Aim Most protist plankton are mixotrophic, with potential to engage in photoautotrophy and phagotrophy; however, the ecology of these organisms has been misdiagnosed for over a century. A large proportion of these organisms are constitutive mixotrophs (CMs), with an innate ability to photosynthesize. Here, for the first time, an analysis is presented of the biogeography of CMs across the oceans. Location Global marine ecosystems. Time period 1970–2018. Major taxa studied Marine planktonic protists. Methods Records for CM species, primarily from the Ocean Biogeographic Information System (OBIS), were grouped by taxonomy and size to evaluate sampling efforts across Longhurst's oceanic provinces. Biases were evaluated through nonparametric tests and multivariate analysis. Biogeographies of CMs from OBIS data were compared with data from studies that specifically targeted these organisms. Results Constitutive mixotrophs of different taxonomic groups, across all size ranges, are ubiquitous. However, strong database biases were detected with respect to organism size, taxonomic groups and region. A strong bias was seen towards dinophytes. Species < 20 µm, especially non‐dinophytes, were least represented, with their recorded distribution limited to coastal regions and to temperate and polar seas. Studies specifically targeting these organisms revealed their distribution to be much wider. Such biases are likely to have occurred owing to a failure to capture and correctly identify these organisms in routine sampling protocols. Main conclusions Constitutive mixotrophs are dominant members of organisms traditionally termed “phytoplankton”. However, lack of routine protocols for measuring phagotrophy in “phytoplankton” protists has led to widespread misrepresentation of the fundamental nature of marine planktonic primary producers; most express both “animal‐like” and “plant‐like” nutrition. Our results have implications for studies of the global biogeography of plankton, of food web dynamics (including models) and of biogeochemical cycling in the oceans.

Mixotrophy is widespread among protist plankton displaying diverse functional forms within a wide range of sizes. However, little is known about the niches of different mixotrophs and how they affect nutrient cycling and trophodynamics in marine ecosystems. Here we built a plankton food web model incorporating mixotrophic functional diversity. A distinction was made between mixotrophs with innate capacity for photosynthesis (constitutive mixotrophs, CMs) and those which acquire phototrophy from their prey (non-constitutive mixotrophs, NCMs). We present simulations of ecosystems limited by different light and nutrient regimes. Our simulations show that strict autotrophic and heterotrophic competitors increased in relative importance in the transition from nutrient to light limitation, consistent with observed oceanic biomass ratios. Among CMs, cells < 20 µm dominate in nutrient poor conditions while larger cells dominate in light-limited environments. The specificity of the prey from which NCMs acquire their phototrophic potential affects their success, with forms able to exploit diverse prey dominating under nutrient limitation. Overall, mixotrophy decreases regeneration of inorganics and boosts the trophic transfer efficiency of carbon. Our results show that mixotrophic functional diversity has the potential to radically change our understanding of the ecosystem functioning in the lower trophic levels of food webs.

Protist plankton are major members of open-water marine food webs. Traditionally divided between phototrophic phytoplankton and phagotrophic zooplankton, recent research shows many actually combine phototrophy and phagotrophy in the one cell; these protists are the 'mixoplankton'. Under the mixoplankton paradigm, 'phytoplankton' are incapable of phagotrophy (diatoms being exemplars), while 'zooplankton' are incapable of phototrophy. This revision restructures marine food webs, from regional to global levels. Here we present the first comprehensive database of marine mixoplankton, bringing together extant knowledge of the identity, allometry, physiology and trophic interactivity of these organisms. This Mixoplankton Database (MDB) will aid researchers that confront difficulties in characterizing life traits of protist plankton, and it will benefit modellers needing to better This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as

Different hypotheses have been proposed explaining plankton community assembly and how changes in biodiversity can impact ecosystem function. Mixoplankton (photo-phago-trophs) are important members of the plankton, but science lacks a clear understanding of their role in plankton succession. Here, we used a modelling approach to evaluate the seasonalities of mixoplankton functional types (MFTs) and to test the hypothesis that functional differences affect their roles in key carbon fluxes. Functional differences were modelled based on cell size and whether mixoplankton possess their own, or acquire, photosystems. Ecosystem simulations incorporated realistic environmental variability and were validated against a 9 yr long-term time series of nutrients, chlorophyll-a, and plankton data from a coastal temperate sea. Simulations, consistent with empirical data, show that mixoplankton of different sizes are present throughout the water column and over time, with seasonal population dynamics differing among the different MFTs. Importantly, the partitioning of production among different size-classes depends on how mixoplankton functional diversity is described in the model, and that merging mixoplankton into one functional type can mask their diverse ecological roles in carbon cycling. Mixoplankton thus play an important role in structuring the plankton community and its dynamics in the simulations.