The role of surface and extracellular phosphatases in the phosphorus requirement of Ochromonas1

Acid and alkaline phosphomonoesterase (PME) activity of phosphorus‐depleted Ochromonas danica could be partially but not completely repressed by exogenous inorganic phosphate (Pi). Acid and alkaline PME activities were found in the plasma membrane as well as the cytosol and acid PME was secreted in the cell’s environment. The extracellular and surface PME could provide the necessary phosphate for the growth of phosphorus‐depleted algae in the presence of organic phosphates; specific organic phosphates, glucose‐1‐phosphate (G‐1‐P) and glucose‐6‐phosphate (G‐6‐P), supported growth with small inocula, other organic phosphates were inactive. G‐1‐P and G‐6‐P induced almost double the normal PME secretion, and this extra secretion is shown to be sufficient to provide for the growth of phosphorus‐depleted cells inoculated in relatively small numbers.