Composition and cycling of marine organic phosphorus

The composition and cycling of dissolved organic phosphorus (DOP) were investigated in samples from the Pacific Ocean, the Atlantic Ocean and the North Sea. Using ultrafiltration and solid-state ³¹P Nuclear Magnetic Resonance (NMR) spectroscopy, the dominant compound classes of marine high molecular weight (HMW) DOP have been characterized. NMR spectra of ultrafiltered dissolved organic matter (UDOM) from all sites and depths reveal that P esters and phosphonates are the major components of ultrafiltered DOP (UDOP). Phosphonates, a chemically stable group of compounds containing a direct C-P bond, were not previously known to be a significant component of marine DOP. P esters (75 percent) and phosphonates (25 percent) are present in similar proportions throughout the ocean. The homogeneity of UDOP from different oceanic regions suggests that processes leading to this chemical composition are ubiquitous. To explore possible sources of phosphonates in the marine environment, three sets of samples were examined by ³¹P NMR: 1) UDOM and particulate organic matter (POM) from cultures of four marine primary producers, 2) particulates from heterotrophic bacterial cultures, and 3) ultrafiltered particulate organic matter (UPOM; 0.1-60 μm size fraction) samples from the Pacific Ocean and the North Sea. P esters were observed in all samples, 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 UDOP, including (1) selective preservation of phosphonates, and (2) production by an unrecognized source of phosphonates. C/P and N/P ratios of UDOM are significantly higher than Redfield ratios for POM. In general, C/P and N/P ratios of UDOM double between surface waters and the deep ocean. Increasing C/P and N/P ratios suggest that P is preferentially remineralized from UDOM relative to C and N throughout the water column. A simplified budget of the marine P cycle is presented, allowing for a first estimate of DOP residence time in the ocean. In the surface ocean, the estimated DOP residence time is 1.5 years, and the deep ocean estimated DOP residence time is 125 years. Calculations reveal that remineralization of DOP in the deep ocean may be a significant source of deep ocean DIP