Mixotrophy and the toxicity of Ochromonas in pelagic food webs

1. Toxic compounds produced by many phytoplankton taxa are known to have negative effects on competitors (allelopathy), anti‐predatory effects on grazers (mortality or impaired reproduction) or both. Although mixotrophs of the genus Ochromonas are known to be toxic to zooplankton, it has often been assumed in studies of plankton community processes that all flagellates in the size range of this taxon are edible to typical zooplankton grazers (i.e. cells ≤30 μm for Daphnia, ≤6 μm for rotifers). 2. We explored the toxicity of a species of Ochromonas to other planktonic taxa, including its competitors (two species of phytoplankton and protists) and consumers (two species of zooplankton). To test if mode of nutrition by this mixotroph influences its toxicity to other taxa, we exposed each test species to Ochromonas cultured in chemostats under four different nutritional regimes: osmotrophy (labile dissolved organic carbon) and phagotrophy (bacterial prey) in both light and dark conditions (i.e. with or without photosynthesis). 3. Filtrate from osmotrophically fed Ochromonas had a significant negative effect on the population growth rate of two obligate phototrophic phytoplankton, Cryptomonasozolini and Chlamydomonas reinhardtii. The protists Tetrahymena tetrahymena and Paramecium aurelia were also negatively affected by Ochromonas filtrate. Ochromonas cells were toxic to both the rotifer Brachionus calicyflorus and the cladoceran Daphnia pulicaria, with the toxic effects significantly more severe when fed at high cell densities (75 000 cells mL−1) than at low densities (7500 cells mL−1). Ochromonas cultured osmotrophically in the light was more toxic to the Daphnia than cells cultured under other conditions. In contrast, Ochromonas from all nutritional conditions was equally highly toxic to Brachionus. 4. Our findings support the view that Ochromonas can be toxic to other components of the food web with which it interacts. It is especially toxic to zooplankton that directly consume it, although the effect depends upon Ochromonas cell density and whether or not a good food source is simultaneously present. Our results call into question the common practice of pooling flagellates into a single ‘functional group’ included in an ‘edible phytoplankton’ category of cells <30 μm in diameter.