The impact of mixotrophs on the microbial food web in lakes

Funding: DACH (FWFDFG)

Leader: Robert Ptacnik


2018-2020: LakeMix – The impact of mixotrophs on the microbial food web in lakes

Original comics by Robert Fischer

Project summary:

Mixotrophic protists, that combine photosynthesis with phagotrophy, are common in marine and freshwater ecosystems. Yet their role in food web dynamics, including trophic interactions, is not well understood. Accumulating evidence from marine environments supports considerable quantitative importance of mixotrophs as bacterivores. Nevertheless, models of the pelagic food web still largely rely on the traditional dichotomy of autotrophic and heterotrophic protists. In lakes, mixotrophic bacterivory has rarely been quantified, and although putatively mixotrophic taxa are numerically abundant there is limited knowledge on how these taxa impact bacterial communities. While it seems obvious that light must play a pivotal role for the potential importance of mixotrophy, experimental evidence on the effect of light on microbial food webs is scarce although light regimes are predicted to change as a result of, for example, altered stratification patterns due to global warming. In LakeMix, we aim to fill these voids by assessing the quantitative and qualitative impact of mixotrophic bacterivory in lakes in the context of climate-related changes.

The project is organized around three major tasks: (1) We will perform laboratory experiments with representative heterotrophic and mixotrophic taxa to address competitive interactions and selective feeding, including their impact on bacterial communities under variable light conditions.  (2) In a regional lake survey, we will identify the relevant mixotrophic bacterivores along a trophic gradient. (3) A European-scale lake survey that covers large geographical and environmental gradients will be carried out to assess the distribution of prevalent mixotrophic taxa.

Our multidisciplinary project combines controlled experimentation in chemostats with comprehensive field observations.  State-of-the-art molecular methods and applications, including high-throughput sequencing technology in combination with stable isotope probing and flow cytometry will be used to identify important mixotrophic taxa, and to characterize their impact on bacterial communities. This project will generate crucial quantitative data that is required for incorporating mixotrophy as a key functional trait in microbial food webs and models of lake ecosystems, and shed light on the ecology of this prevalent yet elusive functional group.