Abstract
The labile fraction of dissolved organic phosphorus (DOP) – predominantly consisting of phosphoric esters – is an important microbial P source in the subtropical oligotrophic ocean. However, unlike phosphate, knowledge for labile DOP is still limited due to the scarcity of broad and intensive observations. In this study, we examined the concentrations and size-fractionated hydrolysis rates of labile phosphoric monoesters and diesters along a >10,000 km longitudinal transect in the North Pacific (23°N; upper 200-m layer). Depth-integrated monoesters decreased westward with a maximum difference of fivefold. Vertical profiles of monoesters in the eastern and western basins showed decreasing and increasing trends with depth, respectively. The monoester-depleted shallow layer of the western basin was associated with phosphate depletion and monoesterase activity was predominant in the large size fraction (>0.8 μm), suggesting that monoesters are significant P sources particularly for large microbes. In contrast, diester concentrations were generally lower than monoester concentrations and showed no obvious horizontal or vertical variation in the study area. Despite the unclear distribution pattern of diesters, diesterase activity in the particulate fraction (>0.2 μm) increased in the phosphate-depleted shallow layer of the western basin, suggesting that the targeted diesters in the assay were also important microbial P sources. Diesterase activities in the dissolved fraction (<0.2 μm) were not correlated with ambient phosphate concentrations; however, cell-free diesterase likely played a key role in P cycling, as dissolved diesterase activities were substantially higher than those in the particulate fraction. The horizontal and vertical variability of labile monoesters in the subtropical North Pacific were therefore predominantly regulated by P stress in particularly large microbes, whereas the distributions of labile diesters and diesterase activities were generally independent of microbial P stress, indicating a more complex regulation of diesters to that of monoesters.
Highlights
Dissolved organic phosphorus (DOP) comprises a significant portion of dissolved phosphorus (P) in the surface layer of the subtropical ocean (Paytan and McLaughlin, 2007; Karl and Björkman, 2015)
We examined the concentrations and sizefractionated hydrolysis rates of monoesters and diesters in the subtropical North Pacific to elucidate the underlying mechanisms influencing the dynamics of labile phosphoric ester
The biomass-normalized diesterase activity (DEA) from these studies and the particulate-associated DEA in this study were both stimulated under phosphate-depleted conditions. These results demonstrate the importance of semi-labile diesters as alternative P sources in the subtropical ocean
Summary
Dissolved organic phosphorus (DOP) comprises a significant portion of dissolved phosphorus (P) in the surface layer of the subtropical ocean (Paytan and McLaughlin, 2007; Karl and Björkman, 2015). DOP comprises various organic P compounds involved in living organisms, including phospholipids, nucleic acids, and sugar phosphates (Karl and Björkman, 2015) Based on their chemical structures, DOP compounds are typically grouped into phosphoric esters and phosphonates and phosphoric esters contribute the highest proportion (80–85%) to the total DOP pool in various oceanic regions (Young and Ingall, 2010). Compiled field studies have demonstrated a distinct elevation of monoesterase activity (MEA) under low phosphate conditions in both the Atlantic and Pacific Oceans (Mahaffey et al, 2014 and references therein) Based on these findings, the labile fraction of phosphoric monoesters has been considered as an important P source for microbes, and studies have increasingly reported the distribution of monoesters in the oligotrophic ocean (Moutin et al, 2008; Duhamel et al, 2010; Suzumura et al, 2012; Hashihama et al, 2013, 2019; Sato et al, 2013; Vidal et al, 2018; Yamaguchi et al, 2019). The examined oceanic regions and depths are still limited, monoester concentrations are mostly below or around the detection limit in the upper 100-m layer and contribute a small proportion of the total DOP pool
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