Arranging organisms into functional groups aids ecological research by grouping organisms (irrespective of phylogenetic origin) that interact with environmental factors in similar ways. Planktonic protists traditionally have been split between photoautotrophic "phytoplankton" and phagotrophic "microzooplankton". However, there is a growing recognition of the importance of mixotrophy in euphotic aquatic systems, where many protists often combine photoautotrophic and phagotrophic modes of nutrition. Such organisms do not align with the traditional dichotomy of phytoplankton and microzooplankton. To reflect this understanding, we propose a new functional grouping of planktonic protists in an eco-physiological context: (i) phagoheterotrophs lacking phototrophic capacity, (ii) photoautotrophs lacking phagotrophic capacity, (iii) constitutive mixotrophs (CMs) as phagotrophs with an inherent capacity for phototrophy, and (iv) non-constitutive mixotrophs (NCMs) that acquire their phototrophic capacity by ingesting specific (SNCM) or general non-specific (GNCM) prey. For the first time, we incorporate these functional groups within a foodweb structure and show, using model outputs, that there is scope for significant changes in trophic dynamics depending on the protist functional type description. Accordingly, to better reflect the role of mixotrophy, we recommend that as important tools for explanatory and predictive research, aquatic food-web and biogeochemical models need to redefine the protist groups within their frameworks.

Azaspiracids (AZAs) are a group of lipophilic marine biotoxins associated with human incidents of shellfish poisoning. During a research cruise to the North Sea, we analysed size-fractionated plankton for AZA by mass spectrometry and successfully isolated an AZA-producing dinoflagellate from the east coast of Scotland. As shown previously, an axenic culture of this dinoflagellate produces AZA 1, AZA 2 and an isomer of AZA 2. Here we give a taxonomic description of this new taxon Azadinium spinosum gen. et sp. nov., as a de novo producer of AZAs. Azadinium spinosum is a small (12–16 µm length and 7–11 µm width) peridinin-containing photosynthetic dinoflagellate with a superficial resemblance under light microscopy to gymnodinioids, but with a thin theca. The large nucleus is spherical and located posteriorly, whereas the single chloroplast is parietal, lobed, and typically extends into both the epi- and hyposome. The Kofoidian thecal tabulation is APC, 4′, 3a, 6″, 6C, 5?S, 6′″, 2″″. This plate pattern has an epithecal affinity to the Peridiniales and a hypothecal affinity to the Gonyaulacales, but is distinctly different from described dinoflagellate genera. The assignment of A. spinosum to the dinoflagellates is supported by molecular phylogenetic analysis of four genes, SSU rDNA, LSU rDNA (D1/D2 region), ITS and cytochrome oxidase (sub-unit 1) (COI). In agreement with the morphological description, phylogenetic analysis did not show any particularly close affiliation to the Peridiniales or Gonyaulacales, nor to any other dinoflagellate order represented in molecular databases. Consequently, we erected a new genus, Azadinium, for this taxon. However the ordinal affiliation of the genus is uncertain. This study represents the first description and confirmation of a new dinoflagellate species capable of producing AZA and is thus an important advance in surveillance programmes for toxigenic microalgae and toxins of human health significance.

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 modelers needing to better appreciate ecology of these organisms with their complex functional and allometric predator–prey interactions. The MDB also identifies knowledge gaps, including the need to better understand, for different mixoplankton functional types, sources of nutrition (use of nitrate, prey types, and nutritional states), and to obtain vital rates (e.g. growth, photosynthesis, ingestion, factors affecting photo’ vs. phago’ ‐trophy). It is now possible to revisit and re‐classify protistan “phytoplankton” and “zooplankton” in extant databases of plankton life forms so as to clarify their roles in marine ecosystems.