Biolunch, 16 Feb 2021
Tuesday, February 16, 2021
Biological Oceanography Lunch Seminar
The ocean methane paradox: back to square one?
University of Puget Sound, Dept of Biology
11:30 A.M., via Zoom
Oversaturation of methane with respect to atmospheric equilibrium values are a persistent feature of near-surface waters in mid-ocean gyres. Most known pathways of microbial methane production are anaerobic, and inhibited by the presence of oxygen and sulfate. Methanogenesis in the high sulfate, well-oxygenated surface ocean has therefore been posed as an “ocean methane paradox”. One solution to the paradox is the aerobic degradation of methylphosphonate (MPn), a simple one carbon, reduced phosphorus compound that is prevalent in marine dissolved organic matter (DOM). MPn in DOM is bioavailable to bacteria containing the carbon-phosphorus (C-P) lyase enzyme, which assimilate the phosphorus and release the methyl group as methane. Because bacteria with C-P lyase are selected for in phosphate-depleted ocean regions, it is possible that methane oversaturation changes in response to phosphate concentrations. In this seminar, I will describe results from several cruises in the North Pacific (Gradients) and in the Western North Atlantic and Sargasso Sea in which we tested this hypothesis. We have also used the stable carbon isotope value (δ13C) DOM MPn to investigate its potential as a source of methane. Dissolved methane δ13C ranges from -44‰ to -47‰ for depths < 300 m and newly produced methane is expected to have a δ13C close to -43‰. However, DOM methylphosphonate has a δ13C value of -35‰. Moreover, C-P lyase is not the only sink of methylphosphonate. Some marine cyanobacteria like Prochlorococcus, and other bacteria, contain oxidative enzymes that convert methylphosphonate to formic acid and phosphate. These results suggest that additional methane production pathways may contribute to the methane paradox. Because anaerobic methanogenesis tends to produce isotopically lighter methane, this may be an important component of the ocean methane cycle, as was proposed originally.