Spatial patterns of bacterial abundance, activity and community composition in relation to water masses in the eastern Mediterranean Sea

To determine the variation of bacterial activity and community composition between and within specific water masses, samples were collected throughout the water column at 5 stations in the eastern Mediterranean Sea corresponding to the regions of the northern Aegean, mid-Aegean, western Cretan, Ionian and southern Aegean Seas. Prokaryotic abundance below 100 m declined with depth at all the stations, while decreasing trends with depth in prokaryotic heterotrophic activity were present only at 2 out of the 5 stations. Bacterial community composition (BCC), determined using both automated rRNA intergenic spacer analysis and terminal-restriction-fragment length polymorphism, was also related to depth although the number of operational taxonomic units was remarkably constant throughout the water column. Overall, the maximums in similarity values of the BCC between water sample pairs decreased with increasing temperature-salinity (T-S) distance of the water samples probably due to distinct biogeochemical characteristics of water masses. However, considerable dissimilarity in the BCC between samples with identical T-S values, and hence within the same water mass, was also observed, possibly reflecting heterogeneity in the organic matter field or in biotic control within a given water body. Thus, we conclude that the richness of bacterial communities is remarkably constant with depth down to bathypelagic waters. The similarity of bacterial communities in water parcels with identical temperature and salinity can range from highly similar to very dissimilar, reflecting variability in substrate supply despite the physical uniformity of water parcels.