The mode of nutrition of mixotrophic flagellates determines the food quality for their consumers

1. Mixotrophic organisms can combine two functionally different modes of nutrition to acquire carbon: (i) by using photosynthesis for inorganic carbon fixation; and (ii) by taking up organic sources. Because of these different metabolic pathways, their biochemical composition, for example, the amount and composition of consumer-relevant compounds such as polyunsaturated fatty acids, varies, and thus, we tested the hypothesis that the mode of nutrition of an osmo-mixotrophic flagellate determines the food quality for their consumers: three rotifer species. 2. Using life table experiments with auto-, mixo- and heterotrophically grown Chlamydomonas acidophila as food sources, we found species-specific differences in life span, fecundity, population growth rate, egg size and maternal effects on the starvation resistance of neonates between the three rotifer species. 3. Population growth of two out of the three rotifers (Brachionus sericus and Elosa worallii) was negatively affected when fed with heterotrophic food, compared to animals fed with autotrophic food. In contrast, Cephalodella sp. was able to use all C. acidophila with similar efficiency. 4. These results explain the vertical distribution of two of the rotifers in their natural habitat, where Cephalodella sp. dominates over E. worallii in deep water layers characterized by light intensities below the compensation point for autotrophic growth of C. acidophila. It is hypothesized that temporal changes in the mode of nutrition of mixotrophs have consequences for the outcome of the competition between consumers and for the carbon transfer in many planktonic food webs.