Heterogeneous Dissolved Organic Phosphorus Composition and Bioavailability in Marine Systems

The critical role of Dissolved Organic Phosphorus (DOP) in supporting primary production has spurred efforts to characterize DOP composition so that insight may be gained into its bioavailability and cycling in aquatic systems. The degree to which DOP is bioavailable to primary producers will determine, in part, the extent of carbon uptake and sequestration.  Ascertaining DOP composition has proven to be an analytically challenging endeavor.  As a consequence, the DOP pool remains poorly characterized, and our predictive power relative to DOP-bioavailability, and coupled carbon cycling, remains limited. Analytical impediments to characterizing DOP composition in natural waters include its low concentration, requiring pre-concentration before compositional features can be probed via spectroscopy, and the fact that organic phosphorus compounds are not easily amenable to standard organic geochemical approaches, such as chromatographic or mass spectrophotometric methods, particularly in salt water. While 31-Phosphorus Nuclear Magnetic Resonance (³¹P-NMR) spectroscopy has provided intriguing information on the distribution of the 2 major DOP compound types (phosphoesters, phosphonates), the crucial question of DOP bioavailability cannot be addressed by this method. We present novel DOP molecular weight distribution and bioavailability data, generated using a coupled sequential ultrafiltration-bioavailability approach from a marine water column depth profile and locations across a gradient in phosphate concentration in the Atlantic and Pacific Oceans.  There is substantial compositional variability in the marine DOP pool, both in the pattern of DOP molecular weight distribution at different sites, as well as the distribution of bioavailable mono- and diesters of phosphate across molecular weight fractions.  In some cases, a substantial fraction of DOP in different molecular weight size classes is non-reactive to the two enzymes used to assay potential bioavailability, raising the interesting possibility of non-bioavailable DOP. The significance of recognizing that the oceanic DOP pool is compositionally heterogeneous, and variably bioavailable, lies in that fact that such information is a prerequisite to building ecosystem models that capture the influence of P biogeochemistry on primary production and carbon cycling in aquatic systems.