• Deep-chlorophyll maximum;
  • Western Mediterranean;
  • Picoplankton;
  • Biodiversity;
  • 16S rDNA


We have applied a simple methodology based on restriction endonuclease digestion of the total bacterial and archaeal 16S rDNA amplified from sea water samples to compare the prokaryotic diversity present along a transect from coastal to offshore waters in the Mediterranean basin (from Barcelona to the Balearic Islands). Samples from the surface and deep-chlorophyll maximum (DCM) were obtained. Temporal variation during a short span was also investigated by repeating the sampling after 48 h. The patterns of digestion bands from the samples were compared to detect major changes in the bacterial or archaeal groups present. For bacteria the main difference was found between the free-living community and that retained by the eukaryotic filter and assumed to be particle attached. By contrast Archaea collected by the same means did not appear as a separate group. The other main variation was depth-related, with remarkably different communities at the surface, at the DCM, and a single sample taken at 400 m depth. On the other hand, the variation along the transect from the continental platform to an offshore (2000 m maximum depth) station was relatively small. Temporal (48 h) and small scale spatial (a few km) variation was minimal.