Model of the iron oxidation electron transport pathway of At. ferrooxidans based partly on references [10, 75]. Electrons are transferred from the membrane-located cytochrome c 2  to rusticyanin and then along one of two paths. The downhill path is via cytochrome c4 (Cyt1) to cytochrome aa3  or the uphill, reverse electron transport path via cytochrome c4 (CytA1) to a bc1 I complex and a NADH-Q oxidoreductase . At. ferrooxidans has up to twelve cytochromes c  and a variety of cytochrome oxidases some of which appear to play different roles depending on whether iron or sulfur is being oxidized . The NADH is responsible for mercury reduction using a MerA mercuric reductase and the cytochrome aa3 is required to reduce mercury via the unique iron dependent mechanism discovered in At. ferrooxidans .