-proteobacterial MP-Betal phylotypes, indicating that it is particularly adapted for success in this extreme environment.