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Abstract

Mixotrophic flagellates can compete with obligately heterotrophic flagellates for the uptake of food particles, whereas mixotrophs and obligately phototrophic phytoplankton can compete for soluble nutrients. Competition for soluble nutrients is expected when photosynthesis covers a significant portion of the mixotrophs' carbon metabolism. When heterotrophy predominates, however, mixotrophs may release soluble nutrients and facilitate phototrophs. Mechanistic resource competition theory predicts that, due to their ability to utilize substitutable C and P sources, mixotrophs should be able to coexist with their more specialized competitors under certain conditions of resource supply. In laboratory experiments, the mixotrophic flagellate Ochromonas excluded heterotrophic flagellates when only phototrophic growth was possible. However, Ochromonas was excluded by heterotrophic flagellates when only phagotrophic growth was possible. Both coexisted when food bacteria and light were supplied simultaneously Ochromonas coexisted with a P-limited phytoplankter, Cryptomonas sp, when soluble reactive phosphorus (SRP) and bacterial phosphorus were available as alternative P sources. In nature, the mixotrophic strategy may be successful when resources are limiting. This is supported by published data on the occurrence of mixotrophic chrysophytes