Revealing the impact of global mass bleaching on coral microbiome through 16S rRNA gene-based metagenomic analysis

Coral bleaching, a phenomenon by which the expulsion of corals' alveolate endosymbiont (zooxanthellae) occurs when experiencing thermal stress is the major cause for devastation of corals. However, apart from this obligate symbiont of Scleractinian corals, there are different kinds of microbes that exist as stable, transient or sporadic members of the holobiont which reside within various microhabitats in the coral structures. Thus, this study aims to profile the coral bacterial community composition among different coral genera (thermally-sensitive (Acropora digetifera and A. noblis) and thermally resistant (Favites abdita) coral genera analyzed by field monitoring surveys) and also in a particular coral genus (thermally sensitive coral-A. digetifera) at two different sampling times (March 2016 and January 2017). A total of about 608695 paired end reads were obtained through Illumina MiSeq Sequencing platform. The alpha diversity indices (ACE, Chao1 and Shannon) were found to be higher in A. nobilis, followed by A. digetifera and Favites abdita, and the corresponding Simpson values were also found to follow the same trend, indicating that the samples are both rich in species diversity and species evenness. Proteobacteria was found to be the most dominant phylum and Gammaproteobacteria was the predominant class present in all the coral genera studied as also during different sampling time periods. As Vibrionaceae was previously reported to increase its abundance during bleaching stress conditions, bacterial profiling among different coral genera showed the presence of 86 % Vibrionaceae in A. digetifera colonies, and it was 93 % in A. digetifera samples collected during March 2016 whereas, it was found to decrease significantly (7 %) in same tagged colonies collected during January 2017. Thus, profiling of microbiome is of prime importance while studying the holobiont organism like the corals. Stress levels experienced by Palk Bay are even depicted in this microbiome study showing high alpha diversity indices that should alarm reef managers to pay attention to this precious stress tolerant reef community.