Novel taxa of the centrohelid heliozoans (Pterocystida and Panacanthocystida) from agricultural soils

Soil protists play key roles in terrestrial ecosystems, influencing microbial dynamics, nutrient cycling, and plant health. In this study, we describe a new genus and species of centrohelid heliozoans, Tellocystis perplexa gen. et sp. nov. and Raphidocystis akmolaensis sp. nov., isolated from agricultural soil habitats, and characterize two other known cyst-forming centrohelid species from soil. T. perplexa gen. et sp. nov., studied using light and electron microscopy, possesses an unusual spine-scale morphology, featuring a small drop-shaped basal plate and a curved shaft. Phylogenetic analysis of the 18S rRNA gene unexpectedly placed this species within the clade previously referred to as ‘Pinaciophora’, which is nested within Pterista, alongside unpublished clones of ‘Choanocystis perpusilla’ from Franz Josef Land and Southern Transbaikalia. Based on the obtained data, we transferred C. perpusilla to the genus Tellocystis. R. akmolaensis sp. nov. is morphologically similar to R. symmetrica, but differs from it by having smaller cell and plate-scale sizes, and by formation of cyst scales that are distinct from those of its trophic phase. Phylogenetic analysis indicated that this species occupies a deep evolutionary position within Raphidocystidae, basal to R. symmetrica, R. tubifera, R. ambigua, and ‘R. brunii’. This study advances current knowledge of the diversity of soil heterotrophic protists, which play an essential role in regulating microbial populations, and clarifies the intricate taxonomy of some centrohelid heliozoans.