Nutrient turnover by freshwater bacterivorous flagellates: Differences between a heterotrophic and a mixotrophic chrysophyte

The patterns of nutrient turnover by the freshwater bacterivorous chrysophycean flagellates Ochromonas sp. and Spumella sp. were compared in laboratory experiments. The exclusive phagotroph Spumella sp, released soluble reactive phosphorus (SRP) and ammonium when feeding on bacteria and stimulated the growth of P-limited algae, obviously by making phosphorus available to them. The mixotroph Ochromonas sp, released SRP and ammonium in the dark or at high bacterial densities in the light when phagotrophic nutrition prevailed, but it took up mineral nutrients when it grew primarily phototrophically. In P-limited situations, it retained bacterial P for its own photosynthetic growth and did not stimulate other phytoplankton. The results suggest basic differences in the patterns of nutrient turnover by mixotrophs and by exclusive phagotrophs.