Seasonality of bacterioplankton community composition is driven by the vertical structure of water column at a coastal Mediterranean site. Weekly dynamics of bacterioplankton community at a coastal Mediterranean site

The weekly dynamics of bacterioplankton community was investigated at the Long-Term Ecological Research Station-MareChiara (LTER-MC) located in the Gulf of Naples (Mediterranean Sea) using flow cytometry and high-throughput sequencing (Illumina) of V4/V5 hypervariable regions of 16S rRNA gene. Flow cytometry counts evidenced pronounced seasonal variations, with peaks in summer and minima in winter, with additional peaks due to freshwater discharges. 16S rRNA gene multivariate statistical analysis at order/family level unveiled two separate clusters attributable to ‘mixed’ and ‘stratified’ water columns. Dynamics of individual taxa using biomarker LefSE analysis at the OTU level evidenced copiotrophs (member of Alteromonadales, Rhodobacterales) dominating the “mixed” period. We also observed persistent, intermittent and transient occurrence of abundant and rare taxa throughout the year with higher diversity, particularly contributed by the rare fraction, during the mixed period. Our results indicate seasonality, reflected by the contrasting structure of water column, as a key factor of temporal β-diversity of bacterioplankton, despite their ability to rapidly profit from point terrestrial discharges intermittently throughout the year.