CAREER: Comparative Biology of Phagocytosis in Green Algae and Other Protists with Emphasis on its Roles in the Endosymbiotic Origin of Eukaryotic Photosynthesis

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Eukaryotes are organisms whose cells have complex internal structures, such as nuclei and mitochondria, unlike the cells of bacteria. Cells of photosynthetic eukaryotes such as plants and algae possess a chloroplast, an organelle that is typically colored green and is responsible for generating cellular energy from sunlight. While a chloroplast is part of a cell and cannot live by itself outside of a cell, it originated from a free-living bacterium, which was engulfed (i.e., phagocytosis) by and formed an intracellular symbiosis with an early host eukaryote. This research aims to characterize the feeding mechanism of several taxonomically diverse groups, including green algae. Feeding is especially poorly understood in these groups. Only a few green algae, for example, still have the capacity to consume food particles. The study has the potential to provide insight into the process of feeding on bacteria that eventually led to the evolution of chloroplasts as organelles within photosynthetic eukaryotes. The work will involve a combination of cell culturing, imaging, and molecular sequencing tools, and will train a cohort of young investigators, including graduate students and undergraduate summer interns. The Principal Investigator recently found definitive evidence for phagocytosis in one lineage of green algae, a group that was once considered to have completely lost such capacity. The research will evaluate the molecular and cellular basis of phagocytosis in green algae as well as in several other lineages of eukaryotes. Major questions driving the proposed research include: 1) Are other "early" lineages of green algae capable of ingesting bacteria? When and how many times was phagocytosis lost during green algal evolution?; 2) Does resource limitation facilitate phagocytosis in mixotrophic green algae? If so, could this have provided sufficient evolutionary pressure for some green algae to still retain feeding capacity?; 3) What genes and proteins are involved in phagocytosis in green algae and other protists? Could lineage-specific traits of phagocytosis have played key roles in the origin of eukaryotic photosynthesis? The project will use a combination of methods, including feeding assays, growth experiments, fluorescence and electron microscopy, transcriptome/genome sequencing, phagosome proteomics, and quantitative PCR. The research will involve the training of one postdoctoral researcher, two graduate students, and five undergraduate interns. In addition, a course on imaging techniques will be developed and offered through the American Museum of Natural History graduate program. This course will train graduate students in the use of current microscopic techniques in comparative cell biology.