Search

Methane is a potent greenhouse gas commonly supersaturated in the oxic surfaces waters of oceans and lakes, yet canonical microbial methanogens are obligate anaerobes. One proposed methane production pathway involves microbial degradation of methylphosphonate (MPn), which can proceed in the presence of oxygen. Directly tracing dissolved methane to its source in oxic waters, however, remains a challenge. To address this knowledge gap, we quantified the carbon isotopic fractionation between substrate MPn and product methane (1.3‰) in lab experiments, which was 1 to 2 orders of magnitude smaller than canonical pathways of microbial methanogenesis (20 to 100‰). Together, these results indicated that microbial catabolism of MPn is a source of methane in surface oceans and lake waters, but to differentiate sources of MPn in nature a further accounting of all sources is necessary. Methane from this pathway must be considered in constraining the marine carbon cycle and methane budget. Plain Language Summary: Each year microbes in the surface of lakes and oceans capture gigatons of carbon dioxide from the atmosphere. Some of this organic carbon is converted to the potent greenhouse gas methane right there in the surface waters, where it may easily escape to the atmosphere. Precisely how much methane is released from one specific microbial pathway that is intimately involved in the cycling of both carbon and phosphorus remains an outstanding question. To enable environmental geoscientists to track this process, we establish isotopic "fingerprint" of this process in the laboratory. Using these lab‐derived constraints, we reinterpreted the limited available C‐isotopic data from methane dissolved in oxygenated ocean and lake waters. Key Points: Carbon isotope fractionation by microbes containing C‐P lyase is near zeroExperimental constraints allow for tracking C‐P lyase produced methane in natureThe C isotopes of surface ocean methane likely track dissolved methylphosphonates [ABSTRACT FROM AUTHOR]