SULFUR CHEMISTRY OF CINDER POOL, YNP: A SEARCH FOR THE CHEMICAL ORIGIN OF LIFE

There are many theories for the origin of life on Earth, but for those where the first cell developed from chemical origins there are two general models. “Heterotroph-first” assumes the early Earth surface contained abundant organic compounds as both the building blocks for the earliest cell and substrate for later growth. “Autotroph-First” (AF) (e.g. Wächtershäuser, 1988) in contrast requires the first cell to be a chemoautotroph capable of metabolism and carbon fixation using chemical energy. The AF model is intriguing, but requires critical abiotic chemical transformations to form alkyl thiols from simple inorganic substances. These transformations have to date not been observed to occur abiotically outside of the lab. We have now documented the formation of methanethiol (aka MeSH) in Cinder Pool, YNP (CP), and have confirmed the reaction as abiotic in the lab.

Evidence for MeSH was found in 2009 while working with Kirk Nordstrom (US Geological Survey) to provide analyses of dissolved gases in the Norris Geyser Basin. CP is a unique feature floored with molten sulfur that provides heat, S-compounds, reactive gases, and small balls of sulfur and sulfides that are the namesake “cinders”. Unidentified peaks on the field gas chromatograms were later found to correspond to the retention times for MeSH and CS₂. Over a 3 year period we confirmed the presence of MeSH and CS₂ in CP and MeSH in one other nearby pool, but not at several other pools in the basin. Conditions at the pool suggested that MeSH forms abiotically.

The abiotic formation of MeSH via CS₂ was confirmed in the laboratory using sterile microcosms with only CO₂ as a source of carbon, H₂ as the reducing compound, and CP cinders as the sole source of S and reactive surface, while FeS and FeS₂ are effective only with CS₂ as S source. CP cinders were found to be an excellent catalytic surface, and the internal gas vesicles are consistent with the requirements of the AF model. The experiments were conducted with synthetic CP water at temperatures typical for the pool.

Our findings suggest that the critical initial prebiotic chemical transformations are occurring in a terrestrial hot spring, although the product compounds are today probably being used as substrate by the native microbial consortium.