methane in the NPSG

Open ocean surface waters are natural sources of methane emissions to the atmosphere. As recently as a decade ago the source of this methane was enigmatic, for methane production was only known to occur in certain anaerobic environments. Production of methane in the aerobic marine water column was posed as a “methane paradox”. The discovery of several metabolic pathways that enable microbes to transform organic matter into methane under aerobic conditions has led to a reexamination of the marine methane paradox and a shift away from the anaerobic methanogenesis paradigm. In this project we propose that the pathway microbes use to make methane depends on the nutrient conditions that prevail in mid ocean surface waters. In the North Atlantic Ocean, phosphorus limits microbial production, and microbes produce methane as a by-product of organic phosphorus degradation. In contrast, nitrogen limits microbial production in the North Pacific Ocean, and we propose that in this ocean basin microbes produce methane as a by-product of organic nitrogen degradation. To test this hypothesis we propose to compare the results of geochemical and biological measurements we made in the Sargasso Sea with a parallel set of geochemical measurements we propose to make in the North Pacific Ocean. Specifically, we propose to measure methane and ethylene, the carbon stable isotope values of methane, the chemical composition of high molecular weight dissolved organic matter with respect to methylamine (MeA) and methylphosphonate (MPn) along with the carbon isotopic value MeA and MPn in high molecular weight dissolved organic matter. We will preform a suite of laboratory experiments with cultured marine microbes to test if they can make methane from methylamine, and conduct parallel sets of experiments in the field. We will bring into cultivation new methane producing microbes from water samples collected as part of our field program.