Bioavailability of representative dissolved organic phosphate compounds to microbial communities in the western North Atlantic.

Dissolved organic phosphate (DOP) typically comprises the major faction (~80%) of the dissolved P pool in the upper waters of the worldwide ocean. Monoester and diester organophosphates (C-O-P), organic phosphoanhydrides (C-O-P-O-P), inorganic polyphosphates (Poly-P) and organophosphonates (C-P) collectively build up the DOP pool, and represent a possible alternative source of P for sustaining marine productivity. Nevertheless, the role of specific DOP compounds metabolism within the marine microbial P cycle still poorly resolved. In the framework of the ZEPHYR (Zooming in on Enzymatic PhosphoHYdrolysis Reactions) cruise, the potential bioavailability of model phosphonates, polyphosphates, and phosphoesters to marine microbial communities was investigated in the western North Atlantic Ocean. Incubation experiments were conducted at end-member stations along a transect going from coastal Georgia to the open ocean. Seawater was amended with either phosphate (Pi), inorganic tripolyphosphate (3Poly-P), adenosine 5’-triphosphate (ATP), adenosine 5’-triphosphate (AMP) or methylphosphonate (Mephn) and an unamended treatment was run in parallel to serve as a control. We determined in situ Pi fluxes from the remineralization of specific DOP pools using the sensitive Liquid Waveguide Capillary Cell method to estimate Pi and DOP concentrations and the ³²P radioactive tracer to track the turnover rate of Pi. At the initial incubation timepoint, turnover rates were lower in DOP amended treatments than in the control, indicating that the naturally occurring microbial assemblages obtained a portion of their required P from a wide range of DOP molecules. Interestingly, the lowest turnover rates were observed following 3Poly-P amended treatments, suggesting it is the most labile DOP source. By the final timepoint (48h), all DOP sources appeared to have been utilized, except methylphosphonate at the nearshore station. Additional short-term incubations showed that 3Poly-P consistently exhibited the highest bioavailability factor compared to ATP, AMP and Mephn, highlighting its preferential utilization by the naturally microbial communities encountered in the western North Atlantic.