Phosphate monoesterase and diesterase activities were measured with soluble reactive phosphorus (SRP) and labile and total dissolved organic phosphorus (DOP) concentrations in the North and South Pacific Ocean, to reveal the microbial utilization of phosphate esters in the Pacific Ocean. Both esterase activities were noticeably enhanced around the western part of 30 degrees N, where the surface SRP concentration was below 10 nM, while they showed no significant correlation with DOP concentration. The proportion of the activity in the dissolved fraction was higher for diesterase than monoesterase, which may support results from previous genomic analyses. Substrate affinity and the maximum hydrolysis rate of monoesterase were the highest at lower concentrations of SRP, showing the adaptation of microbes to inorganic phosphorus nutrient deficiency at the molecular level. The calculated turnover time of monoesters was 1 to 2 weeks in the western North Pacific Ocean, which was much shorter than the turnover time in other areas of the Pacific Ocean but longer than the turnover time in other phosphate-depleted areas. In contrast, the turnover rate of diesters was calculated to exceed 100 days, revealing that diesters in the western North Pacific were a biologically refractory phosphorus fraction. In the present study, it was revealed that both phosphate monoesters and diesters can be a phosphorus source for microbes in the phosphate-depleted waters, although the dynamics of the two esters are totally different.
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