Successful strategies in size structured mixotrophic food webs

This study investigates how food web structures in aquatic microbial communities emerge based on different mixotrophic life strategies. Unicellular mixotrophic organisms that combine osmotrophy and primary production with phagotrophy account for significant amounts of primary production and bacterivory in marine environments, yet mixotrophs are still usually absent in large-scale biogeochemical models. We here present for the first time a thorough analysis of a food web model with a finely resolved structure in both cell size and foraging mode, where foraging mode is a strategy ranging from pure osmotrophy to pure phagotrophy. A trade-off for maximum uptake rates of mixotrophs is incorporated. We study how different factors determine the food web structure, here represented by the topology of the distribution of given amounts of total phosphorous over the cell size-foraging mode plane. We find that mixotrophs successfully coexist with foraging specialists (pure osmo- and phagotrophs) for a wide range of conditions, a result consistent with the observed prevalence of mixotrophs in recent oceanographic surveys. Mixotrophy trade-off and size-dependent parameters have a strong effect on the emerging community structure, stressing the importance of foraging mode and size considerations when working with microbial diversity and food web dynamics. The proposed model may be used to develop timely representations of mixotrophic strategies in larger biogeochemical ocean models.