Archaea
Japanese: 古細菌（アーキア）
A group of prokaryotes phylogenetically distinct from bacteria. Many are known for inhabiting extreme environments, but they are also widely distributed in oceans, soil, and the gut.
Archaea may look similar to bacteria, yet they are a phylogenetically distinct group. They occupy one of life's three domains—Bacteria, Archaea, and Eukarya.
Extremely hot springs, hypersaline lakes, strongly acidic environments—archaea are well known for inhabiting such extreme conditions. Yet in reality, they also exist commonly in oceans, soil, and the human gut.
Their direct involvement in fermented foods is limited, but methanogens (a group of archaea) are essential for biogas production. Inconspicuous yet fundamental, they support Earth's material cycles. One of the important elements supporting fermentation's diversity.