In 1993, bacteria were discovered which do not need oxygen but can oxidize Fe(II) by using energy from light (anoxygenic phototrophic iron-oxidizing bacteria). Studies by Professor Kappler's team in 2005 and 2010 showed that these bacteria transform dissolved ferric iron into iron oxide (rust) -- like the material in the early iron ores. Now, the geomicrobiologists from Tübingen have been able to demonstrate that, by examining the identity and structural properties of the iron minerals, it is possible to tell that the minerals were deposited by iron-oxidizing microbes and not by oxygen made available by the action of cyanobacteria. To do this, the researchers placed different amounts of organic material together with iron minerals into gold capsules and increased the pressure and temperature to simulate the transformation of the minerals over geological time. They ended up with structures of iron carbonate minerals (siderite, FeCO3), just as they occur in geological iron formations. In particular, they were able to distinguish iron carbonate structures which had been formed in the presence of a rather small amount of organic compounds (microbial biomass) from those formed in the presence of a larger amount.