C-Isotopic Fractionation During Methylotrophic Methanogenesis
Roger E. Summons, Peter D. Nichols, and Peter D. Franzmann
Methanogenesis from non-competitive substrates such as trimethylamine (TMA) may be quantitatively important in global methane budgets. This is because choline, glycine betaine and related compounds, precursors of TMA, are produced abundantly and ubiquitously by microbes, particularly those from marine and saline lacustrine environments. These substrates may source methane supersaturating oceanic seawater, possibly formed in anaerobic microhabitats (e.g. zooplankton guts and within particulates) and indicated by carbon isotopic analysis of abundant methanogen-derived hydrocarbons (e.g. 2,6,10,15,19-pentamethyleicosane, PME) found in recent and ancient marine sediments. In order to elaborate on these observations using cultured organisms we measured isotopic fractionation f r methane and lipids formed by methanogens using trimethylamine as their carbon source. Methanosarcina barkerii showed fractionation factors (^egr) of 50.2^pmil for conversion of TMA to methane and 20.2^pmil for TMA-biomass. Moreover PME and phytanyl chains of M. barkerii ether lipid were depleted by as much as 18^pmil compared to biomass. For the Antarctic methanogen Methanococcoides burtonii we measured even greater ^egr values of 71^pmil (TMA-CH4), 49.6^pmil (TMA-biomass) and 79.9^pmil (TMA-phytanyl ether). It should be noted that these values represent maximum or near maximum values for non-limiting substrate concentrations and are significantly higher than those reported for aceticlastic methanogenesis (approx. 21^pmil) or reduction of carbon di xide (32 to 46^pmil). Methylotrophic methanogenesis may be the source of extremely 13C-depleted methane in certain gas accumulations.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California