Exploring the physiological and ecological basis of mixotrophy in marine food webs

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Marine phytoplankton are responsible for about half of global primary production despite being seasonally or chronically nutrient limited. To cope with this, many phytoplankton supplement their nutritional needs through mixotrophy, which involves feeding on bacteria or other algae. These microscopic Venus Fly Traps of the ocean are major players in marine microbial food webs, yet we know so little about when they feed and how their eating is balanced with photosynthesis. This research will shed light on how environmental and cellular factors control mixotrophy, and how mixotrophy and photosynthesis are integrated in the overall metabolism. While understanding the ecological role of mixotrophy in ocean food webs is center to this work, results from this study will also shed light on the evolution of mixotrophy by identifying potential tradeoffs between feeding and photosynthesis. This work will involve collaboration with The Zephyr Education Foundation, a marine science literacy and education program, to design an educational unit to emphasize the importance of mixotrophy in marine microbial food webs, and will utilize boat trips and classroom activities in order to communicate these concepts. Undergraduate students from underrepresented groups will be trained at the Woods Hole Oceanographic Institution (minority fellowship program), as well as local high school summer students, to perform lab research on mixotrophs. This research will also provide training and career development for a postdoctoral scientist. Mixotrophy refers to species that combine some level of phagotrophy and phototrophy, and represents a diverse array of ecological interactions and cellular and metabolic adaptations. While often perceived as an exception to the norm, mixotrophy is commonplace in marine food webs, affording phytoplankton greater ecological fitness during periods of low or limiting nutrients while stabilizing food webs. Many mixotrophs have a low chlorophyll: carbon ratio, which tends to make them poor phototrophic competitors. In turn, feeding allows these species to achieve maximum growth while in some cases also eliminating their competitors. Other mixotrophs are strong phototrophic competitors, and only feed when severely nutrient limited. This research will determine the cellular and environmental factors that lead to feeding by marine phytoplankton, and how the contrasting metabolisms of heterotrophy and photosynthesis are integrated within a cell. This research will involve laboratory-based experiments on model dinoflagellate and chrysophyte cultures. Using microscopy, physiology, proteomics and metabolomics approaches, this work will test hypotheses about the ultimate causes and consequences of mixotrophy. The major objectives are to determine 1) environmental controls for inducing mixotrophy, 2) the role of prey quality on predator selection, 3) cellular and molecular controls of mixotrophy, and 4) nutrient assimilation and integrated metabolism. Using these various research approaches, this work will produce a comprehensive view of several mixotrophs and provide new insights into cellular, ecological, and evolutionary aspects of mixotrophy. Results from this research will improve our understanding of the physiological and ecological role of mixotrophy in marine phytoplankton, and provide much needed molecular markers for studying this process in both the laboratory and field.