Significance of phosphomonoesterase activity in the regeneration of phosphorus in a meso-eutrophic, P-limited reservoir

The significance of phosphomonoesterase (PMEase) activity in the regeneration of phosphorus was investigated in a phosphorus-limited reservoir during the growing season (April–October) using the artificial substrate p-nitrophenylphosphate (pNPP). Despite the relatively high concentration (up to 0.45 μmol l−1) of soluble nonreactive phosphorus (SNP) in the reservoir and the high total PMEase activity (up to 1144 nmol pNP l−1h−1), the rate of phosphate release from SNP accounted for less than 0.6% of the direct orthophosphate uptake by the planktonic community. The high apparent affinity and maximum velocity for pNPP may underestimate the velocities of Pi release by PMEase. More critical studies using natural substrates are needed to assess the role of PMEase as Pi supplier for the planktonic community. PMEase activity was apparently not related to soluble or particulate phosphorus fractions, suggesting that PMEase activity may not be useful as an indicator of P-limitation, unless other phosphorus pools (e.g. intracellular phosphorus) are considered.