Bacterial epibiosis on the invasive seaweed 'Caulerpa cylindracea' (Sonder) in the Mediterranean sea: potential ecological implication

Abstract

In the Mediterranean Sea, biodiversity is strongly threatened by biological invasions. Some introduced seaweeds often have the potential to change sediment biogeochemistry leading to profound alterations into the functioning of ecosystems. Caulerpa cylindracea (Sonder) is an non-indigenous invasive macroalga widely distributed in all Mediterranean habitats, representing a serious threat for native assemblages. The seaweed can harbour microbial communities and, in turn, epiphytes could be a source of bioactive compounds. In the present thesis, C. cylindracea was collected in Torre Guaceto (Brindisi, Italy) on the rocky shallow substrates in four sampling times in order to study the associated epibacterial community. Cultured and uncultured bacteria have been compared to describe the microbial community associated to C. cylindracea. Gamma-Proteobacteria belonging to the genera Shewanella and Vibrio have been found by both approaches on the algal surface consistently in time, along with an unknown species belonging to the Rhodobacteraceae. Other taxa belong to Bacillus, Pseudoalteromonas, Tropicibacter, Photobacterium, Exiguobacterium, Kocuria, Ruegeria and Marinobacter genera have been discovered with the culturable approach. The isolated Vibrio were identified to species level. Most of the observed C. cylindracea-Vibrio associations were consistent in time, by contrast, only few seem to be sporadic. Moreover, the cultural and molecular approaches were used to analyze the microbial metabolic profiles associated to the surface of the seaweed collected in different sites across the central Mediterranean Sea. These results showed that there is a distinct group of bacteria consistently present on C. cylindracea, irrespective of its geographical origin. Finally, in the five sites, the microbial metabolic patterns associated to C. cylindracea surface, as well as to sediments uncolonized and colonized by C. cylindracea, were compared. Moreover, in the same sites the biochemical composition of the organic matter was analysed both in sediment invaded and not-invaded by C. cylindracea. Results revealed greater organic matter concentration in seaweed colonized sediments than in seaweed uncolonized ones. In conclusion, Caulerpa cylindracea appears to host a specialized bacterial community with a functional diversity potentially playing a crucial role in the biogeochemistry of the sediment and in the patterns of algal invasions.