Table 1_The bacteriovorous ciliate Uronema marinum as a natural biological control agent against Vibrio infections in bivalve hatcheries: a sustainable alternative to antibiotics.docx

IntroductionBacterial infections caused by Vibrio spp. are among the main factors limiting the survival of bivalve larvae in hatcheries, and the overuse of antibiotics has promoted resistance and environmental concerns. This study evaluated the free-living marine ciliate Uronema marinum as a natural biological control agent against Vibrio pathogens affecting clam larvae.
MethodsThe ciliate was identified by morphology and 18S rRNA gene sequencing. Its cytopathogenicity was assessed in EPC cell monolayers using Philasterides dicentrarchi as a pathogenic control. Bacteriovory was evaluated by growth assays with several virulent Vibrio species, by comparing active versus heat-inactivated bacteria, and by fluorescence/Nomarski microscopy using FITC-labeled bacteria. Protection assays were performed in larvae of Ruditapes philippinarum and R. decussatus challenged with Vibrio alginolyticus or P. dicentrarchi, with or without U. marinum or gentamicin.
ResultsUronema marinum was confirmed as a non-cytopathogenic, strictly bacteriovorous ciliate. It proliferated efficiently on several virulent Vibrio species, reaching densities above 4 × 104 ciliates mL−1, whereas no growth occurred with heat-inactivated bacteria. Microscopy demonstrated active and selective ingestion of FITC-labeled bacteria and exclusion of microalgal cells. In infection assays, co-incubation with U. marinum significantly improved larval survival after challenge with V. alginolyticus or P. dicentrarchi, maintaining protection comparable to gentamicin. The ciliate also protected larvae across bacterial concentrations of 103–105 CFU mL−1.
DiscussionThese results provide the first direct experimental evidence that a bacteriovorous ciliate can reduce Vibrio-induced mortality in bivalve larvae through selective bacteriovory. The non-pathogenic nature, trophic selectivity, and protective efficacy of U. marinum support its potential as a sustainable and environmentally safe alternative to antibiotics in bivalve hatcheries.