In exposing metabolic features of a heat-resilient coral symbiont, this work casts a bright light in our quest to understand coral thermal adaptability. Corals are foundation species, dominating shallow, tropical marine environments, and forming essential habitat for more than a quarter of all marine species. Belonging to the phylum Cnidaria, most corals live in a symbiotic partnership with unique microalgae, commonly referred to as ‘zooxanthellae’. This relationship allows the coral animal to obtain energy and carbon from the photosynthesis carried out by the microalgae in its tissue, which is essential for its survival in the low nutrient waters of the tropics. However, when water temperatures reach a critical threshold of only a few degrees higher than normal, the corals start to ‘bleach’; that is, they lose their symbiotic algae and become pale, exposing the white skeleton through their translucent tissue (Fig. 1). If the high water temperatures persist, the coral is unable to recolonise with algae and ultimately dies. Recent studies suggest that coral bleaching may be the result of metabolic breakdown between the coral animal host and its symbiotic algal partner (Rädecker et al., 2021; Kemp et al., 2023), where, due to physiological stress, the transfer of carbon from the symbiont to the host is compromised, effectively decoupling the symbiosis and turning the symbiont into a parasite. Understanding the metabolic processes leading to this ‘dysbiosis’ requires knowledge of the often subtle metabolic changes occurring within the cells during heat stress. FTIR is a nondestructive technique that uses the absorption properties of biomolecules within a sample to generate a spectrum that reveals the biomolecular composition of the sample. First used in coral bleaching research by Petrou et al. (2018), the technique proved promising for interrogating coral symbiont metabolism. Using this methodology, Johnston et al. identified the spectral fingerprint of lab-evolved, heat-resistant coral symbionts (strain SS8) and compared it to a WT strain (WT10). Their comprehensive study included both in hospite, bleaching expelled and cultured symbiont cells, providing broad, unique insight into the different physical conditions of zooxanthellae lifestyle. Several key findings emerged: first, strong parallels in spectral fingerprints between the two strains of symbionts and their response to heat stress were found, which is central to demonstrating that metabolic stress responses to heat are conserved and consistent across symbiont phylogeny. Second, by using a genetically evolved heat-resistant strain, they were able to ascertain that the evolved strain SS8 showed a significantly ‘dampened’ response, with minimal protein loss along with other reduced effects. In other words, they were able to show that the evolved strain had the ability to maintain a more stable cellular metabolism during heat stress. Third, they highlight the strength of FTIR for interrogating the consequences of environmental change by resolving subtle metabolic shifts within individual cells. In exposing metabolic features of a heat-resilient coral symbiont, this work casts a bright light in our quest to understand coral thermal adaptability. The data presented here by Johnston et al. identified heat-resistant metabolic responses that may hold the key to understanding how some corals withstand marine heatwaves better than others. This signature of resilience forms an important resource for the coral research community, as only a few studies on evolved heat-resistant symbiont strains exist, and there is little information on the metabolic differences between WT and heat-resistant symbionts. A recent example underlining the potential importance and application of this information can be drawn from a new study that shows how heat stress differentially affects subpopulations of symbionts within corals (Nielsen et al., 2026). In this study, a metabolic profile of one of the subpopulations showed signatures similar to those reported here for the heat-resistant strain SS8, lending empirical support for the author's interpretation of potential resilience. This central contribution has implications also for broader cross-disciplinary discussions on plasticity, adaptation, protection and restoration of coral reef systems. Physiologists, evolutionary biologists and restoration practitioners benefit from understanding the mechanisms behind coral resilience and persistence in warmer waters. In exposing metabolic patterns of heat-resistance in coral symbionts, Johnston et al. offer a scaffold for collecting data on phylogenetically diverse symbionts to better understand thermal resilience at a reef scale. The knowledge gained from using a known heat-resistant strain enables signatures of heat resistance to be sought in natural (unevolved) symbionts. Similarly, this information offers a means for testing resilience in new strains or, equally, validating heat resistance metabolically in lab-evolved strains compared to a nonresistant strain. Beyond their findings on metabolic profiles of heat-resilient strains, the study by Johnston et al. adds to the growing body of work that shows the power and importance of single-cell techniques for understanding biological processes. While traditional metabolomic methods provide high-level details on molecules and pathways of interest, cell-specific information, which has been shown to be critical in multispecies organisms such as corals, is still generally out of reach. By being able to separate cells by type and origin, single-celled approaches can increase the resolution of the specific response and avoid biasing the outcome from unrelated cell types. The more we know about how corals and their symbionts work together and thrive, and what confers resilience, the better equipped we can be for addressing the future needs of coral reef conservation and management. While it may not preserve our reefs as they are today, assisted evolution offers a way forward to help maintain individual coral populations or species that would otherwise succumb to the consequences of ongoing global warming. The authors acknowledge support from the Human Frontier Science Program (HFSP) – award ID RGP014/2025. During the preparation of this work, the authors used Microsoft Copilot to generate Fig. 1. After using Microsoft Copilot, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication. Open access publishing facilitated by University of Technology Sydney, as part of the Wiley - University of Technology Sydney agreement via the Council of Australasian University Librarians The New Phytologist Foundation remains neutral with regard to jurisdictional claims in maps and in any institutional affiliations.
Petrou et al. (Fri,) studied this question.
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