Final answer:
Methanogens get hydrogens from sources like hydrogen gas or internally produced hydrogen to reduce CO₂ to CH₄ during anaerobic respiration. These Archaea live in anaerobic environments and play crucial roles in energy generation and the biological carbon cycle.
Step-by-step explanation:
Methanogens, a group of Archaea, play a key role in the carbon cycle by reducing carbon dioxide (CO₂) to methane (CH₄) as part of their anaerobic respiration process. Methanogens obtain hydrogens involved in this reduction from various sources. For example, they can use hydrogen gas (H₂) directly or produce it internally through the fermentation of substrates like acetate. When CO₂ acts as a terminal electron acceptor, hydrogens are transferred to it, which results in the formation of methane. These processes occur in anaerobic environments, such as the soil, aquatic ecosystems, and the digestive tracts of ruminants like cows and sheep.
Lacking oxygen, methanogens have evolved to obtain energy (ATP) by converting CO₂, which is energetic 'downhill' in the absence of oxygen. Essentially, they use the reduction of CO₂ as a way to oxidize NADH to NAD⁺, a necessary step for energy generation in the absence of oxygen. As methane is a potent greenhouse gas, this anaerobic process is important in terms of both environmental impact and the biological carbon cycle.