Phosphonate cycling supports methane and ethylene supersaturation in the phosphate-depleted western North Atlantic Ocean

In oligotrophic ocean regions, dissolved organic phosphorus (DOP) plays a prominent role as a source of phosphorus (P) to microorganisms. An important bioavailable component of DOP is phosphonates, organophosphorus compounds with a carbon-phosphorus (C-P) bond, which are ubiquitous in high molecular weight dissolved organic matter (HMWDOM). In addition to being a source of P, the degradation of phosphonates by the bacterial C-P lyase enzymatic pathway causes the release of trace hydrocarbon gases relevant to climate and atmospheric chemistry. In this study, we investigated the roles of phosphate and phosphonate cycling in the production of methane (CH4) and ethylene (C2H4) in the western North Atlantic Ocean, a region that features a transition in phosphate concentrations from coastal to open ocean waters. We observed an inverse relationship between phosphate and the saturation state of CH4 and C2H4 in the water column, and between phosphate and the relative abundance of the C-P lyase marker gene phnJ. In phosphate-depleted waters, methylphosphonate and 2-hydroxyethylphosphonate, the C-P lyase substrates that yield CH4 and C2H4, respectively, were readily degraded in proportions consistent with their abundance and bioavailability in HMWDOM and with the concentrations of CH4 and C2H4 in the water column. We conclude that phosphonate degradation through the C-P lyase pathway is an important source and a common production pathway of CH4 and C2H4 in the phosphate-depleted surface waters of the western North Atlantic Ocean and that phosphate concentration can be an important control on the saturation state of these gases in the upper ocean.