Coral reefs are deteriorating under climate change as oceans continue to warm and acidify and thermal anomalies grow in frequency and intensity. In vitro experiments are widely used to forecast reef-building coral health into the future, but often fail to account for the complex ecological and biogeochemical interactions that govern reefs. Consequently, observations from coral communities under naturally occurring extremes have become central for improved predictions of future reef form and function. Here, we present a semi-enclosed lagoon system in New Caledonia characterised by diel fluctuations of hot-deoxygenated water coupled with tidally driven persistently low pH, relative to neighbouring reefs. Coral communities within the lagoon system exhibited high richness (number of species = 20) and cover (24–35% across lagoon sites). Calcification rates for key species (Acropora formosa, Acropora pulchra, Coelastrea aspera and Porites lutea) for populations from the lagoon were equivalent to, or reduced by ca. 30–40% compared to those from the reef. Enhanced coral respiration, alongside high particulate organic content of the lagoon sediment, suggests acclimatisation to this trio of temperature, oxygen and pH changes through heterotrophic plasticity. This semi-enclosed lagoon therefore provides a novel system to understand coral acclimatisation to complex climatic scenarios and may serve as a reservoir of coral populations already resistant to extreme environmental conditions.

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Climate change threatens the survival of scleractinian coral from exposure to concurrent ocean warming, acidification and deoxygenation; how corals can potentially adapt to this trio of stressors is currently unknown. This study investigates three coral species (Acropora muricata, Acropora pulchra and Porites lutea) dominant in an extreme mangrove lagoon (Bouraké, New Caledonia) where abiotic conditions exceed those predicted for many reef sites over the next 100 years under climate change and compared them to conspecifics from an environmentally more benign reef habitat. We studied holobiont physiology as well as plasticity in coral-associated microorganisms (Symbiodiniaceae and bacteria) through ITS2 and 16S rRNA sequencing, respectively. We hypothesised that differences in coral-associated microorganisms (Symbiodiniaceae and bacteria) between the lagoonal and adjacent reef habitats may support coral host productivity and ultimately the ability of corals to live in extreme environments. In the lagoon, all coral species exhibited a metabolic adjustment of reduced photosynthesisto-respiration ratios (P/R), but this was accompanied by highly divergent coral host-specific microbial associations. This was substantiated by the absence of shared ITS2-type profiles (proxies for Symbiodiniaceae genotypes). We Topic