Bacterial diversity in the South Adriatic Sea during winter of 2016

Bacteria represent the most important microorganisms in the world oceans comprising up to 75% of the total biomass at surface and playing fundamental processes for biogeochemistry. Along the water column, they are useful markers of the trophic state of water masses, therefore representing useful ecological indicators in the surveyed areas. In this study in the southern Adriatic Sea bacteria were quantified by flow cytometry and their diversity assessed by high-throughput sequencing (HTS). In addition, the most represented bacterial groups were quantified by qPCR. The samples were collected from the surface to the seabed, at a total of 16 different depths at four stations in the southern Adriatic Sea, during the late winter BIOTA (BIO-Tracing Adriatic water masses) cruise conducted in 2016. The investigated area showed an unusual circulation characterized by a mixed layer down to 200 m, which differed from the usual winter convection conditions, typical of middle-altitude ecosystems and important for the seasonal picoplankton dynamics in the South Adriatic Sea. Heterotrophic bacteria were separated into HNA (relative High Nucleic Acid content) and LNA (Low Nucleic Acid content) subpopulations with abundances up to 1.8 x 105 and 8.8 x 105 cells/mL, respectively. HNA populations dominated at offshore stations reaching their maximum at depths below the euphotic zone. The bacterial community was dominated by Alphaproteobacteria accounting for > 40% of the total, represented by the SAR11 clade (90.84%), followed by Marinimicrobia (18% of the total), mainly represented by clade SAR406 (8.44%). However, distinctive bacterial groups were found in euphotic layer (Bacteroidetes and Actinobacteria) and aphotic layer samples (Deltaproteobacteria, Marinimicrobia, Chloroflexi, Acidobacteria and Planctomycetes). qPCR assays confirmed HTC results with abundances of targeted bacterial groups found to vary between different sampling points and sampling depths and values ranging from 8.7 x 103 to 9.13 x 105 copy number/mL for Alphaproteobacteria, from 1.44 x 104 to 5.11 x 105 copy number/mL for Gammaproteobacteria and from 1.8 x 103 to 2.4 x 105 copy number/mL for Bacteriodetes. For all targeted taxa higher abundances were found in the euphotic than in the aphotic zone. Introducing of different methodological approaches to the study allowed us to get better insight into the structure of the bacterial assemblages in the South Adriatic.