Unusual bacterioplankton community structure in ultra-oligotrophic Crater Lake

The bacterioplankton assemblage in Crater Lake, Oregon (U.S.A.), is differe nt from communities found in other oxygenated lakes, as demonstrated by fou r small subunit ribosomal ribonucleic acid (SSU rRNA) gene clone libraries and oligonucleotide probe hybridization to RNA from lake water. Populations in the euphotic zone of this deep (589 m), oligotrophic caldera lake are d ominated by two phylogenetic clusters of currently uncultivated bacteria: C L120-10, a newly identified cluster in the verrucomicrobiales, and ACK4 act inomycetes, known as a minor constituent of bacterioplankton in other lakes . Deep-water populations at 300 and 500 m are dominated by a different pair of uncultivated taxa: CL500-11, a novel cluster in the green nonsulfur bac teria, and group I marine crenarchaeota. beta -Proteobacteria, dominant in most other freshwater environments, are relatively rare in Crater Lake (les s than or equal to 16% of nonchloroplast bacterial rRNA at all depths). Oth er taxa identified in Crater Lake libraries include a newly identified cand idate bacterial division, ABY1, and a newly identified subcluster, CL0-1, w ithin candidate division OP10. Probe analyses confirmed vertical stratifica tion of several microbial groups, similar to patterns observed in open-ocea n systems. Additional similarities between Crater Lake and ocean microbial populations include aphotic zone dominance of group I marine crenarchaeota and green nonsulfur bacteria. Comparison of Crater Lake to other lakes stud ied by rRNA methods suggests that selective factors structuring Crater Lake bacterioplankton populations may include low concentrations of available t race metals and dissolved organic matter, chemistry of infiltrating hydroth ermal waters, and irradiation by high levels of ultraviolet light.