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.

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We examined the combined effects of grazer infochemicals and nutrient status on colony development of Phaeocystis globosa cultures grown under nitrogen and phosphorus (NP)-sufficient, P-deficient, and N-deficient conditions exposed to high and low Acartia spp. density filtrates. Changes in colony development relative to controls receiving no grazer signals were estimated. P. globosa colony development responded to grazer infochemicals regardless of nutrient status, although the expression of the response varied between nutrients. Significant colony suppression (in terms of percent of cells allocated to colonies) occurred in both NP-sufficient and P-deficient experiments, with the response being dependent on the density of grazers for NP-sufficient cells. The percent of cells in colonial form in N-deficient P. globosa decreased in response to low grazer density filtrates but increased in response to high grazer density filtrates. These opposite results for the N-deficient experiment are related to a high mortality of Acartia in the high grazer density filtrate treatment, which may affect the infochemicals released from such grazers.

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