Image source: H. Huber, M. Hohn, R. Rachel and K.O. Stetter, University of Regensburg, Germany
Archaea; Nanoarchaeota; Nanoarchaeum; Nanoarchaeum equitans
Chromosome: 490,885 bp
Discovered as being tiny dots on another organism, Nanoarchaeum was published as being an organism in need of a new phylum among the Archaea. They have not been able to grown on their own and while a parasitic lifestyle cannot be excluded at present, there have been several observations that promote a symbiotic mode of life. Since the original publication, there have been Nanoarchaeota found all over the world that are related to Nanoarchaeum but need to be classified in different families.
Nanoarchaeum has a tiny genome with only 490 kb, which represents the smallest archaeal genome to date. Comparing ss rRNA sequences, it was noted that sequence identities were more like archaeon than bacterial species. There was no difference, however, in the sequence identity to the Crenarchaeota, Euryarchaeota, and 'Korarchaeota', indicating it represents a new archaeal phylum.
It is very common for parasitic and symbiotic bacteria to have small genomes. They can develop these by the elimination of unneeded genes and the acquisition of new genes by lateral gene transfer. It is possible that N. equitans had a size reduction during adaptation to Ignicoccus or it could be an ancient genome since it is assumed that the genomes of the first microorganisms were small.
These coccus cells are only 400 nm in diameter and are covered by an S-layer. They require cell-cell contact with an actively growing Ignicoccus cell in order to grow. No negative effects have been observed on Ignicoccus cells with the addition of Nanoarchaeum, hinting at a possible non-parasitic lifestyle. This new Ignicoccus species that has been found as a Nanoarchaeum host, is an autotrophic sulfur-reducing thermophile.
This new microorganism was first identified from hot rocks taken at Kolbeinsey ridge, north of Iceland. This hydrothermal system is located at the subpolar Mid-Atlantic Ridge at a depth estimated at 106m. N. equitans can grow in the same temperature range as its host, which is 70-98oC, a pH around 6.0, and salt concentrations of about 2% NaCl. They grow on Ignicoccus cells in what is either a symbiotic or parasitic relationship.
Other nanoarchaeotal 16S rRNA genes have been obtained from the East Pacific Rise (pH 6.5), the Obsidian Pool in Yellowstone National Park (80oC, pH 6.0), and Caldera Uzon in Kamchatka, Russia (85oC, pH 5.5).
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Waters E, Hohn MJ, Ahel I, Graham DE, Adams MD, Barnstead M, Beeson KY, Bibbs L, Bolanos R, Keller M, Kretz K, Lin X, Mathur E, Ni J, Podar M, Richardson T, Sutton GG, Simon M, Soll D, Stetter KO, Short JM, Noordewier M., Proceedings of the National Academy of Sciences of the United States of America 100(22):12984-8 (2003 Oct 28).