To follow one of the main topics of the blog (bacteria-coral relationship), I review this article on bacterial communities associated with a Mediterranean gorgonian coral, Paramuricea clavata. This species is considered a key species for coralligenous assemblages and his presence and abundance is also considered as bio-indicator of high environmental quality. Together with Posidonia oceanica seagrass meadow, coralligenous accretions are the higher biodiversity environments in the Mediterranean Sea. P. clavata is a long-lived aposymbiotic colonial octocoral with long branches and bushy colonies that also contribute to improve environmental eterogenity of corralligenous accretions.
(picture from google)
Authors in this article present a spatial and temporal study of bacterial diversity associated with this octocoral. They also underline that it represent a pioneer baseline work because there is a lack of informations on bacterial communities associated with temperate water octocoral with respect to scleractinian reef-forming corals. They collected 4 years (2007–2010) seasonal sampling in 3 distinct sites in NW Mediterranean Sea, separated by hundreds of kilometers (Provence, Corsican and Catalan coasts), and also exposed to different grade of anthropogenic stressors.
To evaluate which bacteria might be conserved across geographically remote P. clavata populations, authors use three molecular culture-independent approaches (denaturing gradient gel electrophoresis (DGGE), terminal-restriction fragment length polymorphism (T-RFLP) and 16S RNA gene clones library construction).
One of the first presented result is the high grade of consistency between 16S rDNA DGGE banding profiles from all the samples indicating high similarity in bacterial composition between 3 sites. This broadly similar bacterial composition suggest that it might be not driven by local environmental and single colony-related factors. Extracting and sequencing this DNA and relating it to a correspondent ribotype, they found these bacteria belonging to Hahellaceae family within Oceanospirillales order (class Gammaproteobacteria). They note also that this Oceanospirillales-affiliated sequences is the most frequent ribotype found in healthy hexacorals suggesting a specific cnidarian symbiotic complex but with a different host selective control among hexa- and octocoral. In Hahellaceae family, authors also were able to detect Endozoicomonas- (96% similar to bacteria found in tropical gorgonian Gorgonia ventalina) and Spongiobacter-related ribotype. Spongiobacter-related metabolize DMSP in A. millepora (hexacoral) but the potential functioning in gorgonian holobiont are unknow for the moment. Authors also suppose other fisiological roles for Oceanospirillales as the production of extracellular hydrolytic enzymes maybe usefull for trophic relationship among host and symbiont.
Another result presented was a transient compositional community shift during summer 2007 in all 3 sites. Combination of bacteria was different and ribotypes show a higher diversity distributed among 5 bacterial phila: Firmicutes, Actinobacteria, Proteobacteria, Bacteroidetes and Cyanobacteria. Belonging to the first two group respectively, they report prominent Paenibacillus- and Propionibacterium-related sequences (found also in low abundance in healty and/or bleached colonies of cold-water and tropical scleractinian corals). Because authors found this change in microbiota composition only during 1 sampling period they present it like a disruption (without any visible signs of desease of P. clavata colonies) of normal Hahellaceae association. The situation shift back in the following winter sampling with again Hahellaceae dominated community allowing authors to suppose this as normal symbiont for P. clavata. Causes of this transient shift in 3 geographically distant population during 2007 summer are not clear but authors link it to altered physiological state of the holobiont during stress conditions. Interestingly they didn’t find Vibrio coralliilyticus in Paenibacillus-dominated summer 2007 clone libraries, although this Vibrio and other has been implicated in recent disease outbreaks and tissue necrosis in P. clavata populations during climatic anomalies in Mediterranean Sea. Authors didn’t ruled-out hypothesis like anthropogenic and abiotic stressor or bacteriophage infection targeting Hahellaceae.
In conclusion this study was the first on Mediterranean octocoral-associated microbiota and should be very interesting understand something more on the in-situ location of these bacteria within gorgonian tissue and also something more on possible physiologic functions of symbionts.
La Rivière, M., Roumagnac, M., Garrabou, J., & Bally, M. (2013). Transient Shifts in Bacterial Communities Associated with the Temperate Gorgonian Paramuricea clavata in the Northwestern Mediterranean Sea. PloS one, 8(2), e57385.