Antagonistic activity of Bacillus altitudinis VUMS1 against ice-ice-associated Vibrio spp. and its biotechnological potential in seaweed-associated systems.

Ice-ice disease caused by pathogenic Vibrio spp. is a major constraint in the aquaculture systems of Kappaphycus alvarezii. The use of beneficial bacteria as biological control agents represents a sustainable alternative to chemical treatments in seaweed aquaculture. In this study, an endophytic bacterium isolated from healthy K. alvarezii thalli, designated VUMS1, was identified as Bacillus altitudinis based on 16 S rRNA gene analysis. VUMS1 exhibited strong antagonistic activity against Vibrio harveyi, V. owensii, and V. alginolyticus, producing inhibition zones ranging from 9.0 to 16.5 mm on Day 1 and increasing to 17.5-19.0 mm by Day 5. The strain also inhibited Staphylococcus haemolyticus, Shigella dysenteriae, and Enterococcus faecalis, with inhibition halos reaching up to 27 mm after 72 h. Co-culture assays demonstrated substantial suppression of Vibrio growth over time. Relative to the control, V. harveyi, V. owensii, and V. alginolyticus exhibited marked reductions in CFU/mL, reaching 90.2%, 87.5%, and 93.0% reductions by Day 5, respectively. VUMS1 displayed relevant functional traits, including DNase and caseinase activities, phosphate solubilisation (PSI 4.00 ± 0.71), siderophore production (SPI 2.00 ± 0.58) and measurable IAA synthesis. Despite these plant growth-promoting characteristics, VUMS1 treatment alone did not significantly enhance seaweed growth or biomass under the experimental conditions tested. However, two-way ANOVA revealed significant effects of treatment, time, and their interaction on relative biomass (p < 0.01), driven by substantial biomass reduction in the V. owensii-challenged group and mitigation of biomass loss in the VUMS1 co-treated group. Collectively, these findings indicate that B. altitudinis VUMS1 primarily functions as a pathogen-suppressive bacterium rather than a growth-promoting agent in seaweed aquaculture systems.