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.