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The iconic and threatened Caribbean coral,Acropora palmata, is an essential reef-ecosystem engineer. Understanding the processes underpinning this coral’s survival and growth is essential to restoring this foundational species. Here, we compared replicateA. palmatacolonies transplanted along 350 km of Florida’s offshore coral reef to determine holobiont and/or environmental variables that predict transplant success. We found a west-to-east gradient in coral physiology coupled with site-specific coral-associated microbiomes. Interestingly, no variables were linked to coral genet. Our results suggest that the unique oceanographic conditions with periodic upwelling events in the Dry Tortugas provide corals with greater opportunity for heterotrophy that in turn enhances coral growth and survivorship, and positively influences the microbiome. Our findings indicate that restoration efforts in the Dry Tortugas, and other places exhibiting higher food availability, could be most effective forA. palmata.

Coral reefs are declining worldwide, yet some coral populations are better adapted to withstand reductions in pH and the rising frequency of marine heatwaves. The nearshore reef habitats of Palau, Micronesia are a proxy for a future of warmer, more acidic oceans. Coral populations in these habitats can resist, and recover from, episodes of thermal stress better than offshore conspecifics. To explore the physiological basis of this tolerance, we compared tissue biomass (ash-free dry weight cm⁻²), energy reserves (i.e., protein, total lipid, carbohydrate content), and several important lipid classes in six coral species living in both offshore and nearshore environments. In contrast to expectations, a trend emerged of many nearshore colonies exhibiting lower biomass and energy reserves than colonies from offshore sites, which may be explained by the increased metabolic demand of living in a warmer, acidic, environment. Despite hosting different dinoflagellate symbiont species and having access to contrasting prey abundances, total lipid and lipid class compositions were similar in colonies from each habitat. Ultimately, while the regulation of colony biomass and energy reserves may be influenced by factors, including the identity of the resident symbiont, kind of food consumed, and host genetic attributes, these independent processes converged to a similar homeostatic set point under different environmental conditions.

Warming and nutrient limitation are stressors known to weaken the health of microalgae. In situations of stress, access to energy reserves can minimize physiological damage. Because of its widespread requirements in biochemical processes, iron is an important trace metal, especially for photosynthetic organisms. Lowered iron availability in oceans experiencing rising temperatures may contribute to the thermal sensitivity of reef‐building corals, which rely on mutualisms with dinoflagellates to survive. To test the influence of iron concentration on thermal sensitivity, the physiological responses of cultured symbiotic dinoflagellates (genusBreviolum; family Symbiodiniaceae) were evaluated when exposed to increasing temperatures (26 to 30°C) and iron concentrations ranging from replete (500 pM Fe’) to limiting (50 pM Fe’) under a diurnal light cycle with saturating radiance. Declines in photosynthetic efficiency at elevated temperatures indicated sensitivity to heat stress. Furthermore, five times the amount of iron was needed to reach exponential growth during heat stress (50 pM Fe′ at 26–28°C vs. 250 pM Fe′ at 30°C). In treatments where exponential growth was reached,Breviolum psygmophilumgrew faster thanB.minutum, possibly due to greater cellular contents of iron and other trace metals. The metal composition ofB.psygmophilumshifted only at the highest temperature (30°C), whereas changes inB.minutumwere observed at lower temperatures (28°C). The influence of iron availability in modulating each alga’s response to thermal stress suggests the importance of trace metals to the health of coral‐algal mutualisms. Ultimately, a greater ability to acquire scarce metals may improve the tolerance of corals to physiological stressors and contribute to the differences in performance associated with hosting one symbiont species over another.