Award: OCE-1634080

Intellectual merit Methane is a potent greenhouse gas that contributes to planetary warming. The concentration of methane in the atmosphere is increasing, but the reasons behind this increase are unclear. The ocean is a source of atmospheric methane, and this project explored a potentially important pathway of methane formation in the ocean, the degradation of organic phosphorus containing compounds, or phosphonates, into methane. Our analyses showed that phosphonates were abundant in surface waters across the western North Atlantic Ocean, as were the microbes that degrade phosphonates to methane. We found that the abundance of genes that code for the degradation of phosphonates to methane were inversely correlated with phosphate concentration, but positively correlated with methane supersaturation. When phosphate concentrations in the ocean are low, microbes turn on phosphonate degradation in order to acquire phosphorus from phosphonates. In doing so they produce methane, and this production of methane increases the flux of methane from the ocean to the atmosphere. One important factor in the strength of the ocean methane source is therefore the concentration of phosphate. If increasing global temperatures lead to decreased mixing and delivery of nutrients to ocean surface waters, this will likely lead to an increase in marine methane production. Several other aspects of phosphonate biogeochemistry were explored in this project. We used marine genomic and metagenomic databases to determine that phosphonate production was widespread among diverse taxa of marine microbes. We identified a cultured representative of the marine cyanobacteria Prochlorococcus that makes phosphonates, and found that up to half the phosphorus in this strain of Prochlorococus was allocated to phosphonate production. Phosphonates were part of membrane proteins, and we postulate phosphonates are used to protect the cell from viral infection and/or grazing. Curiously, we found that microbes that can make phosphonates are incapable of degrading them. Phosphonate production and phosphonate degradation are catalyzed by very different groups of microbes. We developed a method to measure the activity of the enzyme that degrades methylphosphonate to methane, and found phosphonate degradation, and moast likely methane formation, occurs throughout the water column. Finally, we developed a method to measure the carbon isotopic value of methylphosphonate in dissolved organic matter. Carbon isotopic values of methylphosphonate in dissolved organic matter can now be compared to carbon isotopic values of methane to measure the amount of ocean methane that is generated by phosphonate degradation. Broader impacts The project contributed to the training of two graduate and five undergraduate students. The project significantly increased the number of methane measurements in the ocean and uncovered the connection between methane supersaturation and phosphate concentration. This relationship should prove valuable to future efforts to model atmospheric methane concentrations. Finally, the project led to the development of two new methods to measure phosphonate degrading enzyme activity and the carbon isotope values of methylphosphonates. Future deployment of both methods will contribute to our broader understanding of methane formation and microbial biogeochemical cycling in the ocean Last Modified: 10/05/2020 Submitted by: Daniel J Repeta