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

Seawater samples were collected at various depths accross the water column, down to 1000 m, from two stations within the Cretan Passage, Eastern Mediterranean Sea. The sampling expeditions took place in August 2019 and February 2020. A total of 14 samples were retrieved, covering all three water masses (AW/LSW, LIW, EMDW) which were subjected in high-throughput 16S rRNA gene analysis and 9 were selected for analysis of the 18S rRNA gene to study the diversity and associations between bacteria and unicellular eukaryotes respectively. 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. A higher number of microbial associations in surface waters indicates abundant ecological niches compared to the deepest layer, possibly related to the lack of bottom-up resources in the oligotrophic deep ocean.