Arsenate and microbial dynamics in different phosphorus regimes of the subtropical Pacific Ocean

Abstract Biologically toxic arsenateis physicochemically similar to biologically essential phosphate. Because arsenateand phosphateare indiscriminately incorporated by microbes, their ambient concentration ratios can be an important factor controlling microbial growth and metabolism. This study investigated the spatial distributions of arsenateand phosphateand the associated biogeochemical dynamics in the subtropicalNorth and South Pacific Ocean. Vertical arsenateand phosphateprofiles (≤ 150 m) in most of the study areas showed a nutrient-type distribution where the concentrations increased below the euphotic zone. The arsenateand phosphateconcentrations in the surface watersranged from the detection limits (5 nM and 4 nM, respectively) to approximately 40 nM and 400 nM, respectively. The surface arsenate:phosphate ratios were typically lower than 1, but those in the western subtropicalNorth Pacific (WSNP) were frequently higher than 1 due to phosphatedepletion. In the WSNP surface waters, Prochlorococcusand Pelagibacter arsenicdetoxification and phosphorus acquisition genes were abundant. Results of the onboard bioassays involving the addition of arsenateor phosphateto the surface waterindicated that microbes throughout the study areas possessed arsenateresistance and those in the WSNP during summer were under serious phosphatelimitation. Although phosphatelimitation likely accelerates the relative cellular accumulation of toxic arsenate, the lowest particulate As:P ratios were observed in the summer WSNP, concurrent with the lowest dissolved organic P (DOP) concentrations and the highest alkaline phosphatase activities. These results imply that active As excretion and/or DOP utilization could alleviate As accumulation while maintaining the cellular P quota.