Abstract
Ocean warming threatens corals and the coral reef ecosystem. Nevertheless, corals can be adapted to their thermal environment and inherit heat tolerance across generations. In addition, the diverse microbes that associate with corals have the capacity for more rapid change, potentially aiding the adaptation of long-lived corals. Here, we show that the microbiome of reef corals is different across thermally variable habitats and changes over time when corals are reciprocally transplanted. Exposing these corals to thermal bleaching conditions changes the microbiome for heat-sensitive corals, but not for heat-tolerant corals growing in habitats with natural high heat extremes. Importantly, particular bacterial taxa predict the coral host response in a short-term heat stress experiment. Such associations could result from parallel responses of the coral and the microbial community to living at high natural temperatures. A competing hypothesis is that the microbial community and coral heat tolerance are causally linked.
Highlights
Ocean warming threatens corals and the coral reef ecosystem
Long-lived sessile stony corals are dependent on an endosymbiosis with photosynthetic algal symbionts in the genus Symbiodinium[5] and, more recently, are shown to associate with a diverse set of bacteria that contribute important functions to the coral host[6,7,8]
The composition of the coral holobiont varies across environments, which may entail coral host genomic[10] and Symbiodinium community[11,12] changes that influence the resilience of the coral holobiont to environmental stressors
Summary
Ocean warming threatens corals and the coral reef ecosystem. corals can be adapted to their thermal environment and inherit heat tolerance across generations. Previous studies on A. hyacinthus in the back reef pools of Ofu Island could show that Symbiodinium communities differ between corals from two thermally distinct environments (i.e., a highly variable HV pool and a moderately variable MV pool), under heat stress MV corals bleached irrespective of which symbiont type they harboured[16] This has been explained in part by contrasting genomic adaptation between corals from the HV and MV pool[10,17], but the data show strong acclimatization between the thermal environments after reciprocal transplantation[18]. Our study shows that bacterial community composition and underlying functional profiles associated with reef corals are different across thermally variable habitats and adapt to the new environment when corals are reciprocally transplanted Subsequent exposure of these corals to thermal bleaching conditions changes the microbial community of heat-sensitive corals coming from a more stable, cooler environment. This effect is irrespective of the origin of the corals and the coral genotype, but rather determined by the environment that transplanted corals where exposed to prior to the short-term heat stress experiment
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