Algicidal　bacteria　exhibit　promising　potential　against　harmful　algal　blooms　(HABs);　however,　their　application　has　been　limited　due　to　their　limited　algicidal　activity.　This　study　demonstrates　the　enhanced　algicidal　activity　of　Alteromonas　sp.　FDHY-CJ　bacteria　against　harmful　Skeletonema　costatum　using　a　5　L　fermenter.　Utilizing　this　refined　framework　increased　the　OD600　value　and　algal　cell　mortality　by　6.50　and　2.88　times,　respectively,　compared　to　non-optimized　culture　cultivated　in　a　flask　using　marine　broth　2216E　medium.　The　mechanism　of　action　involves　significant　inhibition　of　algal　photosynthetic　efficiency　with　concurrent　degradation　of　photosynthetic　pigments.　Relative　to　the　non-optimized　group,　the　optimized　bacterial　treatment　led　to　a　significant　increase　in　H2O2　and　MDA　(malondialdehyde)　by　19.54　and　4.22-fold,　respectively,　and　resulted　in　membrane　damage.　The　culture　optimization　procedure　yielded　effectual　algicidal　substances　capable　of　considerably　reducing　the　severity　of　S.　costatum　HABs　through　cell　membrane　disruption.