PROKARYOTIC DISTRIBUTION ALONG A PH GRADIENT AT SHALLOW HYDROTHERMAL VENT OF VULCANO ISLAND (ITALY)

Shallow hydrothermal systems evoke primordial conditions and represent models to investigate the effects of the ocean acidification and warming on coastal environments. Surface sediment samples were collected along a pH gradient, at increasing distance from the acidic primary vent (PV). Prokaryotic community abundance and diversity were examined by 16S rRNA Fluorescence in situ Hybridization and PCR-Denaturing Gradient Gel Electrophoresis (DGGE), respectively. Bacteria numerically dominated over Archaea in all sites, however the archaeal contribution to the entire prokaryotic community increased at PV, where the lowest pH value was recorded. As emerged by DGGE pattern analyses, bacterial and archaeal richness and diversity (Shannon index) increased in the vicinity of PV, indicating that venting conditions result in high diversity of prokaryotic communities. Phylogenetic analyses revealed that Epsilonproteobacteria (chemosynthetic producers) prevailed at PV, whereas Cyanobacteria (photosynthetic producers) and Bacteroidetes (heterotrophs able to degrade complex biopolymers) dominated at increasing distance from PV, according with the increasing of pH. All achaeal sequences from DGGE bands were affiliated with uncultured members of Euryarchaeota and Crenarchaeota. Differently from Euryarchaeota, Crenarchaeota increased with the proximity to the acidic vent, determining a marked shift in the archaeal population composition. Our results revealed the contrasting effects of venting on the benthic bacterial and archaeal communities. The proximity to the principal vent, with a concomitant pH decreasing, seems to provide trophic and ecological interactions resulting in assemblages of different phylogenetic and metabolic microorganisms.