Pocillopora verrucosa metagenome

The functional importance of bacteria to coral holobiont health and functioning is increasingly acknowledged. Yet, we know surprisingly little about bacterial community dynamics and how they relate to holobiont performance under adverse environmental conditions. We assessed coral stress response to excess organic carbon and excess organic nitrogen provisioning over 14 days using an integrative holobiont framework approach. This involved combined physiological measurements and next-generation sequencing of the Pocillopora verrucosa coral host, associated algal endosymbionts, and the bacterial community. Both excess nutrients rapidly altered seawater bacterial communities and disrupted coral holobiont functioning. The latter was reflected in changes in the algal symbiont population structure and abundance, coral bleaching, and ultimately coral mortality (> 90 % tissue loss). Conversely to the coral holobiont breakdown, the associated bacterial communities remained stable and dominated by two Endozoicomonas phylotypes (90 % of sequences on average) throughout the entire experiment, even when coral physiology was clearly impaired and mortality became apparent. The remarkably stable relationship of Endozoicomonas with P. verrucosa suggests an important functional role of Endozoicomonas, potentially rendering them quasi-obligate symbionts of corals, just like the algal endosymbionts. As such, our study extends our current understanding of coral holobiont structure by advocating the concept of quasi-obligate symbiotic bacteria. Given the diversity of hosts and environments that Endozoicomonas associate with, they might serve as a bacterial model genus to further understand host-microbe associations and adaptation.