Abstract
<jats:title>Abstract</jats:title> <jats:p> Mi.cro.cal'dus. Gr. masc. adj. <jats:italic>mikros</jats:italic> , small (may keep ‐o); L. masc. adj. <jats:italic>caldus</jats:italic> , hot; N.L. masc. n. <jats:italic>Microcaldus</jats:italic> , a thermophilic small‐size microorganism. </jats:p> <jats:p>Microcaldota / Microcaldia / Microcaldales / Microcaldaceae / Microcaldus</jats:p> <jats:p> The genus <jats:italic>Microcaldus</jats:italic> currently includes a single species, <jats:italic>Microcaldus variisymbioticus</jats:italic> . It is an obligately symbiotic microorganism that requires host microorganisms for growth. A range of <jats:italic>Sulfolobales</jats:italic> species can be utilized as a host. Cells are ultrasmall cocci observed singly or often attached to and surrounding the cell surface of hosts. Aerobic. Growth under anaerobic conditions has not been tested. Thermoacidophilic. The outermost surface of <jats:italic>M. variisymbioticus</jats:italic> is composed of an S‐layer. The type strain of the species, ARM‐1 <jats:sup>T</jats:sup> , interacts with its original host, <jats:italic>Metallosphaera javensis</jats:italic> AS‐7, via intercellular nanotubes that originate from the host cell. <jats:italic>M. variisymbioticus</jats:italic> must obtain essential nutrients from host species since its small genome (0.81 Mb) lacks the majority of genes for the biosynthesis of essential biomolecules. Known habitats are terrestrial acidic hot springs. </jats:p> <jats:p> <jats:italic>DNA G</jats:italic> + <jats:italic>C content (mol</jats:italic> % <jats:italic>)</jats:italic> : 34.1. </jats:p> <jats:p> <jats:italic>Type species</jats:italic> : <jats:bold>Microcaldus variisymbioticus</jats:bold> Sakai et al. 2022, VL214. </jats:p>