Variable Molecular Weight Distribution and Bioavailability of DOP from Coastal and Open Ocean Waters Suggests Compositional Heterogeneity

Phosphorus (P), an essential nutrient, can be growth limiting in aquatic environments. Organisms synthesize phosphohydrolytic enzymes to convert unavailable forms of P to orthophosphate, which can be directly assimilated. To the extent that DOP is enzyme hydrolyzable, it can be rendered bioavailable and satisfy organismal P-demand. While recognition of the role of DOP in supporting primary productivity is increasing, DOP composition and bioavailability remain poorly constrained. It is the composition of DOP that ultimately will control its bioavailability. To explore this link, DOP from a range of marine environments was segregated into four molecular weight size classes (>100kDa, 1-100kDa, 500Da- 1kDa, and <500Da) via sequential ultrafiltration. Molecular weight segregates were then exposed to phosphohydrolytic enzymes (alkaline phosphatase and phosphodiesterase) and the potential bioavailability of ester-bound P was determined through monitoring the buildup of soluble reactive phosphorus (SRP), the end product of DOP hydrolysis. On average, the sum of all DOP segregates yielded 99 ± 15% recovery. Contrary to previous studies on DOP composition that show striking homogeneity, our results indicate (i) distinct differences in molecular weight distribution, and thus composition, in surface water samples; and (ii) no observed correlation between molecular weight and bioavailability. This latter observation deviates from the size-reactivity continuum hypothesized for dissolved organic carbon (DOC), suggesting that generalizing about DOP based on DOC may not be warranted. Such insights about how DOP molecular characteristics translate into P-bioavailability to marine microorganisms is a prerequisite to building ecosystem models that will capture the influence of P biogeochemistry on oceanic carbon cycling.