110 of glass microscopic slides were cleaned three times using 70% ethanol and laboratory wipes and were tucked into gaps made in styrofoam using a sterile scalpel. The styrofoam carrier was previously cleaned, also with 70% ethanol and placed on the sea surface so the slides were immersed into the sea. More precisely, in Kaštela Bay at the Institute of Oceanography and Fisheries in Split, in late November of 2019. At the moment of immersion, 1200 ml sea sample was collected beneath the surface and filtered through 0.22 µm pore-size MCE membrane filters (Whatman, UK).A formed biofilm was thoroughly scraped from the slides, using a sterile razor combined with frequent gentle rinsing with sterile distilled water. A scraped biofilm from each day separately was also filtered, the same way an initial sea sample was. DNeasy PowerWater Kit (Qiagen, Germany) was used to extract genomic DNA from biofilm and seawater samples. Finally, we prepared four DNA samples. The first one containing DNA from the sea organisms at the moment of immersion (sample S0) and the others containing DNA from early phase biofilm creators at different stated times (samples B1, B2 i B3), giving an insight in three-day biofilm evolution. The concentration and quality of DNA were analyzed using a NanoDrop® Spectrophotometer 1000 (Thermo Scientific, USA). The samples were sent to Novogene Europe (Cambridge, UK) for 16S rRNA amplicon metagenomic sequencing and NGS analysis, with applied primers for the bacterial V3-V4 amplified region.