Archaea

Life on Earth is divided into three main evolutionary lineages: the Archaea, Bacteria and Eukarya domains. Archaeal organisms display a fascinating mixture of features from the other two domains. In particular, the majority of the replication, transcription and translation proteins are homologous to those of eukaryotes despite the fact that archaea, similar to bacteria, do not contain a cell nucleus. The archaea also display unique features, including distinct rRNA motifs, ether-linked membrane lipids and unique metabolic pathways, notably the ability of certain genera to produce methane gas.

Evolution

In the universal Tree of Life, the shortest and deepest branches consist of hyperthermophiles (high-temperature organisms). This suggests that the last common ancestor of all life on Earth may have been a hyperthermophile, and most hyperthermophiles are archaea. Thus, by studying the archaea, it may be possible to deduce properties of the earliest cellular organisms. It has also been suggested that the eukaryotic lineage originated from cellular fusions between different bacteria and archaea. Archaea may therefore provide insights into the origin of the eukaryotes, and act as simple model systems for complex eukaryal processes.

Extremophiles and applied science

Many archaea are extremophiles that thrive under conditions of extreme heat, acidity, salinity and/or pressure, and there is significant industrial interest in archaea as sources of thermostable enzymes and other biomolecules with of unusual properties. In addition, proteins from thermophiles are often easier to crystallize than counterparts from lower-temperature organisms. Thus, a range of important structures, including the Cdc6 and Mcm replication proteins, the multisubunit RNA polymerase and the entire ribosome, have been solved with thermophiles as protein sources.

Ecology and biodiversity

Although many archaea are extremophiles, rRNA gene amplification from environmental samples has made it clear that archaea are widespread also in non-extreme biotopes. Thus, they have ecological significance for large-scale circulation of energy, nutrients and biomass, as well as for global warming, since methanogenic archaea annually release in the order of half a billion tons of methane, a greenhouse gas 20 - 40 times more efficient than CO2. In addition, increasing evidence is implicating archaea as main contributors to nitrogen cycling in one of the largest habitats on Earth, the deep marine biosphere, as well as in many soil environments.

Exobiology

All planets and moons in our solar system, except Earth, display environmental conditions that only extremophilic organisms can endure. Thus, knowledge about the biology of extremophiles is becoming increasingly releveant in searches for extraterrestrial life. We are co-founders of the Swedish Astrobiology Network which deals with astrobiology and exobiology issues.