Effects of prey abundance and light intensity on nutrition of a mixotrophic flagellate and its competitive relationship with an obligate heterotroph

Abstract

The mixotroph Poterioochromonas malhamensis was grown in batch and semi-continuous cultures in order to examine the dependence of phagotrophy versus phototrophy at bacterial densities similar to those found in oligo- and mesotrophic lakes in combination with differing light regimes. In addition, the growth rates and biomass accumulation of the mixotroph were compared to those of the heterotroph Spumella elongata grown under similar conditions. In P. malhamensis, primary production rates and cell-specific chlorophyll a concentrations were significantly higher when grown at low bacterial densities, and the carbon cell-1 gained by photosynthesis corresponded to 79% of the carbon cell-1 gained by bacterial ingestion. Furthermore, due to a longer exponential growth phase, cell densities and biomass of P. malhamensis were significantly higher when the mixotroph was cultured in high light than in darkness. As a result, the cumulative biomass of the mixotroph was significantly higher in the light treatments than the biomass of the heterotroph at similar bacterial densities. These results suggest that photosynthesis may contribute more to the nutrition of P. malhamensis in lakes with low bacterial abundances than previously suggested, and that the mixotroph's capacity for photosynthesis enables it to dominate in terms of biomass over the heterotroph.