design: Surface microlayer sample from the Baltic Sea (500 ml) filtrated onto 0.2 m pore-sized supor filters. For experimental details see corresponding publication. Extraction with Qiagen Plant Easy mini kit. Triple PCR amplification of the V3-V4 region of the 16S rRNA gene using the Bac341F primer (5'-CCT ACG GGN GGC WGC AG-3') and the Bac805R primer (5'-GAC TAC HVG GGT ATC TAA TCC-3') with overhang adapters. The PCR product was cleaned AMPure XP beads and normalized to 4 ng/l.