Heterotrophic bacteria associated with the green alga Ulva rigida: identification and antimicrobial potential

Heterotrophic bacteria associated with the green alga Ulva rigida, collected from the coast of Tunisia, were isolated and subsequently identified by their 16S rRNA gene sequences and by phylogenetic analysis. The 71 isolates belong to four phyla: Proteobacteria (Alpha-and Gamma- subclasses), Bacteroidetes, Firmicutes, and Actinobacteria. Most of the isolates belong to Proteobacteria. The Gram-positive Firmicutes and especially the genus Bacillus were well-represented at the surface of U. rigida, collected from the coast as well as from the lagoon, while Actinobacteria were represented only at the surface of algae collected from the coast of Cap Zebib. Bacteroidetes were more represented at the surface of algae collected from the Ghar El Melh lagoon. The bacterial community of the water surrounding the algae was different from that associated with the surface of the algae. Moreover, the abundance of bacteria in the surrounding water was much lower compared to the density of bacteria associated with the surface of the algae. Bacteria isolated from the algal surface were tested for their antimicrobial potential. The results show that ~ 36% of the algae-associated bacterial isolates possess antibacterial activity whereas free-living bacteria, isolated from the surrounding water, did not show such activity. The surface of U. rigida was colonized by a high diversity of culturable and possibly novel epiphytic bacteria that may be an important source of antimicrobial compounds and are therefore of biotechnological interest.