Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions

Global degradation of coral reefs has increased the urgency of identifying stress-tolerant coral populations, to enhance understanding of the biology driving stress tolerance, as well as identifying stocks of stress-hardened populations to aid reef rehabilitation. Surprisingly, scientists are continually discovering that naturally extreme environments house established coral populations adapted to grow within extreme abiotic conditions comparable to seawater conditions predicted over the coming century. Such environments include inshore mangrove lagoons that carry previously unrecognised ecosystem service value for corals, spanning from refuge to stress preconditioning. However, the existence of such hot-spots of resilience on the Great Barrier Reef (GBR) remains entirely unknown. Here we describe, for the first time, 2 extreme GBR mangrove lagoons (Woody Isles and Howick Island), exposing taxonomically diverse coral communities (34 species, 7 growth morphologies) to regular extreme low pH (<7.6), low oxygen (<1 mg l(-1)) and highly variable temperature range (>7 degrees C) conditions. Coral cover was typically low (<5 %), but highly patchy and included established colonies (>0.5 m diameter), with net photosynthesis and calcification rates of 2 dominant coral species (Acropora millepora, Porites lutea) reduced (20-30 %), and respiration enhanced (11-35 %), in the mangrove lagoon relative to adjacent reefs. Further analysis revealed that physiological plasticity (photosynthetic 'strategy') and flexibility of Symbiodiniaceae taxa associations appear crucial in supporting coral capacity to thrive from reef to lagoon. Prevalence of corals within these extreme conditions on the GBR (and elsewhere) increasingly challenge our understanding of coral resilience to stressors, and highlight the need to study unfavourable coral environments to better resolve mechanisms of stress tolerance.