The mixotrophic protist Ochromonas danica is an indiscriminant predator whose fitness is influenced by prey type

Microbial predator-prey interactions are one of the primary trophic interactions linking biogeochemical cycles to ecosystem dynamics. The mixotrophic flagellate Ochromonas danica was used as a model predator to investigate feeding trends when supplied with actively growing and non-growing bacteria representing a variety of phylogenetic groups. The rate at which bacteria were ingested and the subsequent growth rate (ecological fitness) of O. danica were determined for each type of prey in each growth state. O. danica preferred bacteria between 0.6 and 1.0 mu m(3). It was, in general, an indiscriminate predator in that it readily ingested growing and non-growing bacteria spanning a range of phylogenetic groups, with some notable exceptions. Bacilli/Actinobacteria were ingested at significantly lower rates than Alpha-and Gamma proteobacteria. There were no significant differences among rates at which Proteobacteria (Alpha-, Beta-, and Gamma-) were ingested. The ecological fitness of O. danica could not be predicted from ingestion rates. Predation on Bacilli/Actinobacteria resulted in growth rates significantly lower than when the flagellate preyed upon Gammaproteobacteria. Betaproteobacteria were readily ingested but always resulted in a significant decline in predator fitness. The data reveal that different types of bacteria have different nutritional value to a consumer. If other flagellates respond similarly, then it would seem that trophic transfer efficiency and nutrient regeneration depend on the diet-breadth of predators.