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

Abstract

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.

Downward fluxes of sinking particulate matter in the deep Ionian Sea (NESTOR site), eastern Mediterranean: seasonal and interannual variability

“…A deep chlorophyll maximum is found at around 80-100 m depth (Boldrin et al, 2002;Casotti et al, 2003;Crombet et al, 2011;Karageorgis et al, 2012;Rabitti et al, 1994) exhibiting low seasonality that is related to the stable 7238 S. Stavrakakis et al: Downward fluxes of sinking particulate matter in the deep Ionian Sea oligotrophic conditions of the area (Casotti et al, 2003). In general, previous studies depict that the oligotrophic Ionian Sea is dominated by a complex microbial food web (review by Siokou et al, 2010;Yokokawa et al, 2010), and dissolved organic matter (DOM) is partially bioavailable in both surface and mesopelagic and bathypelagic layers, as witnessed by the important correlation of DOC with AOU and the chemical characteristics of DOM (Meador et al, 2010).…”

Bacterial assemblages of the eastern Atlantic Ocean reveal both vertical and latitudinal biogeographic signatures

“…Although heterogeneous environmental conditions and geographic separation have a strong influence on the biogeographic distribution of species, only recently have we begun to understand how these conditions may define distinct microbial communities in marine habitats (Giovannoni and Stingl, 2005;Martiny et al, 2006b;Pommier et al, 2006;DeLong, 2009;Fuhrman, 2009). For example, ocean water masses are frequently associated with unique microbial communities (Yokokawa et al, 2010;Varela et al, 2007;Galand et al, 2010;Hewson et al, 2009;Agogue et al, 2011). However, it has been much more challenging to assess differences in microbial communities across adjacent oceanographic biomes and, in particular, across those with complex physical hydrographic features.…”

“…Very little is known about the distribution of bacteria in the ocean as it relates to physicochemical parameters or ocean biogeochemistry. Moreover, what little information we do have has been largely regionspecific (Hewson et al, 2006;Galand et al, 2009;Yokokawa et al, 2010), limiting both our understanding of the factors that structure ocean bacterial communities across biomes as well as our ability to study biogeographic patterns.…”

Variations in Microbial Community Structure through the Stratified Water Column in the Tyrrhenian Sea (Central Mediterranean)

“…Studies on meso-and bathy-pelagic Atlantic Ocean metagenomes suggested that microbial population structure could be correlated with particular water masses, determining differences in microbial assemblages thriving at the same depths, but at different geographical location [16][17][18]. The studies performed in the Eastern Mediterranean Sea highlighted the localized distinctions between samples collected within the same water masses from different sampling stations, probably due to local chemical differences, such as organic matter load [12,13].…”

“…Previous studies that have investigated microbial community changes through the water column showed a general trend of decreasing prokaryoplankton abundance with increasing depth, along with a shift from a light-scavenging to a heterotrophic lifestyle [7,12,13].…”