Mollusc-algal chloroplast endosymbiosis. Photosynthesis, thylakoid proteinmaintenance, and chloroplast gene expression continue for many months in the absence of the algal nucleus

Early in its Life cycle, the marine mollusc Elysia chlorotica Gould forms a n intracellular endosymbiotic association with chloroplasts of the chromoph ytic alga Vaucheria litorea C. Agardh. As a result, the dark green sea slug can be sustained in culture solely by photoautotrophic CO2 fixation for at least 9 months if provided with only light and a source of CO2. Here we de monstrate that the sea slug symbiont chloroplasts maintain photosynthetic o xygen evolution and electron transport activity through photosystems I and II for several months in the absence of any external algal food supply. Thi s activity is correlated to the maintenance of functional levels of chlorop last-encoded photosystem proteins, due in part at least to de novo protein synthesis of chloroplast proteins in the sea slug. Levels of at least one p utative algal nuclear encoded protein, a light-harvesting complex protein h omolog, were also maintained throughout the 9-month culture period. The chl oroplast genome of V. litorea was found to be 119.1 kb, similar to that of other chromophytic algae. Southern analysis and polymerase chain reaction d id not detect an algal nuclear genome in the slug, in agreement with earlie r microscopic observations. Therefore, the maintenance of photosynthetic ac tivity in the captured chloroplasts is regulated solely by the algal chloro plast and animal nuclear genomes.