A novel trinuclear iron center is revealed by the structure of a cyclic di-GMP HD-GYP domain phosphodiesterase from Persephonella marina. A detailed view of the HD-GYP domain of P. marina is shown in ribbon presentation with iron atoms in orange and key metal binding residues His221 and Asp222 in stick format. Also shown are the residues of the GYP motif. For further details readers are referred to the article by Bellini et al. on pp. 26-38 of this issue.