The inside cover picture shows a schematic representation of [FeFe]-hydrogenase maturation in a microbial community in a geothermal hot spring of Yellowstone National Park, USA. In the stepwise formation of the active site H-cluster (lower right) of the [FeFe]-hydrogenase, carbon monoxide and cyanide ligands are synthesized from tyrosine through a radical SAM reaction catalyzed by HydG (lower left, shown here as a ribbon diagram based on a homology model). The synthesis and transfer of these diatomic ligands to the H-cluster during hydrogenase maturation is discussed in detail in the Microrevierw by John W. Peters, Joan B. Broderick et al. (p. 935 ff ). Photograph and superimposed artwork by courtesy of Kevin Swanson.