Humic substances modulate bacterial communities and mitigate adverse effects of temperature stress in coral reef organisms.

Aims: In the present study, we tested whether terrestrially derived humic substances (HS) could mitigate the adverse effects of elevated temperature and ultraviolet B (UVB) radiation on the bacterial communities of two hard corals (Montipora digitata and M. capricornis), one soft coral (Sarcophyton glaucum), sediment and water. We also examined the impact of temperature, UVB radiation, and HS supplementation on coral photosynthetic activity, a proxy for coral bleaching.

Methods and results: We performed a multifactorial experiment using a randomized-controlled microcosm setup. Coral photosynthetic efficiency was measured in vivo using a pulse amplitude modulation fluorometer. Bacterial communities were analyzed using 16S rRNA gene sequencing. Corals in HS-supplemented microcosms had significantly higher photosynthetic activities than those in microcosms subjected to elevated temperature and UVB radiation. Additionally, HS supplementation significantly influenced the composition of sediment, water, and host-associated bacterial communities. Reef organisms in HS supplemented microcosms contained distinct bacterial communities enriched with groups of potentially beneficial bacteria. In the hard coral M. digitata, we observed an interactive effect of HS supplementation, UVB radiation, and temperature.

Conclusion: Our findings indicate that HS significantly modulates coral reef bacterial communities and support the hypothesis that these substances contribute to improved reef resistance to the adverse effects of elevated temperature and UVB radiation.