Marine Heatwaves Transform Coral Symbioses With Enduring Effects

Climate change-amplified extreme weather events are reshaping ecological communities globally. On coral reefs, heatwaves typically disrupt the obligate coral-algal symbiosis, with symbiont identity a prime determinant of coral resilience to these thermal extremes. Yet, whether heatwaves have long-term effects on coral symbioses remains unclear due to a lack of longitudinal symbiont data. Here, we report on a decadal coral symbiont survey (2013-2023), spanning the most prolonged tropical marine heatwave on record (2015-2016) and its aftermath. Concomitant with mass coral mortality, we document wholesale transformation of the symbiont assemblages in two coral species, the legacy of which was persistent for 7 years post heatwave. We also found evidence suggestive of a symbiont's local extinction, of local human disturbance impeding symbiont recovery, and of new coral recruits hosting symbiont assemblages distinct from survivors. Our study demonstrates heatwaves can have long-lasting impacts on symbioses raising concern for coral resilience to future heatwaves.