## 1 Introduction  
Bacterial surface layers (S-layers) represent the outermost cell envelope component in _archae_ and many bacteria, being the simplest biological membranes which have developed during evolution [1; 2; 3]. S-layers are composed of (glyco)proteins which self-arrange in nanometric repeating unit cells (of varying lattice symmetries) with sizes of 3 to 30 nm, thicknesses of 5 to 10 nm (up to 70 nm in _archae_) and pore sizes of 2 to 8 nm. One of the most fascinating features of many S-layer proteins is that they can be extracted from the bacterial cell wall, subsequently purified and still maintain the ability of forming recrystallization products with identical lattice properties as on the native bacterial cell [4]. These unique self-assembly properties have been employed to functionalize very diverse supports with such monomolecular crystalline lattices [5; 6]. Furthermore, the introduction of either chemical groups or biologically active moieties via genetic engineering (fusion proteins) opened new biotechnological and biomedical applications [1]. Divalent ions have been described in several studies as a key factor in S-layer formation process, not only during the self-assembly of protein subunits but also for incorporating those S-layer monomers onto the hosting surface [7]. In most of them, CaCl\({}_{2}\) has been by far the most widely used compound to trigger S-layer assembly [8]. Alternatively, some authors reported on the use of other salts like BaCl\({}_{2}\) and MgCl\({}_{2}\) for the particular case of recrystallizing SppA proteins [9; 10] or also [PERSON] et al. [11] who broadened the ion list to SrCl\({}_{2}\), MnCl\({}_{2}\), NiCl\({}_{2}\), AlCl\({}_{3}\), CuCl\({}_{2}\), CdCl\({}_{2}\), ZnCl\({}_{2}\), CoCl\({}_{2}\), NaCl, KCl, LiCl and FeCl\({}_{3}\) for their recrystallization studies on _Caubobacter crescentus_ paracrystalline S-layer, with rather diverse results. Interestingly, none of these studies considered the use of iron in its _ferrous_ state (Fe\({}^{2+}\)) even though its electrochemical similarities with Ca\({}^{2+}\) are known, as well as their comparable ionic radii. A proper justification could be that, despite the large abundance of iron in nature, such _ferrous_ state is unstable under aerobic conditions and oxidizes into the _ferric_ (Fe\({}^{3+}\)) state which, in turn, aggregates into insoluble hydroxides. Therefore, combination of S-layers with iron was uniquely investigated in the field of biosorption and mineralization of heavy metals on biomass [12; 13], or defined in terms of iron acquisition by binding and transferring heme [14; 15]. Indeed, the redox potential of the Fe\({}^{2+}\)/Fe\({}^{3+}\) pair (+0.77 V) makes iron extremely versatile when incorporated into proteins as a catalytic centre or as an electron carrier, acquiring an important role for many biological processes. From a mere 'paleo-geo-chemical' view, it can also be assumed that iron might have played an important role in S-layer self-assembly in an early (anaerobic) stage of biological evolution [1]. Indeed, the redox state of iron on earth is correlated with the emergence and availability of oxygen. In the earliest days of life on earth iron was most often found in the water-soluble Fe\({}^{2+}\) state as a result of an O\({}_{2}\)-free environment. After the oxygenation events caused by cyanobacterial photosynthesis atmospheric oxygen increased, thus limiting iron existence to solely the Fe\({}^{3+}\) state [16]. This might have forced bacteria to find alternative ways in order to activate the S-layer self-assembly, since this complete coverage resulted crucial for their development and perpetuation. In this work, we have investigated for the first time the potentiality of two different iron (II) salts (FeCl\({}_{2}\) and FeSO\({}_{4}\)) for the recrystallization of the well-known S-layer protein SppA from _Lysinibacillus__spahericus_ CCM2177 [17; 18; 19].