Composition of prokaryotic and eukaryotic microbial communities across the water column of the Eastern Mediterranean Sea

The waters of the Eastern Mediterranean Sea (E) present numerous, complex oceanic processes and are stratified into three distinct layers based on thermohaline circulation. The upper surface (0150m) consists of Atlantic Water (AW) entering from the Straits of Gibraltar, the Levantine Intermediate Water (LIW) accumulates in water depths from 150400m moving westwards and the Eastern Mediterranean Deep Water (EMDW) occupies depths beyond 400m. The oceanographic profile of this semi-enclosed basin in combination with its unique properties (high salinity, elevated bottomsea temperatures, phosphorus limitation) drives the selection of distinct microbial taxa and determines the structure of microbial communities. To our knowledge, microbial community studies across the E water column have been performed near the coasts of Israel and the Nile Delta River while in areas south of Crete (Northeastern Mediterranean), molecular studies focused only on eukaryotic communities or on particular strains. For this study, two sampling cruises were performed off South Crete, near Koufonisi (August 2019) and Gavdos (February 2020) during which seawater was retrieved from several depths across the three-water layer system. In total, fourteen (14) samples were dispatched for DNA metabarcoding analysis of prokaryotic taxa and nine (9) for eukaryotic. In addition, CTD data were recorded during the sampling cruises and nutrient analysis was also performed to address which environmental parameters impact the microbial community composition. Finally, correlation network analysis indicated positive and negative interactions between the two life domains of the surface (AW) and deep (EMDW) water layers. Overall, this work ai to enrich our insight regarding the composition and interactions of background prokaryotic and eukaryotic communities in the E water column.