Phosphate release due to excess alkaline phosphatase activity in Trichodesmium erythraeum

Abstract

We investigated the dependence of alkaline phosphatase activity (APA) on temperature and light intensity in cultured strains of the diazotroph Trichodesmium erythraeum isolated from the Kuroshio Current in the East China Sea. In contrast to the low levels of APA observed under phosphorus (P)-replete conditions, cultures grown under Plimited conditions exhibited strong APA, and the levels steadily increased with increases in temperature and irradiance from 22–30°C and 40–260 μmol m−2 s−1, respectively. Based on the chlorophyll a and APA levels in actively growing T. erythraeum, the mean hydrolyzing time required to double the cellular P quota was estimated to be 0.68 and 9.7 times shorter than the reported values under P-replete and P-limited conditions, respectively. This strongly suggests that APA under P-limited conditions is excessively active compared with the cellular P requirement. Furthermore, under P limitation intense bacterial aggregations were observed with fluorescence microscopy around the T. erythraeum trichomes in the same location as the visualized APA, suggesting enhancement of bacterial growth by surplus phosphate due to the excess APA, as well as possible release of dissolved organic matter from T. erythraeum. The present study suggests an important role for T. erythraeum as a phosphate supplier to other organisms in the oligotrophic ocean.