To investigate the potential application of hadal bacteria in offshore shellfish culture, the capacity of biofilms forming and settlement inducing activity on mussel Mytilus coruscus of the eight strains of bacteria from seawater of the Mariana Trench were investigated. It was found that all eight strains of hadal bacteria were able to form biofilms and had the activity to induce settlement of M. coruscus plantigrades, with different activities to induce M. coruscus settlement among different bacterial genera. The biofilms of Pseudophaeobacter arcticus, Halomonas titanic, Halomonas axialensis,Pseudoalteromonas tetraodonis and Pseudoalteromonas shioyasakiensis had moderate inducing activity (>30%). The moderate inducing activity strains had <15% dead or damaged bacteria and extracellular DNA (eDNA) content within 200 ng/mL. Alteromonas stellipolaris, Microbacterium oxydans and Pseudoalteromonas hodoensis had low inducing activity (<30%). The low inducing activity strains had ≥50% dead or damaged bacteria and eDNA levels within 400 ng/mL. There was higher α-polysaccharide content in the bacterial biofilms with medium inducing activity than that in the bacterial biofilms with low inducing activity. Correlation analysis indicated that the inducing activity of M. coruscus plantigrades was significantly positive correlation with the content of extracellular polysaccharides as well as the proportion of live-to-dead bacteria ratio (P<0.05). The content of eDNA was shown to be significantly negative correlation with the proportion of live-to-dead bacteria ratio in the biofilm (P<0.05). The findings provide new ideas for the development and use of hadal microorganisms in offshore shellfish culture.
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