Marine Organic Phosphorus Cycling: Novel Insights from Nuclear Magnetic Resonance

In vast regions of the ocean, a significant fraction of the vital nutrient phosphorus (P) in surface waters is associated with dissolved organic matter (DOM). Oceanic biological productivity in these regions is potentially dependent on the regeneration of bioavailable forms of P from DOM. Using tangential-flow ultrafiltration, high molecular weight (HMW) DOM samples were collected for chemical analysis and 31P Nuclear Magnetic Resonance (NMR) studies. In a profile of HMW DOP from the Pacific, depth related changes in dissolved organic P (DOP) concentration and C/N/P ratios indicate that DOP is preferentially regenerated from HMW DOM relative to dissolved organic carbon and dissolved organic nitrogen. NMR studies of these samples show that P esters (75 percent) and phosphonates (25 percent) were the major compound classes present in HMW DOP. HMW DOP appears to be highly enriched in phosphonates relative to marine organisms, where phosphonates comprise a very small fraction of total P. The origin of phosphonates in the marine environment is largely unknown, and the biogeochemical dynamics by which phosphonates become a major fraction of HMW DOP are not well understood. To explore possible sources of phosphonates in the marine environment, HMW DOM and particulate organic matter (POM) from cultures of four marine primary producers, as well as particulates from heterotrophic bacterial cultures, were examined by 31P NMR. P esters were observed in all cultures, and phosphonates were detected in only two of the heterotrophic bacterial cultures. Several processes may be acting to produce the observed abundance of phosphonate-P in marine HMW DOP, including (1) selective preservation of phosphonates, and (2) production by an unrecognized source of phosphonates.