Abstract
Due to human activities the oceans are currently warming and acidifying. Consequences for the photosynthetic microbes that contribute 50% of global carbon fixation will affect entire ocean food webs. This project addresses a critical knowledge gap concerning these marine microbes: the present and future role of mixotrophs in the ocean. Mixotrophs cannot only photosynthesize, but also feed on other microbes. Thus they fix inorganic CO2 to synthesize their own biomass in a ‘plant-like’ manner, but also release CO2 back into the environment through feeding on other organisms in an ‘animal-like’ manner. Because the relative importance of these two processes depends on environmental conditions, their net impact on carbon cycling can take opposite directions. Quantifying carbon transformations by mixotrophs is technically challenging. Recent estimates suggest they contribute up to 30% to carbon fixation, and 60% to the total consumption of other microbes in the nutrient-poor, subtropical North Atlantic, representative of the largest biome on our planet. Shifts in future ecosystem functions of mixotrophs might thus have dramatic consequences for global carbon cycling. This project aims at providing a predictive understanding of how mixotrophs will respond to ocean acidification and warming. Novel stable-isotope probing techniques will be applied on two oceanographic cruises to provide the first direct measurements of resource acquisition rates via both plant-like and animal-like nutrition by mixotrophs. This will allow quantification of the impacts of ocean warming and acidification on evolutionarily diverse, uncultured mixotrophs within their natural communities. Furthermore, this project will apply in-depth physiological experiments using closely related mixotrophs with recently described contrasting physiologies to resolve how their responses depend on physiological strategies and underlying transcriptional regulation. Thus, by combining oceanographic measurements with physiological experiments, this project will provide the mechanistic basis to predict how these enigmatic mixotrophs will modify carbon cycling in our future oceans.
Netherlands