Stony coral symbioses show variable responses to future ocean conditions

Coral reefs support over a quarter of marine species and nearly a billion people worldwide but are also among the ecosystems most threatened by anthropogenic impacts. There is long-standing debate about whether coral symbioses will be disrupted or respond adaptively under future ocean conditions. Using a factorial 2.5-year future-ocean mesocosm experiment across eight coral species representing the major coral lineages, we tracked symbiont community shifts within replicate fragments from the same individual coral. Some corals exhibited stochastic divergence, whereas others showed deterministic shifts in symbiont community composition. Heat stress generally reduced symbiont diversity and promoted predictable restructuring, consistent with deterministic adaptation to moderate thermal stress but stochastic accumulation of diversity under extreme conditions. We propose that adaptive and stochastic responses represent endpoints along a continuum of host-orchestrated symbiont sorting. This study bridges coral reef ecology with broader host-microbiome theory, offering an integrated perspective on how symbiotic systems may respond to environmental change.