Amicrobial source of phosphonates in oligotrophic marine systems

Phosphonates, compounds with a carbon–phosphorus bond, are a key component of the marine dissolved organic phosphorus pool1. These compounds serve as a phosphorus source for primary producers, including the nitrogen-fixing cyanobacteria Trichodesmium2. Phosphonates can therefore support marine primary production, as well as climate-driven increases in marine nitrogen fixation3, carbon sequestration4 and possibly methane production, through the breakdown of methylphosphonate5. Despite their importance, the source of phosphonates to the open ocean has remained uncertain. Here, we use solid-state nuclear magnetic resonance spectroscopy to screen for the presence of phosphonates in cultured strains of Trichodesmium erythraeum. We show that phosphonates comprise an average of 10% of the cellular particulate phosphorus pool in this species. We therefore suggest that these cyanobacteria produce phosphonates, and might be a significant source of these compounds in the ocean, particularly in nutrientpoor regions, where Trichodesmium blooms occur. Given that Trichodesmium also thrives in a warm, carbon-dioxide-rich environment3, phosphonate production may increase in the future. This, in turn, might select for a microbial community that can use phosphonate, and could have implications for nitrogen fixation, carbon sequestration and greenhouse-gas production. Phosphorus (P) has a key role in constraining the growth of marine primary producers over both modern and geologic timescales6,7. In many regions of the ocean, standing stocks of dissolved inorganic phosphorus are so low that organically bound P dominates the dissolved P reservoir. With future P limitation scenarios predicted from natural (for example, increased N2 fixation) and anthropogenic (for example, increased atmospheric nitrogen deposition) responses to climate change3,4,7, microbial community structure, oceanic carbon export, and hence the oceanic uptake of atmospheric CO2, may be controlled by dissolved organic phosphorus (DOP) concentration and composition5,8. Furthermore, the presence and microbial degradation of methylphosphonate in the upper water column has been suggested to result inmethane release to the atmosphere5. However, these hypotheses, and the ability to model climate-driven changes in oceanic biogeochemical cycles, are limited by the lack of information regarding the chemical composition, production and degradation of the present-dayDOPpool. DOP exists as two main bond classes, phosphoester (P–O–C bond) and phosphonate (P–C bond). 31P NMR analysis of highmolecular-weight (HMW) DOP (the only fraction concentrated enough for study) has shown that both bond classes are a significant and constant percentage (75% phosphoester and 25% phosphonate) of marine DOP (ref. 1). The phosphoester pool