Active restoration strategies targeting corals with elevated heat tolerance have the potential to enhance reef resistance under a warming climate. While stress-tolerant corals have been documented in extreme systems such as mangrove lagoons, it is critical to assess the ability of these corals to maintain tolerance when moved to a more benign habitat. Here, we translocated corals from a mangrove lagoon to an adjacent reef and evaluated the thermal thresholds of corals from both locations before translocation and after 1 year. We demonstrate that mangrove colonies have higher thermal tolerance than reef corals, and, critically, mangrove colonies exhibited no loss in thermal tolerance following 1-year translocation to a less extreme reef habitat. Up-regulation of genes associated with DNA repair, metabolism, and homeostasis indicates the importance of these pathways in helping mangrove corals mitigate thermal stress. Our findings suggest the use of heat tolerant corals from extreme systems holds promise as part of intervention strategies aiming to increase reef resistance.

As coral reefs continue to decline globally, a toolbox of new interventions such as nutritional supplementation are being considered. Low level supplementation of the trace element manganese (Mn) has increasingly been shown to enhance the resilience of photosynthetic Cnidarians to thermal stress, however, questions around the timing of Mn delivery relative to a thermal stress event remain unanswered. Here we explore how the timing of Mn additions influences the response of the common reef building coral Acropora millipora to thermal stress. Coral health was assessed using a variety of different metrics including photochemical efficiency, oxygen dynamics, elementomics and symbiont densities. A. millepora displayed significantly improved thermal tolerance when exposed to either intermittent pulse (19.34 µg L⁻¹ every second day) dosing or constant (4.24 µg L⁻¹) supply of dissolved Mn from 7 days before heat stress exposure (32 °C). Pulse dosed corals maintained higher photosynthetic efficiency and photosynthesis to respiration rates, as well as increased Symbiodiniaceae densities compared to thermally stressed corals without Mn supplementation. Elemental analysis revealed that both coral tissue and Symbiodiniaceae in Mn-treated corals accumulated significantly more Mn than non-Mn treated corals. Collectively, our results suggest that intermittent (pulse) Mn exposure prior to the onset of thermal stress may provide the greatest support for thermal resilience in corals. This study advances understanding on how essential elements like Mn could be strategically delivered as an active intervention to enhance coral tolerance during periods of thermal stress.

Manganese (Mn) is an essential trace element for all photosynthetic life, playing an integral role in their photosystems, metabolism, and antioxidant activity. For corals, most studies focus on the potential toxicity of Mn at high concentrations (e.g. >700 µg L^-1). However, there has been less exploration on beneficial, biologically relevant levels of Mn. Combining promtomics, ICP, and PAM fluorometry, we evaluate how Mn supplementation at increasing concentrations (0.5, 4.8, 11.4, 15.6 µg L^-1) alters the physiology and proteome of the model cnidarian, Exaiptasia diaphana, when subjected to ambient (26 ˚C) and elevated (32 ˚C) temperatures. We demonstrate that Mn from 4.8 to 15.6 µg L^-1 mitigates thermal stress to E. diaphana, resulting in reduced photochemical damage and symbiont expulsion. Derived photobiology and proteomics data contributes to a mechanistic model for how Mn reduces thermal susceptibility, supporting the viability of Mn additions to enhance the protective capacities of photosynthetic cnidarians during heatwaves.

Non-photochemical quenching (NPQ) is a key photoprotective mechanism in Symbiodiniaceae, enabling photosystem II (PSII) to dissipate excess excitation energy under stress. The balance between regulated (Φ_NPQ) and unregulated (Φ_NO) energy dissipation influences thermal tolerance, yet the temperature thresholds at which this balance shifts remain poorly defined. Here, we used the Phenoplate, a high-throughput fluorometric platform integrating rapid light curves with controlled temperature ramping, to examine short-term thermal responses in Breviolum minutum across 6–71 °C. We identified a sharp transition at 41 °C where Φ_NPQ collapsed and was replaced by Φ_NO, indicating loss of regulated photoprotection. This switch coincided with a pronounced drop in PSII effective quantum yield (Φ_II) and substantial reductions in cell density, marking a thermal Achilles’ heel in the photoprotective capacity of this species. Despite this regulatory breakdown, a fraction of cells persisted for at least three days post-exposure. These results demonstrate that B. minutum maintains regulated photoprotection up to a discrete threshold, beyond which unregulated becomes the dominant pathway and survival is compromised. Identifying such thermal inflection points in coral symbionts provides mechanistic insight into their vulnerability under acute heat stress and may inform early-warning indicators for coral bleaching susceptibility.