Associated Bacteria, Particularly Mucilaginibacter Species, Inhibit Sheath Formation in Nostoc commune

The terrestrial cyanobacterium Nostoc commune fixes atmospheric nitrogen and exhibits remarkable desiccation tolerance, making it a promising organism for biomass production on land unsuitable for conventional agriculture. However, reliable artificial cultivation of N. commune remains difficult because hormogonium formation cannot yet be induced reproducibly and mechanically prepared trichome fragments often fail to develop into sheath-enclosed colonies. In this study, we examined factors affecting sheath formation in short non-motile trichome fragments, referred to here as nas-trichomes. Microscopic observation revealed two distinct developmental outcomes: a curling type, in which trichomes formed sheaths and developed into compact colonies, and an elongation type, in which trichomes extended without forming a distinct sheath. Curling-rich cultures were obtained only from colonies subjected to drying and rehydration, and early bacterial proliferation around trichomes was negatively associated with subsequent sheath formation. Microbial community analysis showed that Mucilaginibacter was enriched in elongation-rich cultures. Co-culture experiments further demonstrated that M. segetis, M. oryzae, M. jinjuensis, and M. xinganensis significantly inhibited sheath formation at high bacterial density. These results indicate that associated bacteria, particularly specific Mucilaginibacter species, can inhibit sheath formation in N. commune and suggest that controlling early bacterial proliferation is essential for establishing reliable cultivation methods.