Building the “perfect beast”: modelling mixotrophic plankton

A mechanistic model is described for carbon–nitrogen–phosphorous-based interactions within a protistan mixotroph. The model describes interactions between photosynthesis (with photoacclimation), inorganic nutrient acquisition and the consumption of prey, making use of a flexible structure to allow an exploration of alternative modes of interaction. Operation can be varied with respect to differential growth rates under pure phototrophy, phago-heterotrophy or mixotrophy, substitutional or additive interactions between modes of C acquisition, the suppression of digestion by C flow from photosynthesis (including, if applicable, that from kleptochloroplasts), competition for volume within the cell between chloroplasts and food vacuole, the need for some level of obligatory photosynthetic activity, activation of mixotrophy in response to general growth limitation, or to specific nutrient limitations. Simulations under dynamic conditions include considerations of predation on bacteria, and on microalgae. These show the potential for mixotrophs, but also indicate the importance of using an appropriate description of their physiology, with different mixotrophy configurations having significant effects on system dynamics. The potential value of kleptochloroplasts for support of mixotroph growth is highest when the food vacuole is large, when the ingested phototroph prey is of good nutritional status and when digestion of prey is repressed by photosynthesis. Analyses of the behaviour of the new model demonstrate that simulations which do not consider the stoichiometric implications of mixotrophy cannot reflect the reality of the trophic interaction both for the mixotroph and for the associated ecosystem. We dedicate this work to the memory of Mike Fasham FRS, without whose enthusiasm and guidance none of this would have come to pass.