Diversity of Archaea

Abstract The Archaea are a group of microbes that form one of three domains of life on Earth. Studies with isolated strains have revealed an archaeal metabolic diversity rivaling that found within the domain Bacteria, and molecular surveys have revealed that archaea occupy a broader range of environments than was suspected based on the physiology of isolates. Archaea are also the most abundant and active microbial component in some environments, typically where their adaptations to chronic energy stress provide selective advantage over bacteria. Ongoing studies of uncultured archaea are likely to reveal important impacts on Earth's elemental and energy cycles. Key concepts Comparison of small subunit ribosomal RNA (16S rRNA) gene sequences and other cellular characteristics revealed that Archaea are a distinct group of microbes and one of three domains of life on Earth. The majority of archaeal diversity is composed of two kingdoms: the Crenarchaeota and Euryarchaeota. Most of the cultured archaea are ‘extremophiles’, or organisms that are adapted to living under extreme environmental conditions. The common ecological factor among the Archaea is their propensity to thrive under conditions of chronic energy stress. The ‘uncultured majority’ refers to the abundant, widespread, and highly diverse groups of archaea that currently lack cultured isolates. The metabolic functions of some uncultured archaea have been revealed by combining molecular, biochemical and geochemical techniques.