1. CSIC, Institute of Catalysis, Cantoblanco, 28049 Madrid, Spain
2. Department of Environmental Microbiology, HZI-Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany
3. Institute of Microbiology, Carolo-Wilhelmina Technical University of Braunschweig, 38106 Braunschweig, Germany
4. Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK
5. These authors contributed equally to this work.

Correspondence to: Manuel Ferrer¹ Correspondence and requests for materials should be addressed to M.F. (Email: mferrer@icp.csic.es).

Ferroplasma is a genus of the Archaea, one of the three branches of the tree of life, and belongs to the order Thermoplasmatales (Euryarchaeota), which contains the most acidophilic microbes yet known. Ferroplasma species live in acid mine drainage, acidic pools and environments containing sulphidic ores such as pyrite and characterized by pH values of 0–2 and high concentrations of ferrous iron and other heavy metals^{1, 2, 3}. F. acidiphilum strain Y^T is a chemoautotroph that grows optimally at pH 1.7 and gains energy by oxidizing ferrous iron and carbon by the fixation of carbon dioxide¹. Here we show that 86% of 189 investigated cellular proteins of F. acidiphilum are iron-metalloproteins. These include proteins with deduced structural, chaperone and catalytic roles, not described as iron-metalloproteins in any other organism so far investigated. The iron atoms in the proteins seem to organize and stabilize their three-dimensional structures, to act as 'iron rivets'. Analysis of proteins of the phylogenetic neighbour Picrophilus torridus and of the habitat neighbour Acidithiobacillus ferrooxidans revealed far fewer and only typical metalloproteins. F. acidiphilum therefore has a currently unique iron-protein-dominated cellular machinery and biochemical phylogeny.

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