Abstract
The importance of
Symbiodinium
algal endosymbionts and a diverse suite of bacteria for coral holobiont health and functioning are widely acknowledged. Yet, we know surprisingly little about microbial community dynamics and the stability of host‐microbe associations under adverse environmental conditions. To gain insight into the stability of coral host‐microbe associations and holobiont structure, we assessed changes in the community structure of
Symbiodinium
and bacteria associated with the coral
Pocillopora verrucosa
under excess organic nutrient conditions.
Pocillopora
‐associated microbial communities were monitored over 14 days in two independent experiments. We assessed the effect of excess dissolved organic nitrogen (
DON
) and excess dissolved organic carbon (
DOC
). Exposure to excess nutrients rapidly affected coral health, resulting in two distinct stress phenotypes: coral bleaching under excess
DOC
and severe tissue sloughing (>90% tissue loss resulting in host mortality) under excess
DON
. These phenotypes were accompanied by structural changes in the
Symbiodinium
community. In contrast, the associated bacterial community remained remarkably stable and was dominated by two
Endozoicomonas
phylotypes, comprising on average 90% of 16S rRNA gene sequences. This dominance of
Endozoicomonas
even under conditions of coral bleaching and mortality suggests the bacterial community of
P. verrucosa
may be rather inflexible and thereby unable to respond or acclimatize to rapid changes in the environment, contrary to what was previously observed in other corals. In this light, our results suggest that coral holobionts might occupy structural landscapes ranging from a highly flexible to a rather inflexible composition with consequences for their ability to respond to environmental change.