Carbon isotope fractionation during microbial methane oxidation

Abstract

Methane, a common trace constituent of groundwaters, occasionally makes up more than 20% of the total carbon in groundwaters^1,2. In aerobic environments CH₄–rich waters can enable microbial food chain supporting a mixed culture of bacteria with methane oxidation as the primary energy source to develop³. Such processes may influence the isotopic composition of the residual methane and because ¹³C/¹²C analyses have been used to characterize the genesis of methanes found in different environments, an understanding of the magnitude of such effects is necessary. In addition, carbon dioxide produced by the methane-utilizing bacteria can be added to the inorganic carbon pool of affected groundwaters. We found carbon dioxide experimentally produced by methane-utilizing bacteria to be enriched in ¹²C by 5.0–29.6‰, relative to the residual methane. Where methane-bearing groundwaters discharged into aerobic environments microbial methane oxidation occurred, with the residual methane becoming progressively enriched in ¹³C. Various models have been proposed to explain the ¹³C/¹²C and ¹⁴C content of the dissolved inorganic carbon (DIC) of groundwaters in terms of additions or losses during flow in the subsurface^4,5. The knowledge of both stable carbon isotope ratios in various pools and the magnitude of carbon isotope fractionation during various processes allows geochemists to use the ¹³C/¹²C ratio of the DIC along with water chemistry to estimate corrected ¹⁴C groundwater ages^4,5. We show here that a knowledge of the carbon isotope fractionation between CH₄ and CO₂ during microbial methane-utilization could modify such models for application to groundwaters affected by microbial methane oxidation.