Basin-Scale Variations in Labile Dissolved Phosphoric Monoesters and Diesters in the Central North Pacific Ocean

Abstract

Labile dissolved phosphoric esters act as important phosphorus sources for microorganisms in the low-phosphate oligotrophic ocean. Despite the significant role of labile dissolved phosphoric esters, previous studies have only focused on phosphoric monoesters, and the spatial distribution and dynamics of phosphoric ester species, including diesters, remain unclear. Thus, we aimed to elucidate basin-scale variations in both monoesters and diesters and their role on microorganisms particularly in terms of diazotrophs. This is the first study to report the concentrations of these phosphoric esters in low- to middle-latitudinal areas of the central North Pacific Ocean, concurrent with their hydrolysis rates. Hydrolysable monoester and diester concentrations ranged from undetectable levels to 40 nM, and their sum accounted for <11% of dissolved organic phosphorus on average. While monoester concentrations were significantly low in the low phosphate (<100 nM) region, there was no obvious trend between diester and phosphate concentrations. Monoesterase and diesterase activities showed significantly high values under low phosphate conditions when normalized by chlorophyll a concentrations. This indicated that both esters served as alternative phosphorus sources for microbial communities including phytoplankton. Diester concentrations and diesterase activities were positively correlated with nitrogen fixation activities, whereas such relationships were not observed for monoester concentrations and monoesterase activities. Thus, our results suggest that diesters have high affinity to diazotrophs compared with monoesters, which are utilized by a broad spectrum of microorganisms.