The impact of mixotrophs on the microbial food web in lakes

Mixotrophic protists combine photosynthesis with phagotrophy. Though common to marine and freshwater ecosystems yet their role in trophic interactions is not well understood. Accumulating evidence from marine environments supports a considerable quantitative importance of mixotrophs as bacterivores. Despite this, mixotrophs are not considered in most contemporary models of the pelagic food web. Not considering the significance of mixotrophs likely yields biased estimates about the carbon metabolism of lakes and oceans, as their metabolism differs considerably from that of heterotrophic consumers. The project will generate crucial quantitative data that is required for incorporating mixotrophy as a key aspect of the microbial food web in lake ecosystems, and will shed light on the ecology of this prevalent yet elusive functional group.

In LakeMix, we studied the implications of mixotrophic flagellates for their prey, bacterial communities. Mixotrophs are key consumers of bacteria in lthe pelagial of akes and oceans. By consuming bacteria, they shape their prey communities. Though we are aware about the general importance of mixotrophis as bacterivores, little is known if they have preferences for certain groups, and how they may impact bacterial communities. In LakeMix, we poerformed a number of experiments to solve these questions. We performed controled experiments in the lab with cultures isolated from natural lakes, and we performed field experiments, where we studied the ipact of mixotrophis on their prey in situ under different environmental conditions. We found that mixotrophs have clear preferences for certain bacterial groups, and that more heterotrophic mixotrophs tend to be more selective than phototrophic mixotrophs, which seem to act more as generalists. The results from this project enhance our functional understanding of pelagic communities, since the functional diversity of mixotrophs so far was poorly understood.