ABSTRACT Echinoderms, widely distributed and abundant marine invertebrates, host diverse microbial communities, including subcuticular symbiotic bacteria (SCB). However, the diversity and functional roles of these bacteria remain largely unexplored in intertidal brittle stars. Here, we utilized both culture-dependent and -independent methods to investigate SCBs in three different intertidal brittle star species. Amplicon sequencing revealed distinct subcuticular microbiota among the three brittle star species, with Endozoicomonadaceae dominating in O. gorgonia, Spirochaetota prevalent in O. exigua, and Entomoplasmatales enriched in O. japonicus. Fluorescence in situ hybridization further demonstrated that these bacteria formed microcolonies within the subcuticular space of the arms. We successfully isolated strain ToK13T, which possesses a nearly identical 16S rRNA gene sequence to that of the predominantly detected SCB. Sequence similarity analysis revealed that ToK13ᵀ exhibited 98.29%, 98.22%, and 98.16% 16S rRNA gene sequence identities to Kistimonas asteriae KMD 001ᵀ, K. scapharcae JCM 17805ᵀ, and K. alittae BGP-2ᵀ, respectively. This isolate is an obligate aerobic heterotroph that is capable of utilizing various monosaccharides. Genomic analysis identified genes associated with host interaction and symbiosis, including those involved in the biosynthesis of multiple vitamins, cofactors, and secondary metabolites with potential antimicrobial activity. Competition assays with co-cultured isolates revealed that strain ToK13T inhibits the growth of several bacterial taxa. Collectively, these findings suggest that host-specific SCBs may contribute to the survival strategies of brittle stars by mediating microbial interactions and potentially influencing host fitness.
Kawano et al. (Tue,) studied this question.