Bleaching susceptibility and mortality of corals are determined by fine-scale differences in symbiont type

TLDR

Coral bleaching is a major driver of reef decline, and the distribution of symbiont clades (A–H) is commonly used to explain thermal responses of reef-building corals. The study uses Stylophora pistillata to investigate how fine-scale genetic variability within clade C correlates with bleaching susceptibility. The authors monitored individual tagged colonies in situ over two years, assessing genetic variability and bleaching responses. They found that subcladal types C78 and C8/a are more thermally tolerant than C79 and C35/a, that bleaching temporarily shifts symbiont types but colonies revert to their originals, and that differential mortality of hosts with thermally sensitive versus resistant symbionts drives changes in coral community composition rather than symbiont shuffling.

Abstract

Coral bleaching has been identified as one of the major contributors to coral reef decline, and the occurrence of different symbionts determined by broad genetic groupings (clades A–H) is commonly used to explain thermal responses of reef-building corals. By using Stylophora pistillata as a model, we monitored individual tagged colonies in situ over a two-year period and show that fine level genetic variability within clade C is correlated to differences in bleaching susceptibility. Based on denaturing gradient gel electrophoresis of the internal transcribed spacer region 2, visual bleaching assessments, symbiont densities, host protein, and pulse amplitude modulated fluorometry, we show that subcladal types C78 and C8/a are more thermally tolerant than C79 and C35/a, which suffered significant bleaching and postbleaching mortality. Although additional symbiont types were detected during bleaching in colonies harboring types C79 and C35/a, all colonies reverted back to their original symbionts postbleaching. Most importantly, the data propose that the differential mortality of hosts harboring thermally sensitive versus resistant symbionts rather than symbiont shuffling/switching within a single host is responsible for the observed symbiont composition changes of coral communities after bleaching. This study therefore highlights that the use of broad cladal designations may not be suitable to describe differences in bleaching susceptibility, and that differential mortality results in a loss of both symbiont and host genetic diversity and therefore represents an important mechanism in explaining how coral reef communities may respond to changing conditions.