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 bproteobacteria. 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 β‐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 Aquaspirillum 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 β‐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. b‐Proteobacteria were the most abundant bacterial group, constituting ~49% of the bacteria that were stained by 4'6‐diamidino‐2‐phenylindole (DAPI). α‐ and γ‐proteobacteria accounted for only 2% each, the Cytophaga‐Flavobacterium group accounted for 9%, and the Actinobacteria spp. accounted for 9%. Approximately 63% of the β‐proteobacterial fraction that was found in the melt pools was determined with a newly developed probe to be the recurrent β‐proteobacterial MP‐BetaI phylotypes, indicating that it is particularly adapted for success in this extreme environment.