| Summary: | The bacterium <i>Gemmatimonas phototrophica</i> AP64 isolated from a freshwater lake in the western Gobi Desert represents the first phototrophic member of the bacterial phylum Gemmatimonadota. This strain was originally cultured on agar plates because it did not grow in liquid medium. In contrast, the closely related species <i>G. groenlandica</i> TET16 grows both on solid and in liquid media. Here, we show that the growth of <i>G. phototrophica</i> in liquid medium can be induced by supplementing the medium with 20 mg CaCl<sub>2</sub> L<sup>−1</sup>. When grown at a lower concentration of calcium (2 mg CaCl<sub>2</sub> L<sup>−1</sup>) in the liquid medium, the growth was significantly delayed, cells were elongated and lacked flagella. The elevated requirement for calcium is relatively specific as it can be partially substituted by strontium, but not by magnesium. The transcriptome analysis documented that several groups of genes involved in flagella biosynthesis and transport of transition metals were co-activated after amendment of 20 mg CaCl<sub>2</sub> L<sup>−1</sup> to the medium. The presented results document that <i>G. phototrophica</i> requires a higher concentration of calcium for its metabolism and growth compared to other <i>Gemmatimonas</i> species.
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