Microbiology of sulfur cycling and other biogeochemical processes in Dry Valleys lakes of Antarctica
The permanently ice-covered lakes of the McMurdo Dry Valleys in Antarctica are characterized by having constantly cold temperatures, low light conditions, high concentrations of dissolved gases, and stratified water columns inhabited exclusively by microorganisms. The geochemical conditions within each of the lakes vary considerably, despite the close proximities of these lakes to one another. In this work the chemical parameters that would indicate the presence of a biogenic sulfur cycle were measured in three lakes from Taylor Valley (Lakes Fryxell, Hoare, and Bonney), one lake from Wright Valley (Lake Vanda), one lake from Victoria Valley (Lake Vida), and one lake from Pearse Valley (Lake Joyce). Lakes having detectable levels of both sulfide and sulfate included Lakes Fryxell, Hoare, and Vanda. The unusual geochemistry of Lake Fryxell, in particular, suggested that an established community of sulfur-cycling microorganisms was present.
Novel strains of obligately chemolithoautotrophic sulfur-oxidizing bacteria (SOB) and obligately anaerobic sulfate-reducing bacteria (SRB) were isolated from various depths of Lake Fryxell. Physiological, morphological, and phylogenetic analyses showed that the sulfur-oxidizing strains were most closely related to mesophilic Thiobacillus species, while sulfate-reducing strains were most closely related to Arctic marine species of Desulfovibrio . The Antarctic isolates showed an adaptation to cold temperatures and thus should be active in the nearly freezing waters of Lake Fryxell. Population estimates of SOB obtained by most probable number analysis revealed that cell numbers peak at the oxycline (9.5 m), although viable cells exist well into the anoxic, sulfidic waters of the lake. SRB were isolated throughout the anoxic portions of the water column and in the lake sediments. The SOB and SRB described herein likely play a key role in the biogeochemical cycling of carbon and sulfur in Lake Fryxell.
In addition to sulfur-cycling prokaryotes, sediment enrichment cultures also yielded pure cultures of a lemon-shaped homoacetogenic bacterium closely related to species of Acetobacterium. Other fermentative isolates included two obligately anaerobic strains of a sugar-fermenting bacterium that were isolated from 9-meter Lake Fryxell water. These strains showed the highest degree of cold-adaptation of all isolates obtained in this study.
0307: Molecular biology
0793: Freshwater ecology