Phosphate uptake from phosphomonoesters by phytoplankton in the Chesapeake Bay1

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Abstract
Phosphomonoester concentrations were 0 to 0.09 µg‐atom liter−1 in Chesapeake Bay from December 1972 to December 1973. Alkaline phosphatase activity associated with natural phytoplankton assemblages indicated the cells’ potential to utilize the monoesters as a phosphorus source. However, ecological interpretation of alkaline phosphatase activity data is complicated by the necessity to increase the monoester concentration in order to measure enzyme activity fluorometrically. The half‐saturation constant (K3) was 0.31 for 3‐0‐methyl fluorescein hydrolysis by a natural phytoplankton assemblage and 0.75 µM for glucose‐6‐PO4 by a nanoplankter in culture, and maximum velocities (Vm) were 3.2 and 6.4 nm (µg Chl a h)−1. In one experiment with a natural phytoplankton assemblage, organisms in the 0.8–5‐µm, size range comprised 78% of the plant biomass and were responsible for 70% of the phosphorus uptake from glucose‐6‐PO4 when size fractionation preceded experimental incubations. Phosphomonoesters may contribute to phytoplankton phosphorus nutrition during much of the year, but are in greatest demand in spring in Chesapeake Bay.