BoCP-Implementation: Functional diversity of the smallest predators in the current and future ocean

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Photosynthesis by primary producers is the foundation of Earth’s ecosystems. On land the major producers are plants, but in the ocean most photosynthesis is performed by single-celled microorganisms. Remarkably, many of these tiny ‘plants’ are also predators that eat other microbes. These organisms are the Venus flytraps of the sea and are called ‘mixotrophs’ because of their mixed nutrition. Previous research compared mixotrophs to autotrophs (organisms that only act as plants) and found that mixotrophs are more abundant when ocean waters have a distinct warm layer at the surface (called stratification). Such an increase in mixotroph abundance could have significant implications for ocean ecosystems because the oceans are becoming more stratified globally as the planet warms. As mixotrophs will likely play a larger role in the future ocean, it is critical to study whether mixotrophy changes how ecosystems work. In this project the researchers will address how mixotrophy changes the flow of energy and nutrients through food webs, and what the consequences may be for the future ocean. These questions are important for society’s reliance on the biosphere, because a more mixotrophic future could mitigate or exacerbate the effects of climate change on ocean productivity. This project will support the training of a postdoc, two graduate students, and undergraduate students in integrative biodiversity science that includes field, lab, and modeling components. Ideas about mixotrophs and their role in the ocean will be developed into an educational game for science outreach events in Hawai’i, and the PIs will recruit undergraduates through programs at UH Manoa that serve Pacific Islanders and other under-represented groups. The specific goal of this project is to quantify functional diversity among common mixotrophic consumers and compare them to common heterotrophs that do not photosynthesize. The researchers will use in situ and lab experiments to ask whether diverse mixotrophs and heterotrophs differ in their consumption of the most common bacterial prey: the cyanobacterium Prochlorococcus, the heterotrophic bacterial clade SAR11 (Pelagibacter), and the heterotrophic bacterial family Rhodobacteraceae. They will use lab experiments on diverse isolates to ask whether the mixotroph-heterotroph spectrum leads to consistent shifts in predation rates, the transfer of nutrients up the food chain, and the elemental composition of the plankton. They will analyze global survey data to ask how mixotrophic and heterotrophic taxa shift in relative abundance with increasing stratification and other environmental gradients. Finally, they will use mathematical models to assess whether patterns of community structure are consistent with theoretical predictions based on experimental results. In total this work will quantify functional biodiversity among a globally important but poorly studied group of microbes, project how communities and ecosystems will change due to global warming, and advance the theoretical understanding of microbial food webs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.