Mixotrophy: Now and then

In last years and decades it has become clear that mixotrophic microalgae algae that engage in photosynthesis, but also prey on bacteria or other algae, respectively can have a high ecological importance in lakes and oceans. This challenges a central paradigm in plankton ecology, since the traditional dichotomy between phytoplankton (plant-like plankton; producers) and zooplankton (animal-like plankton; consumers) appears to be outdated. However, the apparent prevalence of mixotrophic protists in the field contrasts our current understanding about their impact on microbial food web dynamics and carbon and nutrient cycles. Moreover, changes to aquatic environments due to global climate change, such as rising water temperatures and increasing input of terrigenous carbon, are likely to promote the growth of mixotrophic plankton. Hence, it appears timely to investigate mixotrophy not only under current conditions, but also under future conditions as projected by climate models, in order to reliably assess future consequences for aquatic ecosystems. The proposed inter-disciplinary research project aims at getting a mechanistic understanding regarding the functional role of mixotrophs in microbial food webs. Understanding the metabolism of mixotrophs is key to understand flows of energy and carbon at the base of aquatic food webs. Specifically the following research hypotheses will be addressed: H1: Dietary sources of carbon (inorganic vs organic) in mixotrophs differ depending on their respective mode of nutrition, some require light for growth, others not. H2: Mixotrophs lock bacterial carbon in the microbial food web, since they do not use it for growth. H3: Inputs of terrigenous carbon may be beneficial only for mixotrophs which dont need light for growth, depending on the relative effects on bacterial activity versus light availability. H4: Climate change promotes conditions favorable for mixotrophs and pushes them more towards being consumers. In lab experiments, key traits of different mixotrophs will be identified and the effect of mixotrophs on microbial food webs will be investigated. In the experiments temperature and the input of carbon will be manipulated. Derived data will be used to validate and enhance models on microbial food web and associated carbon and nutrient cycles. The project has clear interdisciplinary aspects, as it elegantly combines plankton ecology, biochemistry, trait-based modelling and global change research.

In recent years and decades, it has been recognized that mixotrophic microalgae - those algae capable of both photosynthesis and consuming bacteria (or other algae) - can have significant ecological importance in lakes and oceans. The overarching scientific purpose of this project was to investigate how various mixotrophs utilize organic carbon (from their prey, bacteria) compared to inorganic carbon (via photosynthesis), depending on light availability. Particularly for those mixotrophic species believed to primarily feed on bacteria to obtain nutrients, little is known about what happens to the carbon ingested with their prey, such as whether they use it for their growth. We were able to demonstrate that the respective sources of carbon used for growth by mixotrophs - organic (prey) versus inorganic (photosynthesis) sources - depend on the specific types of feeding strategies. Broadly, two types can be distinguished: light-independent and light-dependent feeding strategies. Species of the light-independent type primarily utilize their prey as a carbon source, whereas in species of the light-dependent type, the carbon sources used depend on the amount of available light. In most investigated species of this type, the proportion of carbon utilized from the prey increases as light availability decreases, although not in all cases. Overall, the insights gained from this project contribute to our understanding of mixotroph metabolism, which in turn helps in understanding carbon and energy fluxes at the base of microbial food webs.