Evidence for host specificity among dominant bacterial symbionts in temperate gorgonian corals
Résumé
Gorgonian corals serve as key engineering
species within Mediterranean rocky-shore communities
that have recently suffered from repeated mortality events
during warm temperature anomalies. Among the factors
that may link thermal conditions with disease outbreaks, a
number of bacterial pathogens have been implicated; they
may take advantage of decreases in the defenses and/or
overall health of the gorgonian hosts. Considering the
beneficial role of the resident bacteria in tropical coral
holobionts, a detailed characterization of the gorgonianassociated
microbial populations is required to better
understand the relationships among native microbiota, host
fitness, and pathogen susceptibility. In this study, the bacterial
communities associated with three sympatric gorgonian
species, Eunicella singularis, Eunicella cavolini,
and Corallium rubrum, were investigated to provide insight
into the stability and the specificity of host–microbe
interactions. Natural variations in bacterial communities
were detected using terminal restriction fragment length
polymorphism (T-RFLP) of the 16S ribosomal DNA. No
major differences were identified between individual
colonies sampled in winter or in summer within each
gorgonian species. Although hierarchical cluster analysis
of the T-RFLP profiles revealed that the three species
harbor distinct communities, comparison of the T-RFLP
peaks indicated the presence of common bacterial ribotypes.
From phylogenetic analysis of 16S rDNA clone
libraries, we identified a bacterial lineage related to the
Hahellaceae family within the Oceanospirillales that is
shared among E. singularis, E. cavolini, and C. rubrum and
that dominates the communities of both species of Eunicella.
However, distinct clades of Hahellaceae are harbored
by various gorgonian species from Mediterranean
and tropical waters, suggesting that these bacteria have
formed host-specific symbiotic relationships with gorgonian
octocorals. In addition, the relatedness of symbionts
from host species belonging to the same taxon but occurring
in geographically remote areas is consistent with
codivergence between gorgonians and their associated
bacteria.