Methanogens: vital but threatened members of the human microbiome?

Methanogens are an ancestral group of archaea that occupy a unique niche within the human gut microbiome by virtue of their methane production. In this process, they serve as hydrogen sinks, allowing continued bacterial fermentation and influencing short-chain fatty acid production. Available evidence suggests that methanogen abundance may be declining in parallel with the broader reduction in gut microbial diversity accompanying industrialization. We describe the evolution of methanogens, their ecological roles in the human microbiome, and evidence for their apparent decline. If confirmed, reductions in methanogen prevalence and abundance may have substantial metabolic consequences, reframing these archaea as keystone species in need of scientific attention and conservation efforts.