Abstract
Marine microbes use extracellular phosphatases to hydrolyse phosphate from organic matter. Dissolved organic phosphorus (DOP) is typically present in higher concentrations than phosphate in oceanic surface waters. Yet, the fate and role of different DOP components, such as phosphomonoester and phosphodiester, are poorly understood. Most of the investigations on extracellular enzymatic hydrolysis of marine DOP have focused on phosphomonoesterase (MEA) activity (i.e., alkaline phosphatase), whereas phosphodiesterase (DEA) measurements are scarce. This limits our understanding of the ecological and biogeochemical role of DOP sources other than P-monoesters in the sea. We determined extracellular MEA and DEA activities including their cell-free fractions on a bimonthly basis over 14 months in surface and mesopelagic subantarctic waters, thus covering a wide range of phosphate availability levels (from <0.5 to 2.3 µM). We found that DEA and MEA exhibit similar hydrolysis rates in surface as well as in mesopelagic waters. The MEA : DEA ratio varied between 0.38 and 5.42 during the study period, indicating potential differences in function and/or expression among the two enzyme groups, potentially reflecting differences in the availability and/or utilization of P-monoester and P-diester pools. Interestingly, the MEA : DEA was negatively correlated to phosphate (r=-0.82, p=0.02, R2=0.67) and positively with N:P (r=0.84, p=0.02, R2=0.67), suggesting that the relative importance of DEA versus MEA is linked to inorganic P availability and the N : P ratio. DEA was also related to the N : P ratio, both at the surface and at depth, suggesting DEA alone is sensitive to changes in the N : P ratio. The majority (>70%) of extracellular MEA and DEA was found in the cell-free fraction which increased with depth. Our results indicated that DOP hydrolysis mediated by DEA in the surface as well as in dark ocean is as important as the frequently measured MEA.
Highlights
Microbial communities are the main drivers of the global biogeochemical cycles (Falkowski et al, 2008)
There was a shift in the inorganic nutrient concentrations in July 2018 in the epi- and mesopelagic waters, which separated the study into two periods (Figure 1A)
Mean phosphate and nitrate concentrations increased in July 2018, remaining at about these levels for the rest of the study (Figure 1A)
Summary
Microbial communities are the main drivers of the global biogeochemical cycles (Falkowski et al, 2008). The seasonal dynamics of total and cell-free DEA and MEA are reported for epi- and mesopelagic waters.
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