Which organism obtains energy for growth from the chemical reaction that spontaneously occurs between iron and manganese and dissolved oxygen?

• A. Iron bacteria
• B. Nitrifying bacteria
• C. Sulfate-reducing bacteria
• D. Iron-oxidizing bacteria

Answer: (A)

Chemolithotrophy is at work here: some bacteria harvest energy by oxidizing an inorganic compound, in this case ferrous iron (Fe2+) with oxygen as the electron acceptor. This iron oxidation releases energy that the cells use to grow, so the organisms that perform it are iron-oxidizing bacteria. They’re commonly referred to as iron bacteria in environmental contexts and are a classic example of microbes obtaining energy from an abiotic redox reaction involving iron (and often manganese oxides) and dissolved oxygen. The other options involve different electron donors or electron acceptors and thus don’t fit the described energy source.