Extreme Excitement about Archaea

40 years ago, Woese and Fox described the tree of life as we know it today, with three branches representing Eukaryota, Bacteria, and Archaea. In the last few decades, important findings have shaken our picture of the ecology and importance of archaea and have revealed novel traits beyond archaeal extremophily and supposed ‘‘primitiveness.’’ However, archaea are not only environmentally important microorganisms: they are also substantial components, or even keystone species, in complex plant, animal, and human microbiomes. Archaea have been constantly overlooked in the human microbiome—or not considered important enough, as no pathogenic archaeon has yet been identified. Since archaeal biology is fundamentally different from that of bacteria, we face numerousmethodological problems to properly assess their diversity, abundance, metabolism, and structure. As long as scientists hesitate to increase their efforts to assess the archaeome along with the bacteriome, we miss a big part of the microbiome. Single methanogenic archaeal species can easily reach up to 10% of all anaerobic microorganisms in the gastrointestinal tract, outnumbering the most abundant bacterial species we have identified thus far. Besides the gut, known and novel archaea thrive in the oral and vaginal cavities, on the skin, and in the lungs. Discovering this unexpected diversity of archaea interacting so closely with us, I was oncemore completely overwhelmed and surprised by this fascinating domain of life, which appears to have found a lifestyle dedicated to syntrophy and mostly positive interactions—at least until we identify the first archaeal pathogen. Epigenetics in Boiling Acid