Fluorometric measurement of alkaline phosphatase activity in algae

SUMMARY. Using both cultures of algae and natural populations, alkaline phosphatase activity located on the cell surface has been measured by a fluorometric procedure. This was done in order to establish optimum standard conditions for the measurement of this activity as an indicator of phosphorus deficiency and to provide a means of interpreting alkaline phosphatase measurements on natural phytoplankton populations. A concentration of 10 μM o-methylfluorescein phosphate saturates or nearly saturates the reaction in a variety of situations. In most trials, rates increased with temperature to or beyond 35°C. Optimum pH values in the range 7–10 were observed. In six of the algae examined, maximum alkaline phosphatase activities were dependent on external calcium at 100 μM or more. One alga, Synura uvella , showed acid phosphatase activity, peaking at pH 5–6, induced by phosphorus deficiency. Based on comparisons between P-sufficient and P-deficient cultures, alkaline phosphatase activities in excess of 0.1 μmol o-methylfluorescein phosphate hydrolysed per mg dry weight per h or 0.1 μmol per μg ATP per h are suggested as indicative of phosphorus deficiency.