Characterization of alkaline phosphatase and organic phosphorous utilization in the oceanic dinoflagellate Pyrocystis noctiluca

Phosphate depleted Pyrocystis noctiluca (Murray) Schuett 1895 has at least one phosphomonoesterase (EC 3:1:3:1) which is triphasic between 0.1 and 222 mol P. The enzyme has a broad temperature range with maximum activity at 50 °C and a Q₁₀ of 1.4 to 1.5. A break in the Arrhenius plot at 35 °C implies the enzyme is membrane-bound. Cytological staining of whole cells and cell fractionation studies (showing 26 times higher specific activity in the particulate compared with the cytoplasmic fraction) suggest the enzyme is plasmalemma-bound. The enzyme has an absolute metal requirement which would be satisfied by Mg⁺⁺ but not Mn⁺⁺, Zn⁺⁺, Fe⁺⁺, or Co⁺⁺ at seawater concentrations. Alkaline phosphatase is a stable enzyme whose activity is not altered by inhibitors of protein synthesis. Orthophosphate inhibition of enzyme activity was largely eliminated in the presence of these inhibitors. Apparently, a protein induced by PO₄ ^3-, rather than PO₄ ^3- itself, inhibits alkaline phosphatase. Cell-free alkaline phosphatase can hydrolyze a variety of phosphate esters and linear polymers of inorganic phosphorus as well as disolved organic phosphorus from tropical oceanic waters. These same hydrolysable organic and inorganic phosphorus compounds support the axenic culture growth of P. noctiluca, suggesting that naturally occurring hydrolysable organic phosphorus compounds may also support the growth of this alga.