The precipitation of iron sulfides potentially offers enough energy and reducing power to sustain life but organisms harnessing this reaction have not to our knowledge been previously described. We isolated a bacterial strain, capable of forming the iron sulfide minerals troilite (FeS), greigite (Fe3S4), and pyrite (FeS2), from subsurface, microbial mats in Mangalia, Romania. This strain, most closely related to strains ofThiomonassp., forms pyrite only if the redox conditions remain negative (<−60 mV), sulfides are provided continually (≈1 mM), and the concentration of iron remains low (≤0.08 mM) but constant. Pyrite formation by this microbial strain is proposed as an example of biologically controlled mineralization because it is controlled by uncouplers of oxidative phosphorylation, it is larger in living than in dead cells, it is additive (controlled less by the amount of cell surfaces and more by reagents), and it results in the formation of ATP. This study indicates that precipitation and crystal formation can represent an energy resource for life and provides support for the“iron-sulfide world hypothesis”regarding the early evolution of life on Earth.