Does Mixotrophy in ciliates compensate for poor-quality prey? Experiments with heterotrophic–mixotrophic species pairs

Abstract Fatty acid and sterol profiles play a large role in determining the food quality of prey in aquatic food webs. Ciliates have limited ability to synthesize essential polyunsaturated fatty acids (PUFA) and sterols, and must obtain these from their food. Many mixotrophic ciliates, however, are in a symbiosis with Chlorella, and this may compensate for prey that is low in PUFA and sterols. Moreover, when PUFA-rich prey are P-limited, mixotrophs may be less dependent than heterotrophs on having the optimal mixture of PUFA-rich and P-rich prey. To test these hypotheses, experiments were conducted using heterotrophic-mixotrophic species pairs and prey mixes with varying proportions of good and poor quality prey. Prey proportions had clear effects on ciliate growth rates, often with a clear threshold. In only one species, however, did mixotrophy appeared to be advantageous, with growth independent of the proportion of PUFA-rich prey. In the other two species pairs, mixotrophy gave at best a small advantage over heterotrophy when there was a large proportion of PUFA-poor prey, independent of whether the PUFA-rich prey was P-limited or P-replete. While PUFA- and sterol-rich prey are important for heterotrophic ciliates, mixotrophy cannot be universally employed as an alternate source of these required nutrients.