Growth, grazing, and inorganic C and N uptake in a mixotrophic and a heterotrophic ciliate

We compared growth, grazing and inorganic carbon and nitrogen uptake in two ciliates, the mixotrophic Strombidium rassoulzadegani and the heterotrophic Strombidinopsis sp. The mixotroph had over 2-fold higher gross growth efficiencies (fraction of ingestion devoted to growth; GGE) for C at low food concentrations, while the heterotroph's GGE was 2-fold higher under food saturation. Inorganic carbon uptake did not vary significantly with food concentration in the mixotroph, but its importance for growth was highest at low food concentrations. Although there was measurable inorganic carbon uptake in the heterotroph due to still-active algae in food vacuoles, it did not contribute significantly to growth. The two ciliates took up inorganic nitrogen (both nitrate and ammonium) at similar biomass-specific rates, but inorganic nitrogen did not contribute significantly to their N requirements. Mixotrophy with retained chloroplasts provides a significant energy subsidy, especially at low food levels, but maximum growth rates were similar for the mixotroph and the heterotroph we compared, suggesting that the advantage of chloroplast retention diminishes when food concentrations are high.