Dissimilatory ferrous iron oxidation at a low pH: a novel trait identified in the bacterial subclass Rubrobacteridae

A novel iron-oxidizing acidophilic actinobacterium was isolated from spoil material at an abandoned copper mine. Phylogenetic analysis placed the isolate within the Rubrobacteridae subclass of the Actinobacteria. Its optimum temperature and pH for growth are 30-35 degrees C and pH 3.0, respectively. Although it could catalyze the dissimilatory oxidation of ferrous iron, growth yields declined progressively in media containing ferrous iron concentrations >100 microM. The isolate, Pa33, did not grow or oxidize iron in the absence of organic carbon, and appeared to be an obligate heterotroph. Specific rates of iron oxidation were much smaller than those determined for the autotrophic iron-oxidizing proteobacterium Acidithiobacillus ferrooxidans and the heterotrophic iron-oxidizing actinobacterium Ferrimicrobium acidiphilum. Iron oxidation by isolate Pa33 appears to be a defensive mechanism, in which iron oxidation converts a soluble species to which the bacterium is sensitive to an oxidized species (ferric iron) that is highly insoluble in the spoil from which it was isolated. This is the first report of acidophily or dissimilatory iron oxidation within the Rubrobacteridae subclass and one of very few within the Actinobacteria phylum as a whole.