Effects of macro metals on alkaline phosphatase activity under conditions of sulfide accumulation

Alkaline phosphatase (AP) is commonly found in aquatic ecosystems as an extracellular enzyme closely related to the biogeochemical cycling of phosphorus. Although the AP activity (APA) is conventionally thought to be a main response to PO₄^3- starvation, significant effects of macro metal elements (Al, Fe, and Ca) and S on the APA were found in this study. The APA was reduced by Al primarily through the adsorption of the enzyme onto AlOOH colloids. Fe²⁺ inhibited the APA via a mechanism involving free radical oxidation. The main mechanism by which Ca²⁺ inhibited the APA was by competing with Mg²⁺ and Zn²⁺ for the active sites of the enzyme. Excessive S^2- could reduce the APA by removing Zn²⁺ from the active sites of the enzyme. The inhibition of APA could be reversed if some metal ions (e.g., Fe²⁺) were precipitated by S^2- under reducing conditions. Therefore, in anaerobic ecosystems, the effects of macro metals on APA under conditions of sulfide accumulation may have innovative implications for phosphorus management.