Distribution patterns of bacterioplankton in the oligotrophic south-eastern Mediterranean Sea

The spatial variability in the composition of bacterioplankton communities from the Levantine basin offthe Israeli coast (south-eastern Mediterranean Sea) was studied using water samples from nine stations down to 1880 m depth. An nMDS (non-metric multidimensional scaling) ordination plot of ARISA (automated rRNA intergenic spacer analyses) fingerprints performed on 39 water samples revealed three groups of communities based on depth (epi, meso and bathypelagial waters), while the stations' geographical location did not seem to have an effect on the distribution of bacterioplankton assemblages. Interestingly, communities from the two deep pelagic zones were further grouped depending on the three prevailing water masses in the Levantine basin, and depth, salinity, temperature and silicic acid were significantly related to the variations of the bacterial community compositions. Cloning of internal transcribed spacer (ITS) sequences of dominant ARISA peaks showed that abundant bacteria in all depths were ubiquitous uncultured Alpha- and Gammaproteobacteria. The results further indicate that distinct microbial community profiles in the Levantine basin are associated with prevailing water masses that have formed due to different physico-chemical parameters and thus might act as physical barriers rather than physiological discriminators, as has been suggested in studies from other ocean regions.