Zooxanthellae are tiny, single-celled algae that live inside the tissues of many marine organisms, particularly coral. These algae perform photosynthesis, converting sunlight into energy, which benefits both themselves and their host. In return, the host provides a protected environment and nutrients for the zooxanthellae. The relationship between zooxanthellae and their hosts is known as symbiosis. This partnership is crucial for the health of coral reefs, as it helps corals grow and thrive in nutrient-poor waters. When environmental conditions change, such as increased temperature, zooxanthellae can be expelled, leading to coral bleaching and potential reef decline.