Misuse of the phytoplanktonzooplankton dichotomy: the need to assign organisms as mixotrophs within plankton functional types

The classic portrayal of plankton is dominated by phytoplanktonic primary producers and zooplanktonic secondary producers. In reality, many if not most plankton traditionally labelled as phytoplankton or microzooplankton should be identified as mixotrophs, contributing to both primary and secondary production. Mixotrophic protists (i.e. single-celled eukaryotes that perform photosynthesis and graze on particles) do not represent a minor component of the plankton, as some form of inferior representatives of the past evolution of protists; they represent a major component of the extant protist plankton, and one which could become more dominant with climate change. The implications for this mistaken identification, of the incorrect labelling of mixotrophs as phytoplankton or microzooplankton, are great. It extends from the (mis)use of photopigments as indicators of primary production performed by strict photoautotrophs rather than also (co)locating mixotrophic activity, through to the inadequacy of plankton functional type descriptions in models (noting that mixotrophic production in the individual organism is not a simple sum of phototrophy and heterotrophy). We propose that mixotrophy should be recognized as a major contributor to plankton dynamics, with due effort expended in field and laboratory studies, and should no longer be side-lined in conceptual food webs or in mathematical models.