18S V4 amplicon sequencing of BioMarKs (2009-2010) DNA/cDNA samples

Seawater samples were collected through the BioMarKs consortium in six European coastal stations: offshore Blanes (Spain), Gijon (Spain), Naples (Italy), Oslo (Norway), Roscoff (France), and Varna (Bulgaria). Water samples were taken with Niskin bottles attached to a CTD rosette at surface and deep chlorophyll maximum (DCM) depths. These samples were pre-filtered through 20-μm filters and afterwards, they were sequentially filtered through 3- and 0.8-μm polycarbonate filters (diameter: 142 mm). Filtration time did not surpass 30 min to avoid RNA degradation. Filters were flash-frozen and stored at −80 °C. Sediment samples were taken with sediment cores and small aliquots were frozen at −80 °C for downstream molecular analysis.