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

The use of coral probiotics, i.e. beneficial microorganisms for corals (BMCs), is a novel approach to enhancing coral health under heat stress. While BMCs mitigate coral bleaching and mortality during prolonged heat stress conditions, their effectiveness in mitigating short-term acute heat stress remains understudied. This study investigates the effects of BMCs on two Red Sea hard coral species, <i>Acropora</i> cf. <i>hemprichii</i> and <i>Pocillopora verrucosa</i>, during short-term heat stress. Twelve coral fragments per species were allocated to each treatment across two temperature regimes (26°C and 32°C) for 48 hours, with half receiving BMC inoculation and half serving as controls. Results show BMC supplementation significantly prevented mortality in <i>Acropora</i> cf. <i>hemprichii</i> at 32°C, contrasting with a 100% mortality observed in the control group. Specifically, probiotic-inoculated <i>Acropora</i> cf. <i>hemprichii</i> at 32°C exhibited preserved primary production, a 12-13 fold increase in algal cell densities, 4-5 times higher <i>F_V/F_m</i> ratios, and 4-5 and 2-3 times higher chlorophyll <i>a</i> and <i>c₂</i> concentrations, respectively, compared to their untreated conspecifics. All <i>P. verrucosa</i> colonies survived the 32°C exposure without tissue loss or reduced holobiont function in both control and BMC treatments. These findings underscore the rapid effects of BMC inoculation, initiated just 2 hours prior to acute heat stress, in protecting heat-sensitive <i>Acropora</i> cf. <i>hemprichii</i> against mortality and adverse photo-physiological changes, with beneficial effects visible within 2 days. Recognizing the critical timeframe for beneficial effects is paramount for management strategies to address heat-sensitive corals on natural reefs, such as implementing probiotic interventions before anticipated marine heatwaves.