Abstract
<jats:title>Abstract</jats:title> <jats:p> Mi.cro.cal.do'ta. N.L. masc. n. <jats:italic>Microcaldus</jats:italic> , type genus of the class Microcaldia; L. neut. pl. n. suff. <jats:italic>‐ota</jats:italic> , ending to denote a phylum; N.L. neut. pl. n. <jats:italic>Microcaldota</jats:italic> , the phylum of the class <jats:italic>Microcaldia</jats:italic> . </jats:p> <jats:p> The phylum <jats:italic>Microcaldota</jats:italic> , previously known as <jats:italic>Candidatus</jats:italic> Micrarchaeota or ARMAN (Archaeal Richmond Mine Acidophilic Nanoorganisms), was proposed in 2022 and validated in 2023. According to the GTDB (Genome Taxonomy Database) taxonomy (release 220), this phylum contains the following six order‐level taxa: <jats:italic>Ca</jats:italic> . Anstonellales, <jats:italic>Ca</jats:italic> . Burarchaeales, <jats:italic>Ca</jats:italic> . Fermentimicrarchaeales, <jats:italic>Ca</jats:italic> . Gugararchaeales, <jats:italic>Ca</jats:italic> . Micrarchaeales ( <jats:italic>Microcaldales</jats:italic> ), and <jats:italic>Ca</jats:italic> . Norongarragalinales. This phylum belongs to the kingdom <jats:italic>Nanobdellati</jats:italic> , previously known as the DPANN (an acronym of the phyla <jats:italic>Ca</jats:italic> . Diapherotrites, <jats:italic>Ca</jats:italic> . Parvarchaeota, <jats:italic>Ca</jats:italic> . Aenigmarchaeota, <jats:italic>Ca</jats:italic> . Nanohaloarchaeota, and “ <jats:italic>Nanoarchaeota</jats:italic> ”) superphylum. Although most members in the phylum <jats:italic>Microcaldota</jats:italic> are uncultivated or yet‐to‐be‐cultivated, three cultivated strains, <jats:italic>Ca</jats:italic> . Micrarchaeum harzensis A_DKE, <jats:italic>Ca</jats:italic> . Mancarchaeum acidiphilum Mia14, and <jats:italic>Microcaldus variisymbioticus</jats:italic> ARM‐1 <jats:sup>T</jats:sup> , have been established. All three strains are obligate symbionts having tiny cells and small genomes, whose host species belong to the orders <jats:italic>Thermoplasmatales</jats:italic> or <jats:italic>Sulfolobales</jats:italic> . Notably, <jats:italic>M. variisymbioticus</jats:italic> , the sole validly named species in this phylum, can utilize five different <jats:italic>Sulfolobales</jats:italic> species as a host, suggesting a relatively wide range of host specificity in this group. The cultivated strains are mesophilic to thermophilic, whereas acidophilicity is a common characteristic. In contrast, a free‐living (or relatively independent) lifestyle is predicted among several uncultivated taxa (i.e., <jats:italic>Ca</jats:italic> . Anstonellales, <jats:italic>Ca</jats:italic> . Burarchaeales, <jats:italic>Ca</jats:italic> . Fermentimicrarchaeales, and <jats:italic>Ca</jats:italic> . Gugararchaeales) based on genomic information, although their physiology has not been experimentally examined. The DNA of this taxon has been detected in various natural environments characterized by diverse geochemical characteristics and temperatures. The genome size of the members of this phylum, including uncultivated taxa, ranges between 0.64 and 1.47 Mb. </jats:p> <jats:p> <jats:italic>DNA G + C content (mol%)</jats:italic> : 28.3–62.2 (Values adapted from Chen et al. (2018), Kadnikov et al. (2020), and Vázquez‐Campos et al. (2021)). </jats:p> <jats:p> <jats:italic>Type genus</jats:italic> : <jats:bold>Microcaldus</jats:bold> Sakai et al. 2022, VL214. </jats:p>