Mixotrophic organisms use a combination of photosynthesis and heterotrophy to meet their energetic requirements. Broadly, aquatic protistan mixotrophs exhibit one of two strategies for phototrophy: Some taxa manufacture "native" plastids (that is, have permanently obtained photosynthesis in their evolutionary past), while others acquire phototrophy within their lifetimes through kleptoplastidy (the theft and retention of photosynthetically active chloroplasts from algal prey). While mixotrophic life history confers flexible metabolism that may allow organisms to adapt to fluctuating environmental conditions, mixotrophy also incurs costs such as the metabolic expense of maintaining photosystems and the risk of failing to capture algal prey. Under what circumstances, then, are these "jacks of all trades" competitively dominant to strict photo- or heterotrophs? Here, we present a mathematical model describing the interactions and growth of mixotrophs, their prey, and their competitors in a spatially explicit context. Specifically, we construct a one-dimensional model of a water column with variable light intensity, nutrient supply, and vertical mixing. We use this model to test how environmental conditions constrain the competitive fitness of mixotrophic strategies.
Results/Conclusions
Our results highlight the importance of light availability, and light acclimation, to the relative fitness of aquatic protistan mixotrophs. Interestingly, our model suggests that mixotrophs that acquire phototrophy (acquired phototrophs) are most abundant in low-light environments, a prediction that is consistent with field observations of their distribution. This finding is a reflection of interacting factors: First, because digestion rates increase with increasing light, acquired phototrophs in low light environments retain the photosynthetic benefits of their acquired plastids for longer periods of time. Second, by acquiring plastids locally from low-light acclimated algae, acquired phototrophs obtain photosynthetic machinery that has been "pre-adapted" to their light environment. In contrast, mixotrophs with native photosystems are more abundant in higher light environments, though, in keeping with published models, their population sizes are constrained by competition with strict phototrophs for nutrients and strict heterotrophs for prey. The model also makes predictions about the degree to which mixotrophs should rely on either photo- or heterotrophy under different environmental conditions, with implications for the evolution of metabolic strategies, including selection for permanent plastid acquisition through endosymbiosis.