The phylum nanoarchaeota

Abstract

In 2002 a novel phylum within the Archaea was proposed, the Nanoarchaeota. It was represented by a single strain, Nanoarchaeum equitans, a nano-sized hyperthermophilic symbiont that grows obligately attached to the surface of an Ignicoccus species, I. hospitalis. Together, they form a unique, intimate association of two Archaea which is mandatory for N. equitans but not for the host I. hospitalis. N. equitans was obtained from a sample of submarine hot rocks of the Kolbeinsey Ridge, north of Iceland. It has a cell diameter of only 350-500 nm and grows under strictly anaerobic conditions at temperatures between 75 °C and 98 °C. First molecular investigations revealed that N. equitans harbors a highly divergent 16S rDNA sequence which exhibits several base exchanges even in previously universal sequence signatures (often used as universal rRNA polymerase chain reaction (PCR) or DNA sequencing primer sequences). With about 490 kb N. equitans has one of the smallest genomes known so far, lacking nearly all known genes for lipid, cofactor, amino acid, or nucleotide biosynthesis. Lipid analyses of N. equitans and I. hospitalis revealed that N. equitans derives all its membrane lipids from its host. The same is true for its amino acids, although the mechanism of transportation from the host is still unknown. Whole-cell proteomics of I. hospitalis in the presence of N. equitans on its surface gave no evidence of a significant transport of Ignicoccus biosynthetic enzymes to N. equitans, suggesting that, at least under laboratory conditions, N. equitans diverts some of its host's metabolism and cell cycle control to compensate for its own metabolic shortcomings. During the last decade, environmental 16S rDNA sequences have been obtained from marine and continental (mostly high temperature) biotopes all over the world which also belong to the proposed phylum Nanoarchaeota, demonstrating a worldwide and ubiquitous distribution of this group. However, no further representative has been cultivated axenically or in coculture under laboratory conditions thus far. Despite of the increasing amount of (mostly 16S rDNA) sequences, the exact branching point of the Nanoarchaeota within phylogenetic trees is still under ongoing scientific discussion and strongly dependent on the molecule investigated and the phylogenetic metric used. This contribution is a modified and updated version of a previous description of the Nanoarchaeota.