In anaerobic respiration, where do methanogens obtain the hydrogen atoms used to reduce CO_2 to CH_4?

• A. From molecular hydrogen (H_2) produced by other anaerobic microbes
• B. From water (H_2O) through photolysis
• C. From atmospheric hydrogen gas
• D. From organic acids such as acetate

Answer: (A)

Verified step by step guidance

1. Understand that methanogens are a group of archaea that produce methane (CH_4) by reducing carbon dioxide (CO_2) in anaerobic conditions.
2. Recognize that the reduction of CO_2 to CH_4 requires a source of hydrogen atoms (H) to act as electron donors in the biochemical reaction.
3. Identify the common sources of hydrogen in anaerobic environments, including molecular hydrogen (H_2) produced by other anaerobic microbes, water (H_2O), atmospheric hydrogen gas, and organic acids such as acetate.
4. Recall that methanogens typically rely on molecular hydrogen (H_2) generated by other anaerobic microbes during fermentation processes, as this H_2 serves as the direct electron donor for reducing CO_2 to methane.
5. Conclude that the hydrogen atoms used by methanogens come from molecular hydrogen (H_2) produced by other anaerobic microbes, rather than from water, atmospheric hydrogen, or organic acids.