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 temperatures 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 on the resilience of species under ocean warming conditions.