Abstract :
[en] Marine macroalgae are highly colonized by microorganisms, with which they maintain a close relationship characterized by both beneficial and detrimental interactions. Alga-associated bacteria have notably developed a range of enzymes enabling them to colonize the host surface and to use algal biomass as a carbon source. The hydrolytic potential of these bacteria, however, has been investigated almost solely at individual scale. Studies have shown the ability of some seaweed-associated bacterial strains to hydrolyze lipids, algal-cell-wall polysaccharides, and other sugars.
In this work we aimed to investigate the hydrolytic potential of the bacterial microbiota associated with the brown alga Ascophyllum nodosum. For this we employed two complementary function-based approaches: functional metagenomics applied to this microbiota and functional analysis of the cultivable fraction thereof.
By functional metagenomics, we identified numerous esterase genes, a beta-glucosidase gene, and an endocellulase gene. The cellulase was purified and biochemically characterized, showing interesting biotechnological features such as halotolerance and activity at low temperature. Furthermore, we assigned tentative origins to the identified genes, thus getting a glimpse of the bacterial taxa associated with the studied alga.
Secondly, we investigated the cultivable surface microbiota associated with three A. nodosum samples. More than 300 bacteria were isolated, assigned to a bacterial taxon and screened for algal-polysaccharide-degrading enzymes (agarase, iota-carrageenase, kappa-carrageenase, and alginate lyases). This allowed the identification of several polysaccharolytic isolates, some of them likely to be new strains or novel species, belonging to two classes: the Flavobacteriia and the Gammaproteobacteria. Subsequently, we constructed and screened two plurigenomic libraries, each produced with the genomes of five representative isolates of each class, and identified several functional genes.
With this work we highlight the presence of A. nodosum-associated bacterial taxa likely to entertain a privileged relation with seaweeds and having developed a range of hydrolytic activities assumed to enable them to associate with algae. We also provide information (taxa, abundances, genomic potential) on macroalgal-polysaccharide-degrading bacteria, in which interest has grown over the last ten years.
