Short-term heat stress assays resolve effects of host strain, repeat stress, and bacterial inoculation on Aiptasia thermal tolerance phenotypes

The ongoing loss of corals and their reef ecosystems hastens the need to develop approaches that mitigate the impacts of climate change. Given the strong reliance of corals on their associated prokaryotic and microalgal symbionts, microbiome-targeted interventions in the form of probiotics or microbiome transplants are emerging as potential solutions. Although inoculation with beneficial microorganisms was shown to improve coral bleaching recovery, the mechanistic underpinnings and extent to which microbiomes can be manipulated are largely unknown. Research progress in this regard is often hindered by coral holobiont complexity and a lack of standardized diagnostics to assess physiological and phenotypic changes following microbial manipulation. Here we address these shortcomings by establishing short-term acute thermal stress assays using the Coral Bleaching Automated Stress System (CBASS) as a standardized and reproducible experimental platform to assess stress tolerance phenotypes of the coral model Aiptasia. We show that thermal tolerance phenotypes following acute heat stress assays are highly reproducible, host species-specific, and can exert legacy effects with consequences for long-term thermal resilience. We further demonstrate the ability to resolve phenotypic differences in thermal tolerance following incubation with the coral bleaching pathogen Vibrio coralliilyticus , providing an avenue for screening bacteria for their ability to affect holobiont thermal performance. By employing acute heat stress assays in conjunction with a tractable model organism, we posit CBASS assays as a standardized experimental platform that allows functional screening for microbes that affect thermal stress tolerance. Such effort may accelerate the discovery of microbes and microbial mechanisms mediating thermal tolerance and our ability to harness them to increase stress resilience.