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Bacteria known to induce settlement of larvae of Hydroides elegans are rare in natural inductive biofilm

Nidhi Vijayan, Michael G. Hadfield*

*Corresponding author:

ABSTRACT: Like those of many benthic marine invertebrates, larvae of the polychaete Hydroides elegans settle in response to complex, multi-species biofilms in the field and to certain mono-specific bacterial biofilms in the laboratory. This study characterized the bacterial diversity of complex natural biofilms from Pearl Harbor, HI, using high-throughput 16S rRNA-amplicon sequencing and further characterized those bacterial species that could be isolated using standard culture methods. Community analysis of the biofilms revealed dominance of Bacteroidetes, Verrucomicrobiae, Alpha- and Gamma-proteobacteria and, at lower taxonomic levels, Rubritalea spp., Hyphomonas spp. and members of Sphingomondaceae and Rhodobacteraceae. These natural biofilms induced settlement in nearly 100% of larvae of H. elegans exposed to them. We identified 1636 different amplicon sequence variants in the field biofilm from the normalized dataset. The culture-based methods with one media type produced only 40 isolates of which 36 were unique based on 97% sequence similarity cut-off. The cultured isolates were assigned to 11 genera. Single-species biofilms of only 6 of the 40 isolates, all in the genus Pseudoalteromonas, induced settlement by larvae of H. elegans. However, the genus Pseudoalteromonas was present at very low relative abundance (<0.01%) in the biofilm. Crude suspensions of 0.22 µm-filtered supernatant from cultures of these 6 Pseudoalteromonas strains were not settlement inductive, while those from P. luteoviolaceae strain HI1 were inductive as previously reported. These findings reinforce the conclusion that larvae of H. elegans respond to different cues from different bacteria.