Award: OCE-1851194

Biology textbooks typically classify organisms into two categories: plant-like (photosynthetic species that use the energy of sunlight to feed themselves) and animal-like (heterotrophic species that make a living by consuming other organisms or their byproducts). Yet aquatic ecosystems are full of metabolic hybrids called "mixotrophs" that combine these two fundamentally different forms of metabolism. In so doing, these "jacks of all trades" can sometimes make a living where metabolic specialists cannot, such as in the low-resource "deserts" of the ocean, the oligotrophic gyres. As the oceans become warmer and more stratified, mixotrophs are expected to become more common, because their hybrid metabolism lets them simultaneously mine nutrients by feeding on other organisms, and harness the power of sunlight as a source of energy. This project asked how marine, planktonic mixotrophs will respond to rising ocean temperatures. Both photosynthesis and heterotrophy accelerate with temperature, but heterotrophy accelerates faster. As a result, theory predicts that mixotrophs should become more heterotrophic over time. But mixotrophs are also small, fast-growing, and have large population sizes, so they may rapidly evolve to adapt to new environments. We examined the effects of evolution on mixotroph metabolism to understand the implications for the marine carbon cycle using a combination of laboratory experiments and mathematical models. Both eco-evolutionary models and multi-year laboratory evolution experiments confirm that evolution will drive mixotrophs towards more heterotrophy at hotter temperatures. This can affect the marine carbon cycle by reducing the amount of carbon dioxide that mixotrophs draw out of Earth’s atmosphere through photosynthesis. But these evolutionary responses are not universal: Some mixotroph species don’t show strong evolutionary responses and rely on short-term metabolic flexibility instead. And all mixotrophs are ultimately limited by the availability of resources like food. We used ecosystem models to extend our findings to mixotrophs’ impacts on ecosystem structure and function, and showed that mixotroph ecology is further constrained by competition with phytoplankton and heterotrophs. In addition to advancing scientific knowledge (with results detailed in seven peer-reviewed publications), this project contributed to the training of numerous early career scientists. Four undergraduate researchers led first-author publications, and three additional undergraduates and one high school student also participated in the research. One graduate student and one postdoctoral fellow led evolution experiments and global ecosystem modeling, respectively. We also partnered with the Santa Barbara Museum of Natural History to provide outreach to the general public as part of Underwater Parks Day. Lab members introduced aquarium visitors to the "invisible biodiversity" that supports the more charismatic species found in our marine protected areas, played "Plankto" (an evolutionary simulation centered around a modified Plinko board), and demonstrated the use of lab flow cytometry. Last Modified: 08/25/2023 Submitted by: Holly V Moeller