Underground Cells Make ‘Dark Oxygen’ Without Light

3/4 of the way down they finally give a half-ass description of the process:

While working in a lab in the Netherlands in the late 2000s, Strous noticed that a type of methane-feeding bacteria often found in lake sediments and wastewater sludges had a strange way of life. Instead of taking in oxygen from its surroundings like other aerobes, the bacteria created its own oxygen by using enzymes to break down the soluble compounds called nitrites (which contain a chemical group made of nitrogen and two oxygen atoms). The bacteria used the self-generated oxygen to split methane for energy.