Dynamic symbioses reveal pathways to coral survival through prolonged heatwaves

Danielle C Claar,Ross Cunning,Andrew C Baker,Ruth D Gates,Julia K Baum,Kim M Cobb,Hannah E Epstein,Samuel Starko,Kristina L Tietjen

doi:10.1038/s41467-020-19169-y

Abstract

Prospects for coral persistence through increasingly frequent and extended heatwaves seem bleak. Coral recovery from bleaching is only known to occur after temperatures return to normal, and mitigation of local stressors does not appear to augment coral survival. Capitalizing on a natural experiment in the equatorial Pacific, we track individual coral colonies at sites spanning a gradient of local anthropogenic disturbance through a tropical heatwave of unprecedented duration. Unexpectedly, some corals survived the event by recovering from bleaching while still at elevated temperatures. These corals initially had heat-sensitive algal symbiont communities, endured bleaching, and then recovered through proliferation of heat-tolerant symbionts. This pathway to survival only occurred in the absence of strong local stressors. In contrast, corals in highly disturbed areas were already dominated by heat-tolerant symbionts, and despite initially resisting bleaching, these corals had no survival advantage in one species and 3.3 times lower survival in the other. These unanticipated connections between disturbance, coral symbioses and heat stress resilience reveal multiple pathways to coral survival through future prolonged heatwaves.

Highlights

Prospects for coral persistence through increasingly frequent and extended heatwaves seem bleak
Prior to the 2015–2016 heatwave, coral symbioses varied significantly with exposure to chronic local human disturbance: corals on highly disturbed reefs were typically dominated by heat-tolerant symbionts in the genus Durusdinium, while those on reefs exposed to lower levels of local disturbance tended to be dominated by heat-sensitive symbionts in the genus Cladocopium
There was a strong signal of human disturbance on coral symbioses across Kiritimati’s reefs

Summary

Introduction

Prospects for coral persistence through increasingly frequent and extended heatwaves seem bleak. These corals initially had heat-sensitive algal symbiont communities, endured bleaching, and recovered through proliferation of heattolerant symbionts This pathway to survival only occurred in the absence of strong local stressors. Corals in highly disturbed areas were already dominated by heattolerant symbionts, and despite initially resisting bleaching, these corals had no survival advantage in one species and 3.3 times lower survival in the other These unanticipated connections between disturbance, coral symbioses and heat stress resilience reveal multiple pathways to coral survival through future prolonged heatwaves. Contrary to previous work, corals dominated by heat-sensitive symbionts at the onset of the heatwave had higher (Platygyra ryukyuensis) or similar (Favites pentagona) survivorship through the heatwave compared to those that started with thermotolerant symbionts This survival pathway–recovering from bleaching during a temperature anomaly–was only observed in corals at sites without very high levels of local anthropogenic disturbance. Our results demonstrate that corals have multiple pathways to survival through prolonged heatwaves—resistance and recovery— the relative importance of which depends on coral species, heatwave duration, and the prevalence of other anthropogenic stressors in the system

Methods

Results

Conclusion