Biochemical and Molecular Autonomy of Symbiotic Chloroplasts

Photosynthesis provides the energy that drives all plant growth, productivity and life on earth. A marine sea slug, Elysia chlorotica, has acquired the ability to carry out photosynthesis like a plant as a result of forming a symbiotic association with chloroplasts of the alga, Vaucheria litorea. Juvenile sea slugs feed on the filamentous alga and retain only the chloroplasts, incorporating them into cells of the digestive epithelium. The chloroplasts in the now dark-green animals are functional, i.e. they evolve oxygen and fix carbon dioxide and actively synthesize proteins from DNA contained in the chloroplasts. Once the symbiosis is established, the animal can live and reproduce in culture without eating for the rest of its normal life span of nine to ten months. About 90% of all the proteins required to keep chloroplasts functioning and carrying out photosynthesis are encoded by DNA in the nucleus of the plant or alga. These proteins must be continually synthesized in the cell and transported into the chloroplast to sustain activity. Thus, considering that the sea slugs only acquire the algal chloroplasts and not any other part of the algal cell including the nucleus, the level of sustained chloroplast activity observed in the sea slugs is unique and quite remarkable.Understanding how these chloroplasts are able to remain photosynthetically active outside of their normal cellular environment for months when higher plant chloroplasts survive only a few hours in isolation, forms the basis of the specific objectives of this proposal. These include: 1) Characterizing the structural and functional long-term stability of isolated plastids, the stability of plastid proteins, and the activity of chloroplast proteases in sea slugs vs. algae, 2) Determining if the sea slug nuclear genome codes for and targets any proteins to the symbiotic chloroplasts and elucidating the general mechanism of protein import in chloroplasts, and 3) Characterizing the genetic autonomy of the chloroplasts by mapping and sequencing the chloroplast genome of the alga.This symbiotic organism provides a unique opportunity to determine how the chloroplasts from one organism (alga) can form a long-term functional photosynthetic union with the cell of an extremely divergent organism (sea slug). This represents a process molecular data indicate occurred repeatedly over evolutionary history resulting in the diversity of plants and algae on earth today. On a much broader scale, this project possibly represents lateral gene transfer between two diverse organisms and endosymbiosis in action. The Solar-Powered Sea Slugs are a fascinating teaching tool eliciting excitement and curiosity from people all around the world from scientists of several disciplines to graduate and undergraduate students discovering them in their readings in journal clubs, to young children who have seen these crawling leaves on the Internet or researched them for science fair projects. The endosymbiotic theory has generated and continues to generate much interest at all levels, but to actually observe a symbiotic association in a potentially evolving situation and imparting such a major new function to the other partner, is indeed rare and a unique opportunity for study