Stability of the coral SML microbiome under heat stress
Thermal stress decreased coral-algal physiological performance and caused significant compositional and functional changes in the microbiome of P. strigosa , indicating holobiont dysbiosis (Levy et al. 2017). According to the AKP for animal microbiomes, stressful conditions disrupt the ability of the holobiont to regulate the microbial community composition, resulting in an increase in dissimilarity (i.e., dispersion, β-diversity) among microbiomes experiencing dysbiosis (Zaneveld et al. 2017). Therefore, the AKP predicts an unstable and stochastic microbiome composition on hosts that were exposed to acute stress or disease, including corals under heat stress (Zaneveld et al. 2016, 2017; Ahmed et al. 2019). However, our results showed that β-diversity decreased in heat-stressed microbiomes at the level of bacterial genera, stress response, nitrogen metabolism, and sulfur metabolism genes (Fig.1). Heat-stressed coral SML microbiomes formed a cluster with the highest similarity among all metagenomes analyzed. The relative abundance of specific bacterial genera (Fig. 2) and gene functions (Fig. 3) changed consistently across coral replicates that were exposed to the heat treatment, while in the ambient treatment these changes were more variable and not significantly different from pre-treatment coral microbiomes. Diversity (H’) remained high both in ambient and heat conditions (Supplemental Tab. 2). Here we present a case for heat stress having a more stable and deterministic effect on coral microbiomes at taxonomic and functional level, in opposition to the AKP.