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Trebouxia (division Chlorophyta) A genus of unicellular, non-motile green algae which are the commonest phycobionts in lichens; free-living Trebouxia cells are found on trees, fences, etc. Phycobiont cells show some differences from their free-living counterparts, e.g. in the proportion of photosynthetic products formed, and in the amount of such products released from the cells (which is greater in the phycobiont).