Microbial Processing of Sulfur Gases from a Terrestrial Cave Ecosystem

P.C.  Bennett, A.S. Engel, L.A. Stern

Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712

The sulfur cycle is one of the basic chemical indicators of the presence of life on Earth. The marine sulfur system is extremely complex, where in addition to recycling of inorganic sulfur compounds, marine biota also produce a variety of volatile organic sulfur compounds (VOSC). In contrast, the continental surface and subsurface sulfur cycle is widely regarded as ‘simple’, with only transformations between sulfate and sulfide considered significant. However, we find unexpected complexity in the sulfur cycle in a chemoautotrophically-based cave ecosystem. Dissolved sulfide provides the energy source to aquatic primary-producing sulfur-oxidizing bacteria, which in turn supports anaerobic microorganisms including fermenters and sulfate-reducing bacteria. In culture, these anaerobes produce H2S, as well as COS, DMS, and methanethiol, which may serve as important energy substrates for sulfur-oxidizers or methanogens within aquatic microbial habitats, or for subaerial cave-wall biofilms due to degassing into the cave atmosphere. Biogenic VOSCs are important to the global sulfur cycle, and may be indicators of microbial activity in subsurface environments. Moreover, as an extreme environment, the cave lacks photosynthetically-fixed carbon, has toxic gases and locally extreme acidity, and extends our understanding of the limits of life.

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