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• At an optimal concentration of 25 ppm, Padina pavonica and Sargassum tenerrimuman obtained 75 and 73% inhibitory efficiency, respectively. • The Extracellular Electron Transfer (EET) of Bacillus megaterium was inhibited using the methanolic extraction of seaweeds. • The bioactive compounds of seaweeds are more potential to inhibit bacterial growth and prevent the biofilm layer. • MEPP and MEST revealed the existence of organic compounds like N, S, O blocking electron transfers reduce the corrosion rate. Biocorrosion is an aggregation of microbes on the metal surface that induces ion degradation by oxidation and reduction of chemical and electrochemical reactions. Extracellular electron transfer (EET) process was a significant phenomenon in ion deterioration. To block the EET process, the inhibitor was used to destroy or prevent extracellular electron transfer during microbial metabolism. In the present study, methanolic extraction of two different seaweeds acts as a bio-inhibitor against the corrosive biofilm forming bacteria B.megaterium SKR7 on the mild steel. The optimized concentration of biofilm inhibition efficiency of methanolic extract P.pavonica (MEPP) and S.tenerrimum (MEST) was found to be 75% and 73% at 25 ppm respectively. It was confirmed by assay of antimicrobial activity and weight loss measurement which is represented the reduction of corrosion rate S3 (A) 0.355 mm/y and S3 (B) 0.442 mm/y of both seaweed extracts than control (1.598 mm/y). This observation was confirmed by the electrochemical studies by the significant decrease in charge transfer resistance (R ct) and the surface analysis of FTIR and SEM analysis were confirmed the seaweeds inhibit the biofilm. MEPP and MEST revealed the existence of organic and bioactive compounds like N, S, O blocks the electron transfers during the biofilm formation on a metal surface and thus inhibits corrosion rate. The study concluded seaweed extracts act as mixed-type of inhibitors, thus effectivley inhibit the biofilm formation and are easily accessible at low cost, eco-friendly. [ABSTRACT FROM AUTHOR]