Iron is an essential element for many organisms, because it constitutes reaction centers of a variety of catabolic enzymes, such as cytochromes and iron/sulfur proteins in respiratory electron-transport chains (Wandersman & Delepelaire, 2004). This is particularly true for DMRB, such as Shewanella, as multiheme c-cyts are the main components of the EET pathway (Shi et al., 2007). In the environment, ferric iron (Fe3+) forms ferric-oxide hydrate complexes (Fe2O3·nH2O) in the

presence of oxygen and water under neutral and basic Z-VAD-FMK research buy conditions. These complexes are very stable, leading to very low free Fe3+ concentrations (10−9 to 10−18 M; Miethke & Marahiel, 2007). Ferrous iron (Fe2+) is soluble in water at neutral pH and can be directly incorporated into living cells by a siderophore-independent

system (e.g. FeoA/FeoB; Andrews et al., 2003). As Fe2+ is stably present under anaerobic conditions, it is reasonable that intracellular iron content was not affected by the SO3030 disruption under fumarate-reducing condition (Table 1). Fe2+ is however spontaneously and rapidly oxidized to Fe3+ in the presence of molecular oxygen, and chelating agents (e.g. siderophores) and associated chelated Fe3+ uptake systems are therefore necessary for bacteria to acquire iron signaling pathway under aerobic conditions. Besides, this study found that Inositol oxygenase the cellular iron content is remarkably low when Shewanella cells were grown under anaerobic MnO2-reducing conditions (Table 1), suggesting that the presence of MnO2 causes iron deficiency of Shewanella cells even under anaerobic conditions. This result can be explained by observations that ferrous iron is oxidized by MnO2 (Myers & Nealson, 1988b; Schippers & Jørgensen, 2001). It is therefore likely

that soluble Fe2+ is scarcely present in the presence of MnO2, and the siderophore-deficient cells are difficult to utilize insoluble iron(III) generated under MnO2-reducing conditions. In support of this idea, we found that ΔSO3030 reduced MnO2 as fast as WT when 50 μM soluble iron(III)-citrate was added in media as an iron source (data not shown). The transcription of the OM-cyt genes (omcA and mtrC) was repressed under iron-limiting and MnO2-reducing conditions, and this repression was pronounced in the siderophore-deficient mutant (Figs 4 and 5). These results suggest that iron availability and metal-reducing activities are coordinately regulated in S. oneidensis MR-1 under metal-reducing conditions. Iron-dependent expression of OM-cyt genes has been reported for Shewanella cells grown under aerobic conditions (Yang et al., 2008, 2009), while we also indicate that iron is an essential factor for OM-cyt expression even under anaerobic conditions.