A novel centrohelid protist from Siberian permafrost.

Permafrost ecosystems serve as unique reservoirs for ancient microbial life, yet the diversity and long-term viability of protists, particularly centrohelid heliozoans, remain poorly understood. In this study, we investigated an enrichment culture of a centrohelid heliozoan isolated from a Siberian permafrost core sample retrieved from a depth of 9 m and dated to the Late Pleistocene (approximately 39,000 years BP). Using an integrative taxonomic approach combining light and scanning electron microscopy with 18S rRNA gene phylogeny, we describe a new species, Acanthocystis yamallongha sp. nov. This novel centrohelid is characterised by a unique spiral-like ridge ornamentation on its plate-scales, a morphological feature that has not been detected in species from contemporary aquatic or soil habitats, as well as by spine-scales with bifurcated tips. It exhibits a high level of genetic divergence from known species and likely represents an independent evolutionary lineage sister to the clade comprising freshwater representatives of A. takahashii and soil-derived environmental sequences. Our findings provide evidence for the ability of Acanthocystidae cysts to retain viability over geological timescales. The identification of A. yamallongha highlights that a close investigation of permafrost ecosystems is essential for uncovering the hidden diversity of ecologically important protists and clarifying their phylogenetic relationships.