Mixotrophic protists and a new paradigm for marine ecology: where does plankton research go now?

TLDR

Mixotrophic protist plankton, which combine phototrophy and phagotrophy, are increasingly recognized as key players in freshwater and marine ecosystems, yet their taxonomic identification, functional measurement, and the entrenched separation of trophic processes pose significant challenges. The authors propose the term “mixoplankton” for protists capable of both phototrophy and phagotrophy and introduce the “mixoplankton paradigm” to reorganize the plankton trophic web. They define phytoplankton as purely phototrophic, protozooplankton as purely phagotrophic, and outline that revising surveys, experiments, and models will require a comprehensive multidisciplinary approach. The inclusion of mixoplanktonic activities fundamentally alters the conceptual base of the plankton trophic web, prompting the authors to propose the mixoplankton paradigm.

Abstract

Many protist plankton are mixotrophs, combining phototrophy and phagotrophy. Their role in freshwater and marine ecology has emerged as a major developing feature of plankton research over recent decades. To better aid discussions, we suggest these organisms are termed “mixoplankton”, as “planktonic protist organisms that express, or have potential to express, phototrophy and phagotrophy”. The term “phytoplankton” then describes phototrophic organisms incapable of phagotrophy. “Protozooplankton” describes phagotrophic protists that do not engage in acquired phototrophy. The complexity of the changes to the conceptual base of the plankton trophic web caused by inclusion of mixoplanktonic activities are such that we suggest that the restructured description is termed the “mixoplankton paradigm”. Implications and opportunities for revision of survey and fieldwork, of laboratory experiments and of simulation modelling are considered. The main challenges are not only with taxonomic and functional identifications, and with measuring rates of potentially competing processes within single cells, but with decades of inertia built around the traditional paradigm that assumes a separation of trophic processes between different organisms. In keeping with the synergistic nature of cooperative photo- and phagotrophy in mixoplankton, a comprehensive multidisciplinary approach will be required to tackle the task ahead.