Mixotrophs, organisms that combine photosynthesis and heterotrophy to gain energy, play an important role in global biogeochemical cycles. Metabolic theory predicts that mixotrophs will become more heterotrophic with rising temperatures, potentially creating a positive feedback loop that accelerates carbon dioxide accumulation in the atmosphere. Studies testing this theory have focused on phenotypically plastic (short-term) thermal responses of mixotrophs. However, as small organisms with short generation times and large population sizes, mixotrophs may rapidly evolve in response to climate change. Here we present data from a 3-year experiment quantifying the evolutionary response of two mixotrophic nanoflagellates to temperature. We found evidence for adaptive evolution through increasing growth rates in the obligately mixotrophic strain, but not in the facultative mixotroph. All lineages showed trends of increased carbon use efficiency, flattening of thermal reaction norms, and a return to homeostatic gene expression. Generally, mixotrophs evolved reduced photosynthesis and higher grazing with increased temperatures, suggesting that evolution may act to exacerbate mixotrophs' effects on global carbon cycling.

This thesis would not have been possible without the help and support of many people. First, I would like to thank my advisor, Holly Moeller for the opportunity and for her guidance throughout my degree. I gratefully acknowledge my committee members, Erika Eliason and Debora Iglesias-Rodriguez for sharing their time and expertise in developing this work. I would also like to thank my lab mates in the Moeller Lab, as well as many friends and colleagues in the Ecology, Evolution, and Marine Biology department for their feedback, support, and camaraderie throughout my time at UCSB. I am also grateful to my friends who supported me virtually from afar. Finally, my deepest thanks to my parents, my sister, and my partner, for their unending encouragement and support. This written thesis is the culmination of my graduate work which involved planning and executing the experiment, developing protocols, analyzing and synthesizing the data, and writing the manuscript. The final version of this thesis was coauthored by Holly Moeller,