Nanoarchaeota is a phylum within the domain Archaea, characterized by its remarkably small size. These microorganisms are among the smallest cellular organisms ever discovered. Their existence challenges previous understanding of how compact life can be for a self-sustaining organism.
Unique Biology and Symbiotic Dependence
Nanoarchaeota are exceptionally small. Individual cells, such as Nanoarchaeum equitans, measure approximately 0.4 micrometers in diameter, about one-hundredth the size of a common bacterium like Escherichia coli. These organisms are also hyperthermophilic, thriving in extremely hot environments with an optimal growth temperature around 90°C.
A distinguishing feature of Nanoarchaeota is their obligate symbiotic relationship, particularly with another archaeon called Ignicoccus hospitalis. This means Nanoarchaeum equitans cannot survive or replicate without being physically attached to the surface of its Ignicoccus host. The host provides essential metabolic functions and cellular components that the Nanoarchaeota cannot produce on its own.
The dependency of Nanoarchaeota is reflected in their remarkably small genome. Nanoarchaeum equitans possesses one of the smallest known genomes, consisting of approximately 0.49 million base pairs (Mbp). This compact genome is highly streamlined, with about 95% of its DNA coding for proteins or stable RNA, indicating a high gene density.
Despite encoding machinery for information processing and repair, the Nanoarchaeum equitans genome lacks many genes for metabolic pathways. These missing genes include those required for the biosynthesis of lipids, cofactors, amino acids, and nucleotides. This genetic reduction highlights their profound reliance on the host for these fundamental building blocks and potentially for energy.
Discovery and Extreme Habitats
Nanoarchaeota were first discovered in 2002 in active hydrothermal vents. These vents are found in volcanically active areas on the seafloor, often along mid-ocean ridges, where cold seawater seeps into cracks and becomes superheated by magma-heated rocks. The heated water then percolates back up through vent openings, releasing a chemical-rich fluid.
These hydrothermal vents represent extreme environments characterized by high temperatures, immense pressure, and a unique chemical composition lacking sunlight. The initial discovery of Nanoarchaeum equitans occurred in a submarine hydrothermal vent, with subsequent findings in terrestrial hot springs.
Scientists utilized molecular and phylogenetic analysis of 16S ribosomal DNA to identify these early colonizers of nascent hydrothermal chimneys. The detection of these organisms in diverse geothermal systems, including those in Yellowstone National Park, suggests a widespread distribution.
Scientific Significance
The study of Nanoarchaeota holds scientific importance, particularly in understanding the limits of cellular life. Their incredibly small cell size and highly reduced genome provide insights into what constitutes a “minimal genome”—the absolute fewest genes required for an organism to survive.
Nanoarchaeota also contribute to our understanding of the evolution of symbiotic and parasitic relationships. Their obligate dependence on Ignicoccus offers a unique model for studying the molecular and cellular mechanisms of such intimate inter-archaeal associations. The comparison of their streamlined genomes with those of their hosts sheds light on how organisms adapt through gene loss when relying on another for survival.
Studying Nanoarchaeota expands our knowledge of the diversity within the Archaea domain. Their deep branching position in the archaeal phylogenetic tree, based on 16S rRNA sequences, suggests they diverged early in archaeal evolution. Given their extreme habitat, Nanoarchaeota provide insights into potential early life forms on Earth, as hydrothermal vents are theorized to be sites where life may have originated.