STIMULATION OF PHOSPHORUS-LIMITED PHYTOPLANKTON BY BACTERIVOROUS FLAGELLATES IN LABORATORY EXPERIMENTS

The influence of the bacterivorous flagellate Spumella sp. on the interactions between P-limited phytoplankton and bacteria was examined in the laboratory. In batch experiments, bacteria from a lake inoculum were allowed to grow and to use up the phosphate in P-limited media with glucose as a source of C. Then algae with or without Spumella were added. In all experiments, algae were only able to grow when the protozoans reduced bacterial numbers and made bacterial P available to them. This P transfer occurred without detectably raised soluble reactive P concentrations. In two chemostat experiments without an additional source of C in the medium, nutrient-limited algae and their bacterial contaminants attained stable densities. The addition of Spumella resulted in reduced bacterial numbers and an increase of P-limited algae. As expected, a Si-limited diatom did not respond to the grazing activity of the protozoans. The presence of Spumella led to biomass increases that surpassed their own direct contribution and that were largely due to the indirect effect of making P available to the phytoplankton.