Bioactivity of bacterial strains isolated from marine biofilms in Hong Kong waters for the induction of larval settlement in the marine polychaete Hydroides elegans

In the present study, 38 bacterial isolates were obtained from a marine biofilm, identified by the comparison of 16S rRNA gene sequences, and investigated by laboratory bioassays for their effects on larval settlement of the marine polychaete Hydroides elegans (Haswell). The bacterial isolates belonged to 3 phylogenetic branches: gamma-Proteobacteria (26 isolates), Gram-positive (8 isolates) and Cytophaga-Flexibacter-Bacteroides (4 isolates). Most of the isolates were affiliated to the genera Vibrio (7 isolates), Alteromonas (8 isolates) or Pseudoalteromonas (8 isolates), which are in the gamma-Proteobacteria branch. According to their efficacy to induce larval settlement of H. elegans in laboratory bioassays, the isolates were categorized as strongly, moderately, and non-inductive for larval settlement, About 42% of the isolates were categorized as non-inductive and the rest of the isolates contained equal numbers of highly and moderately inductive strains. The results indicated that larval settlement of 14, elegans could be induced by bacteria in a wide range of taxa, The isolates that induced high and moderate levels of larval settlement belonged to the genus Cytophaga in the Cytophaga-Flexibacter-Bacteroides branch; the genera Bacillus, Brevibacterium, Micrococcus and Staphylococcus in the Gram-positive branch; and the genera Alteromonas, Pseudoalteromonas and Vibrio in the gamma-Proteobacteria branch, Results also showed that isolates affiliating to the Same genus might have tremendously different activities for the induction of larval settlement. For example, isolates that were affiliated to the genus Alteromonas or Pseudoalteromonas distributed over the 3 categories of activity for induction of larval settlement.