The role of multiheme c-type cytochromes in Shewanella sp. ANA-3 with respect to iron (III) reduction

Fe(III) reducing and As(V) respiring prokaryotes contribute to arsenic mobilization in aquifers contaminated with arsenic, specifically in places such as Bangladesh. Under oxic conditions As(V) predominates and is often adsorbed onto mineral surfaces such as amorphous iron oxide (hydrous ferric oxide, HFO). However, under anoxic conditions prokaryotes can respire As(V) by reducing it to As(III), which becomes the predominant form of arsenic. As(III) sorbs to fewer minerals and has a greater hydro-logic mobility compared to As(V). Microbial mobilization of arsenic from subsurface material most likely involves a combination of respiratory processes specific to iron reduction and arsenate respiration. Arsenic reduction and mobilization from subsurface material most likely involves a combination of metabolic pathways encoded in clusters such as mtr/omc and arr. The mtr/omc gene cluster encodes a number of c-type cytochromes important for iron reduction and the arr gene cluster encodes an arsenate respiratory reductase. The mechanism of iron reduction is not completely elucidated. We are currently exploring the molecular genetics underlying Fe and As geochemistry using a model Fe(III) and As(V) respiring bacterium, Shewanella sp. ANA-3. Specifically, we are investigating the role of c-type cytochromes encoded by the mtr/omc gene cluster with respect to iron reduction. Strains carrying multiple gene deletions were more severely impaired in iron reduction abilities than single gene deletions, although not completely eliminated. An ANA-3 strain carrying a complete deletion in the mtr/omc gene cluster was still capable of reducing iron oxide to Fe(II). These results suggest additional pathways or other cytochromes are involved in iron reduction. Work is currently underway to identify addition genes essential to iron reduction.