Effects of phosphorus supply on phagotrophy by the mixotrophic alga Uroglena americana (Chrysophyceae)

The mixotrophic alga Uroglena americana forms a dense 'bloom' in Lake Biwa, Japan, in spring when dissolved inorganic phosphorus is in short supply relative to the requirements for algal growth. To assess the possibility that this alga uses bacteria as a substitutable P source via phagotrophy, feeding experiments were conducted using bacteria-sized fluorescent microspheres as tracer particles. The experiments revealed that the ingestion rate of U. americana on the food particles (bacteria + microspheres) is significantly affected by the concentrations of phosphate as well as food particles in the lake water. The response of ingestion rate to phosphate concentration indicated that P levels ≥0.4 μM P were enough to reduce bacterivory by this alga. However, phosphate concentration had no effect on ingestion rate in heterotrophic nanoflagellates. These results suggest that U. americana can utilize bacteria as a substitutable P source. Such an ability favors the development and maintenance of dense blooms by this alga under P-limited conditions.