Organic Phosphorus Compound Class Utilization by Marine Microorganisms in the Amazon River Plume

Abstract

Dissolved marine phosphorus (P) consists of inorganic P (essentially phosphate, Pi) and organic P (DOP), encompassing P-esters, P-anhydrides, and phosphonates. Marine microorganisms use DOP compounds as a source of P, particularly in regions where Pi is scarce, though it is unclear which bond classes of DOP are preferentially utilized by natural microbial assemblages. Here we explored the potential microbial utilization of different DOP compounds at three stations with background Pi concentrations ranging from 50 to 70 nmol L-1, along the Amazon River Plume. We carried out seawater incubations by adding 20 µmol L-1 DOP as either adenosine monophosphate (AMP, P-ester bond), 3-polyphosphate (3PP, P-anhydride bonds), adenosine triphosphate (ATP, containing both P-ester and P-anhydride bonds), or methylphosphonate (MnPh, a phosphonate). Our results showed that most of the added AMP and ATP were hydrolyzed in 48 hours, as evidenced by a decrease in DOP concentrations and an increase in Pi concentration amounting to approximately 18 µmol L-1. These compounds were hydrolyzed by microbial communities since we ruled out autohydrolysis based on stable DOP compounds concentrations measured in MilliQ control samples. In contrast, the hydrolysis of 3PP and MnPh was low or negligible. While recent studies in culture have demonstrated that some phytoplankton species can break down and preferentially use P-anhydrides, our results from natural samples suggest that the microbial community as a whole degrades P-esters in larger amounts. We found that picophytoplankton and bacterial communities both benefited from the hydrolysis of P-esters, specifically the AMP and ATP that were tested, resulting in increased cell abundance. The findings carry significance for both the bioavailability of marine DOP and the broader process of ocean nutrient recycling within dissolved organic matter.