Acetylene fermentation: An Earth-based analog of biological carbon cycling on Titan

Details Acetylene fermentation: An Earth-based analog of biological carbon cycling on Titan Acetylene fermentation: An Earth-based analog of biological carbon cycling on Titan

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p51g1190m&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P51G-1190

Biology

Quantitative Biology

Genomics

[0343] Atmospheric Composition And Structure / Planetary Atmospheres, [0406] Biogeosciences / Astrobiology And Extraterrestrial Materials, [0456] Biogeosciences / Life In Extreme Environments, [0465] Biogeosciences / Microbiology: Ecology, Physiology And Genomics

Scientific paper

Acetylene (C2H2) is present in part per million quantities in the atmosphere of Titan; conceivably as an intermediate product of methane photolysis. Currently, Earth’s atmosphere contains only trace amounts of C2H2 (~40 pptv), however higher concentrations likely prevailed during the Hadean and early Archean eons (4.5 - 3.5 Ga). We isolated C2H2-fermenting microbes from various aquatic and sedimentary environments. Acetylene fermentation proceeds via acetylene hydratase (AH) through acetaldehyde, which dismutates to ethanol and acetate, and if oxidants are present (e.g., sulfate) eventually to CO2. Thus, the remnants of a C2H2 cycle exists today on Earth but may also occur on Titan and/or Enceladus, both being planetary bodies hypothesized to have liquid water underlying their frozen surfaces. We developed a molecular method for AH by designing PCR primers to target the functional gene in Pelobacter acetylenicus. We used this method to scan new environments for the presence of AH and we employed DNA sequencing of the 16S rRNA gene in order to positively identify pelobacters in environmental samples. Acetylene fermentation was documented in five diverse salt-, fresh-, and ground-water sites. Pelobacter was identified as the genus responsible for acetylene fermentation in some, but not all, of these sites. Successful probing for AH preceded the discovery of acetylene consumption in a contaminated groundwater site, demonstrating the utility of functional gene probing. A pure culture of a C2H2-fermenting pelobacter was obtained from an intertidal mudflat. We also obtained an enrichment culture (co-cultured with a sulfate reducer) from freshwater lake sediments, but neither was pelobacter nor AH detected in this sample, suggesting that an alternative pathway may be involved here. Slurry experiments using these lake sediments either with or without added C2H2 or sulfate showed that sulfate reduction and acetylene fermentation were independent processes. In general, the ubiquity of acetylene fermentation as well as the presence of AH (an enzyme specific to acetylene) begs the questions; 1) why has this ability persisted on Earth for so long in the absence of significant atmospheric acetylene? 2) does C2H2-fermentation represent a possible means of sustaining growth in the anoxic, aqueous subsurface regions of Titan (and Enceladus)?

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