Thermal stress reveals contrasting bleaching and trophic patterns in two Southwestern Atlantic octocorals

Abstract Most shallow-water octocorals engage in a symbiotic relationship with photosynthetic dinoflagellates of the family Symbiodiniaceae. Their inherently mixotrophic nature, combining autotrophic and heterotrophic feeding, provides a flexible energy acquisition that supports their metabolic demands. This nutritional plasticity depends on the functional integrity of symbiosis, which can be disrupted by thermal stress, potentially disturbing oxidative balance and causing symbiosis breakdown (i.e., bleaching). Although temperature effects are well known in scleractinian corals, octocoral ecophysiological responses remain poorly understood, especially in South Atlantic reefs. In this study, we used two octocoral species, Neospongodes atlantica and Plexaurella grandiflora , to test the hypotheses that elevated temperature (+ 3.5 °C, 14 days) induces bleaching by triggering oxidative stress, and that enhanced heterotrophic feeding occurs after heat stress. Symbiosis was evaluated through symbiont density and chlorophyll- a content; trophic ecology was assessed using fatty acid markers of autotrophy and heterotrophy; and oxidative status was analyzed through total antioxidant capacity and lipid peroxidation. Both species bleached under thermal stress, but via distinct mechanisms. Plexaurella grandiflora showed a reduction in symbiont density, which led to a decline in autotrophic input that was compensated by an increase in heterotrophic feeding. In contrast, N. atlantica decreased chlorophyll- a content, but its trophic balance remained stable. Oxidative status was unaffected in both species, suggesting that bleaching may have mitigated potential oxidative imbalances, or that bleaching occurred independently of oxidative stress—possibly linked to impaired photosynthetic carbon translocation. These findings underscore octocoral susceptibility to warming oceans and reveal species-specific physiological strategies.