Biologically toxic arsenate is physicochemically similar to biologically essential phosphate. Because arsenate and phosphate are 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 arsenate and phosphate and the associated biogeochemical dynamics in the subtropical North and South Pacific Ocean. Vertical arsenate and phosphate profiles (≤ 150 m) in most of the study areas showed a nutrient-type distribution where the concentrations increased below the euphotic zone. The arsenate and phosphate concentrations in the surface waters ranged 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 subtropical North Pacific (WSNP) were frequently higher than 1 due to phosphate depletion. In the WSNP surface waters, Prochlorococcus and Pelagibacter arsenic detoxification and phosphorus acquisition genes were abundant. Results of the onboard bioassays involving the addition of arsenate or phosphate to the surface water indicated that microbes throughout the study areas possessed arsenate resistance and those in the WSNP during summer were under serious phosphate limitation. Although phosphate limitation 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.
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