Probiotics prevent mortality of thermal-sensitive corals exposed to short-term heat stress

Coral reefs are increasingly threatened by marine heatwaves, leading to mass bleaching and mortality. While long-term probiotic interventions have shown promise, the efficacy of using beneficial microorganisms (BMCs) to protect corals against short-term, acute heat stress—the kind typical of sudden heatwaves—remained understudied. This study tested whether pre-inoculating two Red Sea hard coral species with tailored bacterial consortia could mitigate mortality and physiological decline during a 48-hour period at 32°C.

Researchers collected two species, the heat-sensitive Acropora cf. hemprichii and the apparently heat-tolerant Pocillopora verrucosa, from a natural reef. Fragments were randomly assigned to either control or probiotic-treated tanks. Inoculation occurred two hours before the temperature was rapidly ramped up to 32°C. The team evaluated outcomes using survival assessments, primary production measurements (oxygen flux), pigment analysis (chlorophyll a and c2), Symbiodiniaceae cell counts, and photosynthetic efficiency (FV/Fm) measurements.

For A. cf. hemprichii, the results were dramatic: 100% of the untreated fragments died under acute heat stress, while 100% of the probiotic-treated fragments survived. Beyond mere survival, the treated fragments maintained primary production rates and pigment concentrations comparable to corals kept at ambient temperatures. In contrast, P. verrucosa showed high inherent resilience, experiencing no significant mortality or physiological decline regardless of whether they received the probiotic treatment, indicating that the benefits of BMCs are species-specific and may be most critical for less thermally tolerant species.

This study provides evidence that coral probiotics can act as a rapid-response tool to enhance resilience during short-term thermal shocks. By confirming that even a brief window of pre-inoculation can prevent mortality, the research offers a potential management strategy for reef conservation, suggesting that probiotics could be applied strategically ahead of forecasted marine heatwaves to protect vulnerable species.