Role of light for consumer-dynamics and nutrient turnover in microbial food webs of the marine pelagial

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The bulk of marine bacterial and primary production is consumed by unicellular grazers (‘protists’). Recent research revealed that pigmented grazers (‘mixotrophs’) represent an important consumer group of bacteria and phytoplankton. Especially in oligotrophic waters, mixotrophs play a pivotal role as consumers of the pico-plankton. These mixotrophs combine autotroph photosynthesis with ingestion of particulate food items. This dual mode of nutrition provides clear advantages under limitation of either energy or essential nutrients. However, the combination of multiple strategies exerts additional costs for building dual cell machineries compared to specialized competitors.Given the tremendous importance of mixotrophs for marine ecosystems, it is mandatory to develop a mechanistic model predicting the relative importance of mixotrophs in the environment, as well as their implications for food webs. Based on preliminary results and existing data, it is postulated that light intensity and grazing pressure represent major factors controlling the relative importance of mixotrophy in microbial food webs. We will study the competition between mixotrophic and heterotrophic grazers systematically in artificial food webs, applying gradients of light and loss rate. The collected data will eventually be the basis for a stoichiometric model of the microbial food web, taking the specific role of mixotrophy into account.