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

Abstract

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 "inductionrepression" 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. Enzymelabeled 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 ELFApositive; 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 cooccurrence 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.

Presence and patterns of alkaline phosphatase activity and phosphorus cycling in natural riparian zones under changing nutrient conditions

“…8). It was consistent with Cao et al (2010) that, the so-called induction-repression mechanism held true with lower DRP and it can be swamped or reversed with relatively higher DRP status (Nausch, 1998;Sebastian et al, 2004). The APA-DRP relationship was not significant for bacteria (Fig.…”

Joint effect of freshwater plume and coastal upwelling on phytoplankton growth off the Changjiang River

“…Under such a condition of high nutrient availability, the inverse relation between APA and Chl a indicates that phytoplankton growth might not suffer from P limitation. We suggest that the highly allochthonous APA is likely sourced from limnetic heterotrophic bacteria (Cao et al, 2010). Studies had confirmed that bacteria would excrete AP to utilize C from DOM in nutrient-replete but lightlimited environments in order to satisfy their C demand instead of P demand (Van Wambeke et al, 2002).…”

Effects of turbulence on alkaline phosphatase activity of phytoplankton and bacterioplankton in Lake Taihu

“…Alkaline phophatase activity (APA) is generally regulated by external Pi concentration (Tanaka et al, 2008;). Also, several studies indicated that cellular (phosphorus cell quota) rather than external phosphorus controlled APA (Litchman & Nguyen, 2008;Meseck et al, 2009;Cao et al, 2010). As an inducible enzyme, the level of APA can therefore be used as a convenient indicator of P deficiency, although several studies also observed that the relationship between APA and P concentration was random or insignificant in some aquatic systems (Jamet et al, 1997;Cao et al, 2009a).…”

“…Also, several studies indicated that cellular (phosphorus cell quota) rather than external phosphorus controlled APA (Litchman & Nguyen, 2008;Meseck et al, 2009;Cao et al, 2010). As an inducible enzyme, the level of APA can therefore be used as a convenient indicator of P deficiency, although several studies also observed that the relationship between APA and P concentration was random or insignificant in some aquatic systems (Jamet et al, 1997;Cao et al, 2009a). Most studies have shown that phytoplankton and bacteria are the most known to have significant APA (Labry et al, 2005;Nedoma et al, 2006;Cao et al, 2009b), however, some evidence also suggest that AP can also be released by large zooplankton (Jansson et al, 1988).…”