The effects of dissolved organic matter supplements on the metabolism of corals under heat stress

Octocorals represent a major alternative group in the benthic community of reefs that have diverged from hexacoral dominance. Despite their phototrophic symbionts, supplementing their diet with heterotrophic sources may promote their growth, particularly when compared to hexacorals in bleaching conditions. However, limited comprehensive data exists on octocorals' trophic ecology, especially regarding their ability to feed on dissolved organic matter (DOM), which comprises the largest pool of organic matter in reefs. This study aims to investigate the ability of two octocorals (Sarcophyton glaucum and Lobophytum sp.) to feed on DOM and compare this ability to that of hexacorals, such as Stylophora pistillata and Turbinaria reniformis. The study measured the net fluxes of DOM under varying DOM concentrations and under heat stress. The results demonstrate that all coral species were net producers of DOM at ambient concentrations, but became net consumers once seawater was supplemented with DOM. Furthermore, our study highlights a relationship between coral physiological responses to heat stress and DOM uptake. Corals that increased (S. pistillata) or maintained (S. glaucum and Lobophytum sp.) their DOM uptake rates at high temperature were the most resilient to heat stress. In contrast, T. reniformis exhibited lower DOM uptake rates at high temperatures, which was associated with significant bleaching. Understanding the ability of corals to feed on DOM may, therefore, provide insight into the resilience of species under ocean warming conditions.