Minor Actinides Can Replace Essential Lanthanides in Bacterial Life**

Abstract Certain f‐block elements—the lanthanides—have biological relevance in the context of methylotrophic bacteria. The respective strains incorporate these 4 f elements into the active site of one of their key metabolic enzymes, a lanthanide‐dependent methanol dehydrogenase. In this study, we investigated whether actinides, the radioactive 5 f elements, can replace the essential 4 f elements in lanthanide‐dependent bacterial metabolism. Growth studies with Methylacidiphilum fumariolicum SolV and the Methylobacterium extorquens AM1 Δ mxaF mutant demonstrate that americium and curium support growth in the absence of lanthanides. Moreover, strain SolV favors these actinides over late lanthanides when presented with a mixture of equal amounts of lanthanides together with americium and curium. Our combined in vivo and in vitro results establish that methylotrophic bacteria can utilize actinides instead of lanthanides to sustain their one‐carbon metabolism if they possess the correct size and a +III oxidation state.