(Submitted Abstract to the 2003 International Symposium for Environmental Biogeochemistry (ISEB16), Amori, Japan)
Annette Summers Engel*, Philip C. Bennett, Libby A. Stern
Jackson School for Geosciences, Research Group for Microbial Geochemistry,
University of Texas at Austin, Austin, Texas 78712
Reduced sulfur gases, including hydrogen sulfide (H2S) and volatile organosulfur compounds (VOSC), are important components of the global sulfur cycle. While H2S is volumetrically the most important sulfur gas in both marine and freshwater settings, recent work suggests that carbonyl sulfide (COS), methanethiol (MT), and dimethyl sulfide (DMS) may have significant impacts on the flux of sulfur into the atmosphere from near-surface environments. But while VOSC production and degradation rates have been widely studied from marine settings, little is known about organosulfur gases from the terrestrial subsurface. This study provides the first approximation of VOSC abundance and production rates from microbial mats in a cave environment.
This work is part of an ongoing investigation of carbon and sulfur biogeochemical cycling in Lower Kane Cave (Wyoming, USA), a system with H2S-bearing springs that discharge into the cave passages. Phylogenetic analyses from 16S rRNA gene sequence clone libraries reveal that mats are dominated by “Epsilonproteobacteria” (68% of the clone libraries), with lesser abundant organisms closely related to Desulfocapsa (15%), Thiothrix (10%), and Thiobacillus (2%), and rare groups within Acidobacterium, Cytophaga, Bacteriodes, and Pantoea genera (< 5% total).
Anaerobic microorganisms from the cave water and mats were enriched in PRAS media specific for metabolic guilds, including fermenting bacteria, and chemoautotrophic, lactate-/formate- (Group I) and acetate-utilizing (Group II) sulfate-reducing bacteria (SRB). Enrichment cultures and strain isolates were screened for sulfur gas production (H2S, COS, MT, DMS, SO2, CS2) with a gas chromatograph (GC). Acetate-utilizing SRB dominated all microbial mats sampled, while the abundance of lactate- and formate-SRB generally increased as mat thickness increased. Chemoautotrophic SRB were found in low abundance in the mats, but were present in all the spring waters. Fermenting bacteria were detected in every sample. Mixed populations of fermenters and SRB produced varying concentrations of H2S, COS, and MT. However, DMS was only detected from lactate-/formate-supplemented SRB cultures.
Sulfur gases were measured directly in the cave using a field-deployable GC to determine the contribution of in situ gas produced by anaerobic populations compared to the allochthonous sulfide entering in the spring waters. Dissolved sulfide decreased more rapidly along the cave stream than can be accounted for by abiotic autoxidation or volatilization. The overall decrease in dissolved sulfide corresponded to an increase in H2S gas directly over the microbial mats, suggesting that the in situ sulfur gas production is due to the anaerobic microbial consortium.
We propose that the VOSC production by anaerobic populations in sulfidic groundwater habitats provides an example of active sulfur cycling in the subsurface and represents an important terrestrial source for reduced sulfur gases. We are currently attempting to understand the mechanisms for sulfur gas production and the ecological role of VOSC to the microbial ecosystem.
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