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Premise of the study Chaetopeltidales is a small understudied order in the OCC clade of the green algal class Chlorophyceae, slowly expanding by occasional discoveries of novel algae. Here we demonstrate that hitherto unrecognized chaetopeltidaleans exist also among previously described but neglected and misclassified species. Methods The strain Characium acuminatum SAG 40.91, with previous preliminary evidence for its affinities to the OCC clade, was investigated with light and electron microscopy to characterize its morphology and ultrastructure. Sequence assemblies of the organellar and nuclear genomes were obtained and utilized in bioinformatic and phylogenetic analyses to address the phylogenetic position of the alga and its salient genomic features. Key results The characterization of the strain SAG 40.91 and a critical literature review led us to reinstate the forgotten genus Hydrocytium A.Braun 1855, with SAG 40.91 representing its type species Hydrocytium acuminatum . Independent molecular markers converged on placing H. acuminatum as a deeply diverged lineage of the order Chaetopeltidales, formalized as the new family Hydrocytiaceae. Both chloroplast and mitochondrial genomes shared characteristics with other members of Chaetopeltidales and were bloated by repetitive sequences. Notably, independently of Volvocales the mitochondrial cox2a gene was transferred into the nuclear genome in the H. acuminatum lineage. The nuclear genome data from H. acuminatum , as well as from another chaetopeltidalean reported by others, revealed endogenized viral sequences corresponding to novel members of the phylum Nucleocytoviricota . Conclusions The resurrected genus Hydrocytium expands the known diversity of chaetopeltidalean algae and provides the first glimpse into their virosphere.This article is protected by copyright. All rights reserved.
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