고세균, or "archaea," are a group of single-celled microorganisms that are distinct from bacteria and eukaryotes. They are known for their ability to thrive in extreme environments, such as hot springs and salt lakes, where most other life forms cannot survive. Archaea have unique biochemical properties, including distinct cell membranes and metabolic pathways. These microorganisms play essential roles in various ecosystems, particularly in nutrient cycling and decomposition. Some archaea are involved in processes like methane production, which is important for energy production and greenhouse gas emissions. Their study helps scientists understand the diversity of life and the origins of cellular life on Earth.