• algal-epiphyte assemblage;
  • alkaline phosphatase;
  • Cladophora;
  • dissolved organic phosphorus;
  • limitation;
  • phosphorus

Extracellular alkaline phosphatase enzyme activity (APA) is important for algal phosphorus (P) acquisition in P-limited freshwater ecosystems and is often used as an indicator of P deficiency. APA allows access to organic P (monophosphate esters), but the regulation of APA in response to availability of both PO43− and organic P is poorly characterized. This study aimed to examine the regulation of APA in freshwater Cladophora-epiphyte assemblages in response to PO43− and a hydrolyzable organic P source, and for the first time to apply enzyme linked fluorescence (ELF) to localize APA within freshwater macroalgal-epiphyte assemblages. In response to elevated PO43− concentrations, a component of net APA was suppressed, but there was also a constitutive APA, which was maintained even after prolonged exposure to nearly 1,000 μM PO43− and saturation of internal P pools. When supplied with organic glycerol P as the sole P source, the algae maintained APA in excess of needs for supplying PO43− for uptake, resulting in PO43− release into the medium. Constitutive APA may be adaptive to growth under chronic P limitation in oligotrophic freshwater habitats. Excess APA and release of PO43− could benefit different algal and bacterial partners within assemblages. APA in both Cladophora sp. and epiphytic algae was localized with ELF only when ethanol fixation was omitted. In algal subsamples exposed to different P treatments, there was no correlation between bulk APA (using 4-methylumbelliferyl phosphate [MUP] substrate) and % cell labeling with ELF, suggesting that ELF labeling of APA was at best semiquantitative in the algal assemblages.