Zooxanthellen

Zooxanthellae are photosynthetic dinoflagellates that live as endosymbionts within the tissues of many marine invertebrates, most notably scleractinian corals. They reside inside the gastrodermal cells of the host, forming a mutualistic relationship that is central to coral reef ecosystems.

Taxonomically, zooxanthellae associated with corals are part of the family Symbiodiniaceae and encompass several genera, including Cladocopium, Durusdinium, Breviolum, and Symbiodinium. The term has historically referred to the symbiotic dinoflagellates within corals, and it remains in common usage even as taxonomic revisions have clarified greater diversity among the symbionts.

In exchange for a protected habitat and inorganic nutrients from the host, zooxanthellae photosynthesize and transfer a substantial portion of the fixed carbon to the coral. This energy supply supports metabolic needs and contributes to the calcification processes that build reef structures, enabling corals to grow in nutrient-poor tropical waters.

The relationship is influenced by environmental conditions. While beneficial under stable conditions, heat stress, light fluctuations, pollution, and acidification can disrupt photosynthesis and lead to bleaching, a stress response in which corals expel their symbionts. Prolonged bleaching can reduce growth and increase mortality, though some corals can reacquire symbionts or shift to more tolerant strains.

Diversity and flexibility in host–symbiont pairings affect reef resilience. Corals with flexible symbiont communities may better withstand environmental change, making zooxanthellae a focal point in studies of climate impacts on coral ecosystems.