Conclusion
The future of coral reefs depends on a healthy coral microbiome, as coral bleaching and disease outbreaks are some of the leading causes of an unprecedented loss in coral cover worldwide, particularly in response to rising seawater temperature (Miller et al. 2009; Maynard et al. 2015; Heron et al. 2016; Precht et al. 2016). We used shotgun metagenomics in an experimental setting to understand the dynamics of microbial taxa and genes in heat-stressed corals, in the context of the AKP. The metagenomes of corals exposed to heat stress showed greater stability, therefore, not conforming to the AKP. Anti-AKP effects are expected when blooms of specific microbes or strong environmental filters are shaping animal microbiomes (Zaneveld et al. 2017). We hypothesize that this stability is the result of a selective pressure towards a beneficial microbiome that supports the holobiont to withstand stress (Fig 4.). Despite the significant decline in physiological performance, the coral colonies of P. strigosa from Bermuda showed an overall tolerance as none of the coral replicates bleached after exposed to high temperatures for one week. The microbiome of heat-stressed corals could have played a role in preventing bleaching by controlling opportunistic pathogens and providing nitrogen and sulfur to the algal symbiont and energy to the coral host (Fig. 4.).
Other factors could help to explain the anti-AKP effects seen here. AKP in coral microbiomes under heat stress were reported after long-term exposure (2 – 3 years) (Zaneveld et al. 2016; Ahmed et al. 2019), while this study tested for a shorter heat exposure (1 week). There could be host species-specific differences in the microbiome response of P. strigosa (present study) compared to Siderastreasiderea , Porites spp., and Agaricia spp. (Zaneveld et al. 2016) and to anemone Aiptasia used as a coral model (Ahmed et al. 2019). Also, these studies used amplicon metagenomics (16S rRNA) at the taxonomic level of family and order, contrasting with the shotgun metagenomics pipeline at the level of microbial genus and gene functions used here. Therefore, the AKP for coral microbiomes under heat stress need to be further investigated across different methodologies and systems.
Promising conservation efforts have been focusing on promoting and maintaining coral microbiome health (Damjanovic et al. 2017; Epstein et al. 2019; van Oppen and Blackall 2019), including the development of coral probiotics (Peixoto et al. 2017; Rosado et al. 2019; Santoro et al. 2021). Here we support these efforts by showing that heat stress can lead to stable and potentially beneficial coral microbiomes.