Biogeography of planktonic microbial communities across the whole Mediterranean Sea

The M84/3 cruise recently held onboard of R/V Meteor represented a great and rare opportunity for the scientific community to realize a multidisciplinary survey on the whole Mediterranean Sea. In this context, molecular microbiology investigation, realized by applying Automated Ribosomal Intergenic Sequence Analysis (ARISA) and microscope evaluation of prokaryotic abundance, were performed on seawater samples aiming to identify the environmental factors driving planktonic bacterial community composition across both vertical and longitudinal transects. Prokaryotic abundance decreased along with depth in all the stations and presented similar values in sub-surface, meso- and bathypelagic layers across the whole Mediterranean basin. On the contrary, peculiar bacterial assemblages were selected along a longitudinal transect in the surface layers of the eastern and western sub-basins. Sharp vertical profiling of the bacterial communities was observed only considering the boundary of the water column, while the study of bacterial β-diversity at finer scale across the water column displayed higher variability at the intermediate layers. Nonetheless, different physico-chemical factors were significantly related to microbial zonation both according to geographic position and across the water column in the whole Mediterranean Sea. These results demonstrated that bacterial diversity is putatively correlated to different water masses across the water column of the complex hydrographical systems of the eastern and western Mediterranean sub-basins.