Limitations of using extracellular alkaline phosphatase activities as a general indicator for describing P deficiency of phytoplankton in Chinese shallow lakes

Extracellular phosphatase can be produced by phytoplankton to utilize organic phosphorus under phosphorus (P) deficiency. However, there is a controversy about its use as an indicator of P deficiency in natural phytoplankton community inferred by such an “induction–repression” mechanism. Size-fractionation of alkaline phosphatase activity (APA), soluble reactive phosphorus (SRP) concentration, algal density, and composition were determined in six Chinese shallow lakes ranking in gradient of trophic status, where a positive relationship between SRP concentration and algal density was observed. Enzyme-labeled fluorescence (ELF) method was used to localize phosphatase on cell membrane of algae. The so-called algal APA that associated with coarser particle (>3.0 μm) accounted for the largest part of total APA. Within a lake with lower SRP concentration, the “induction–repression” mechanism held true. Contrastingly, both algal and total APA were positively related to SRP concentration based on the data across all study lakes with statistical significance, which may be explained firstly by algal composition. The lakes with higher SRP concentration were dominated by diatoms and green algae, while they easily produced extracellular phosphatases as evidenced by ELFA labeling. In parallel, the lakes with lower SRP concentration were dominated by cyanobacteria, while it was never ELFA-positive; secondly, ELFA-positive dots or structures suggested that, in lakes with higher trophic status, attached bacteria or heterotrophic microorganisms could substantially contribute to extracellular phosphatases for hydrolyzing organophosphoric compounds but probably utilizing the organic moiety as an organic carbon source. This process simultaneously produces inorganic P, leading to the co-occurrence of high phosphate concentration and APA. So, the contributor of APA are complex, which may produce extracellular phosphatase species-specific or not exclusively for P nutrient and consequently make it difficult to normalize APA with the exact biomass estimators. Therefore, it is not reasonable to use APA, normalized or not, as a general indicator for describing P deficiency of phytoplankton in shallow lakes especially eutrophic ones.