Darwin performed innovative observational and experimental work on the apparently paradoxical occurrence of carnivory in photosynthetic flowering plants. The nutritional use of particulate organic material which also supplies other elements is now known to be widespread in free-living algae as well as in organisms with endosymbiotic algae and with kleptoplastids. In addition to this direct nutritional role, phagotrophy, in the broad sense of internalisation of photosynthetic organisms by a eukaryote, is essential for the occurrence of present-day endosymbiotic algae and kleptoplastid containing protists, and was essential for the origin of plastids themselves. The endosymbiotic phenomena involving photosynthetic organisms clearly played a major role in combining genomes providing different metabolic functions, but, in our opinion, does not demand a re-appraisal of evolution by natural selection. That the balance of physiological optimisation for competition for resources and minimization of losses (e.g., through predation) is a fine one, and thus subject to a complex selective process, is illustrated by the diversity of mixotrophic strategies in extant phytoplankton
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