Fig. 3.

Metal-dependent oxidation of ¹³C-labeled CH₄ to ¹³CO₂ by an enrichment culture of AAA. (A) Fe³⁺-citrate addition at the start of the experiment and after 150 h (arrow) stimulates methane oxidation. Control incubations of the enrichment culture without any addition, sulfate or citrate only, as well as an archaea-free enrichment culture of M. oxyfera (34) incubated with Fe³⁺-citrate were not active. Data are the average of two incubations containing 1 ± 0.1 mg protein each. (B) Stoichiometric coupling of CH₄ oxidation to Fe²⁺ production. Methane oxidation was deduced from ¹³CO₂ production; data points represent the average of four incubations of 77 ± 3.6 µg protein each. (C) Stimulation of ¹³CO₂ production from ¹³CH₄ by the addition of nanoparticulate suspensions of ferrihydrite (Fe³⁺) and birnessite (Mn⁴⁺). Protein content in these experiments was 77.0 ± 3.6 µg per vial. Error bars indicate SD.