Micrarchaeum acidiphilum is an ultra-small archaeon in the DPANN group—a deep-branching clade of archaea with highly reduced genomes. It was discovered in acidic mine drainage and appears to be dependent on a symbiotic relationship with other archaea or bacteria for survival due to its extremely limited metabolic capacity. Its tiny cell size (less than 0.5 µm) and minimal genome make it one of the smallest known cellular organisms.