Ecological distribution and environmental preference of the deep-branching archaeal lineage in high-temperature hot springs of Central Kyushu, Japan.

Uncultured microorganisms represent a significant portion of the biodiversity of Earth and play crucial roles in ecosystem functioning, particularly in extreme environments. Among these, a novel archaeal lineage, the "OYS group," was recently identified via 16S rRNA gene sequencing from actively boiling hot springs in northeastern Japan. This lineage forms a deep-branching clade, an early-diverging group that is phylogenetically distinct from all the known archaeal phyla. However, no additional reports of the OYS group have emerged from other geothermal environments, raising questions about its ecological niche and geographic distribution. In this study, we conducted a comprehensive 16S rRNA gene-based survey of 17 hot spring sites in Central Kyushu, Japan, covering a broad range of temperatures (30-95 °C) and pH values (4.1-8.8), to assess the distribution and environmental preferences of the OYS group archaea. The OYS group was detected at five high-temperature sites with relative abundances of up to 16.9% and was particularly dominant in the Tsuetate hot spring. Phylogenetic analyses confirmed that these sequences formed a monophyletic clade, a single origin group with the previously reported OYS group sequences. Multivariate analyses revealed co-occurrence with other thermophilic taxa, indicating a possible preference for high-temperature niches. These findings expand the known habitat range of the OYS group and highlight their potential ecological significance in geothermal ecosystems.