Enrichment and Isolation of Iron-Oxidizing Bacteria from an Ancient Earth Analogue

Abstract

Fe2+ was an abundant component of ancient anoxic oceans and could have acted as a respiratory electron donor. The overall goals of this study were to test whether anaerobic microbial growth could occur with Fe2+ as the electron donor in Fe2+-rich sediments from an ancient ocean analogue (Lake Matano, Indonesia) and to determine the taxonomic identity of the bacteria. Sediments were incubated with Fe2+ sulfide as the electron donor in a nitrogen:carbon dioxide (90/10%) atmosphere. Manganese (III), nitrate, nitrite, and oxygen were provided as electron acceptors. With Mn3+ as the electron acceptor, cultures showed some evidence of growth near the middle of the gradient tube. However, orange Fe3+ oxides were absent, suggesting that anaerobic Fe2+ oxidation had not occurred. Ferric oxides were also absent in tubes containing nitrate and nitrite. A white precipitate was present in cultures with Mn3+, which indicated that the microbes reduced Mn3+ to Mn2+. The precipitate was not present in uninoculated controls. With oxygen as the electron donor, a layer of orange Fe3+ oxide minerals formed near the water-air interface, indicative of growth of microaerophilic Fe2+-oxidizing bacteria. This layer did not form in uninoculated controls. Our preliminary results suggest that anaerobic Lake Matano enrichments are capable of Fe2+ oxidation using oxygen but not alternative electron acceptors. After subsequent transfers of the enrichments that showed growth of microaerophilic Fe2+-oxidizing bacteria, the bacteria were isolated and their 16S rRNA gene was sequenced. Sequences were most similar to the Betaproteobacteria genus Comamonas and the Alphaproteobacteria genus Skermanella. Some species of Comamonas are known to oxidize Fe2+, while the exact mechanism of the metabolism of Skermanella are not well known. The presence of microaerophilic Fe2+ oxidizing bacteria from Lake Matano, Indonesia serves as a link between understanding the transition from an anoxic to an oxic world.