The coral microbiome under climate change: shifting symbiotic interactions

Host–microbe symbioses are central to organismal health, yet these complex partnerships are often strongly shaped by the environment. Climate change is increasingly altering environmental conditions, disrupting the balance of host–microbe interactions and pushing them along a continuum from mutualism (both partners benefit) toward parasitism (one partner gains at the other’s expense). Reef-building corals provide a clear example of this vulnerability, as the coral holobiont, an ecological unit comprising diverse photosynthetic algal symbionts (Symbiodiniaceae) and other microbial partners, depends largely on these mutualistic interactions for nutrition, immunity, and stress tolerance. Increasingly, evidence suggests that under heat stress, the algal symbionts retain more photosynthetic products and proliferate, shifting the relationship more toward parasitism as the net benefits to the host decrease. Similarly, heat stress also restructures coral-associated bacterial communities, allowing opportunistic or pathogenic taxa to increase. These changing interactions highlight that symbiosis is not a fixed state and should instead be viewed as a dynamic continuum that will be highly influenced by climate change. This has important conservation implications – including the intentional integration of heat-evolved algal symbionts and beneficial bacteria in the form of probiotics to enhance coral stress tolerance. Understanding when and how host–microbe interactions can vary along the mutualism–parasitism continuum is essential for designing conservation strategies that enhance resilience under climate change.