Distinct communities of Bacteria and unicellular eukaryotes in the different water masses of Cretan Passage water column (Eastern Mediterranean Sea)

Understanding the diversity and dynamics of marine microbiota holds significant importance due to their role in maintaining vital ecosystem functions and services including climate regulation and bioremediation. Here, we studied the diversity and associations between Bacteria and unicellular eukaryotes in the different water masses of the Cretan Passage water column in the Eastern Mediterranean Sea (EMS). Samples were collected from two stations in the Hellenic Exclusive Economic Zone (EEZ) at various depths down to 1000 m during two sampling expeditions in August 2019 and February 2020. Through high-throughput 16S and 18S rRNA gene analysis, we unveiled vertical variations in both bacterial and unicellular eukaryotes diversity respectively. Additionally, interspecies co-occurrence patterns were evaluated between the top and bottom water masses. Our results revealed species fluctuations indicative of seasonality in the surface water mass while the deepest water layers were enriched in heterotrophic taxa and grazers related to organic matter degradation and nutrient cycling. Finally, we found a higher number of microbial associations in surface waters indicating abundant ecological niches compared to the deepest layer, possibly related to the lack of bottom-up resources in the oligotrophic deep ocean.