The role of thermal history in shaping the microbiome of Red Sea corals

Coral reefs are immensely vulnerable to climate change and particularly the effects of ocean warming; in efforts to understand whether and how reef systems will survive into the future, research is increasingly focusing on present day populations acclimatized to thrive under relatively extreme conditions. Whilst corals thrive along a range of environmental conditions, including relative extremes, within the Red Sea, these coral populations are still considered not well explored of the genetic and physiological signatures throughout this system. Corals microbiota communities (the “microbiome”) are recognized as a major component as to how corals “acclimatize” to different environmental conditions; therefore, this work aimed to investigate the historical thermal variability along the Red Sea and subsequently identify the relative role of coral microbiome associated with differences in coral thermal tolerance. Remotely sensed data (1982-2012) demonstrated migration of Sea Surface Temperature anomalies (i.e. DHW) from the south to the north during this time frame. Analysis of historical bleaching records indicated that coral populations were more tolerant to bleaching in the northern compared to the central/southern Red Sea. Symbiodinium clade type (ITS2) and microbial community (16S rRNA metagenomics) associated with six key coral species persisting across five sites of the northern Red Sea (29°-20°N) were then examined. Symbiodinium clade identity associated with each coral species generally remained highly conserved throughout the sites sampled. In contrast, microbial communities were variable within and between species across the Red Sea sites. Corals from two sites (central-Jeddah and northern-Hurghada) were exposed to a thermal stress experiment which confirmed that corals were more heat resistant at Hurghada (summer SST mean is 3.3 °C less) than Jeddah; however, symbiont ITS2 clade types were the same at both sites. Conversely, microbial community changed in heat stressed samples at Jeddah compared to the control group, while it remained stable at Hurghada. This work provides for the first time genetic analysis on corals’ microbiome inhabiting extreme thermal resistant region (i.e. the northern Red Sea) that contradict the global bleaching pattern. Our findings suggest that plasticity of microbial community may play the key role in acclimation of corals experience thermal anomalies in the Red Sea suggesting presence certain microbial phylotypes fill specific thermal niche. Finally, the higher latitudes of the Red Sea will broadly serve as a potential corals refugia which highlights the importance to conserve and implement a regional management policy to improve corals thermal tolerance of this region to be used as a genetic reservoir.