Project: Collaborative Research: NSF OCE-BSF: Coupling organic nutrient cycling to methane production in the oligotrophic North Pacific Ocean

NSF abstract:

Open ocean surface waters are natural sources of methane to the atmosphere. As recently as a decade ago the source of this methane was a mystery, because methane production was only known to occur in certain environments without oxygen. Recently, the discovery of several metabolic pathways that enable microbes to transform organic matter into methane in the presence of oxygen has led to a shift away from the idea that methane can only be produced in anaerobic (oxygen-free) environments. The investigators propose that the pathway microbes use to make methane depends on the nutrient conditions that prevail in open ocean surface waters. In the North Atlantic Ocean, phosphorus limits microbial production, and microbes produce methane as a by-product of getting the phosphorus they need from organic compounds that contain phosphorus. In contrast, nitrogen limits microbial production in the North Pacific Ocean. The team proposes that in the North Pacific Ocean microbes produce methane as a by-product of organic nitrogen degradation. To test this hypothesis, they propose to compare the results of geochemical and biological measurements previously made in the North Atlantic with a parallel set of geochemical measurements they propose to make in the North Pacific Ocean. The award will support collaborations between an early career professor at a primarily undergraduate institution (PUI) and a senior scientist, and between US and Israeli scientists. Undergraduate students will participate in interdisciplinary research spanning oceanography, isotope biogeochemistry, and genome science and will conduct research at sea. The microbiology and genomic research will be integrated into course-based undergraduate research experiences at the University of Puget Sound enabling diverse students to participate directly in authentic research. Results will also be integrated into a graduate level course in marine organic geochemistry available on-line through the MIT Open Courseware website. This is a project jointly funded by the National Science Foundation's Directorate of Geosciences (NSF-GEO) and the Israel Binational Science Foundation (BSF) in accord with the language in the Memorandum of Understanding between the NSF and the BSF. This Agreement allows a single collaborative proposal, involving US and Israeli investigators, to be submitted and peer-reviewed by NSF. Upon successful results of the NSF merit review and recommendation by the cognizant NSF Program of an award, each Agency funds the proportion of the budget and the investigators associated with its own country.

The guiding hypothesis of this study is that although surface seawater in the North Atlantic and North Pacific Subtropical Gyres are both sources of methane to the atmosphere, the underlying microbial processes that produce methane in the two basins are fundamentally different. Microbial production in the Sargasso Sea is chronically phosphorus-limited. To mitigate this limitation, some microbes degrade methylphosphonate that is incorporated into the high molecular weight fraction of dissolved organic matter (HMWDOM) into methane and phosphaote. Bacteria expressing the carbon-phosphorus (C-P) lyase enzyme pathway for phosphonate catabolism dominate the Sargasso Sea microbial community and mediate this form of methane production making it the principal route through which excess methane is produced in the Sargasso Sea. In contrast, microbial production in the North Pacific Subtropical Gyre (NPSG) is chronically nitrogen limited and the proposal postulates that nitrogen acquisition through the degradation of methylamines in HMWDOM is a major route through which excess methane is produced. Methylamines are twenty-fold more abundant than methylphosphonate in marine HMWDOM and the aminotransferase gene linked to the conversion of methylamine into methane in freshwater lakes has closely related sequences in marine bacterial genomes. These sequences are abundant and widespread in marine metagenomes. Although the cycling of methylphosphonate and methylamine in oligotrophic surface waters both produce methane, the study postulates that the two processes will yield methane with distinct and characteristic carbon isotopic values. To test this hypothesis, the team will measure the stable carbon isotope value of the methane produced from HMWDOM methylamine and methylphosphonate. The team will also conduct laboratory experiments that test the capacity of diverse oligotrophic and copiotrophic marine bacterial isolates to convert HMWDOM methylamines to methane. This objective is complemented by a field study in the NPSG northwards from Hawaii along 158°W, the longitude of Station ALOHA, to 25-28°N to conduct geochemical and biological measurements associated with each methane production pathway. The team will obtain water column profiles of methane and ethylene concentration (two products of C-P lyase), methane carbon isotopes, and concentrations and carbon isotope values of HMWDOM methylamine and methylphosphonate. The investigators will quantify the rates of methane production from methylamine and methylphosphonate using stable carbon isotope tracers, C-P lyase activity, and the ratio of C-P lyase to aminotransferase gene abundance and expression in the NPSG. Lastly, the team will compare the bioavailability of HMWDOM methylamine and methylphosphonate to natural microbial communities in the NPSG using a metatranscriptomics approach to examine changes in microbial metabolic functions in response to HMWDOM additions. Together, these data will resolve the relative contribution of the methylamine and methylphosphonate pathways to aerobic methane production in the NPSG and the microbial groups and ecosystem properties underlying methane production. Through this interdisciplinary approach, the study will enhance our understanding of processes controlling aerobic methane production in the environment.