Comparative effects of 405 nm and 450 nm blue light on Halomonas pacifica biofilms: implications for marine antifouling

Marine biofouling poses a significant challenge for ocean engineering, with microbial biofilm formation being a critical prerequisite for macrofouling colonization. Traditional chemical antifouling strategies face increasing restrictions due to ecological toxicity and microbial resistance, creating an urgent demand for eco-friendly physical alternatives. This study aimed to systematically compare the wavelength-dependent effects of 405 nm and 450 nm antimicrobial blue light on Halomonas pacifica biofilm formation. Biofilms were constructed on glass slides and continuously irradiated at 50 mW/cm2 for 4 days (cumulative radiant exposure about 17,280 J/cm2), with a non-irradiated control. Total biomass was quantified by crystal violet staining, three-dimensional morphology was analyzed by white light interferometry, and bacterial viability was visualized by confocal laser scanning microscopy with DMAO/PI staining. The 450 nm treatment significantly reduced total biofilm biomass by 20.9% (P < 0.05), while 405 nm decreased biomass by 15.5% without statistical significance. In contrast, 405 nm strongly suppressed vertical growth, reducing average dry thickness by 77.2% (P < 0.01) and arithmetic mean roughness (Ra) by 55.0% (P < 0.01), whereas 450 nm optimized overall micromorphology by reducing maximum height (Sz, P < 0.05). Confocal imaging revealed dense continuous biofilms in controls, sparse fragmented structures in the 450 nm group, and intermediate density in the 405 nm group. These results indicate that the two wavelengths regulate biofilm formation through distinct mechanisms: 450 nm excels in inhibiting proliferation and reducing biomass, while 405 nm is superior in suppressing vertical thickening and minimizing attachment sites for subsequent macrofouling. This study demonstrates that 405 nm and 450 nm blue light are promising complementary strategies for eco-friendly marine antifouling.