Archaeal Cells

Abstract

At a first glance, Archaea are structurally seen quite similar to Bacteria, and for a long time they were named ‘Archaebacteria’. They can form cocci, rods, spirals or irregular shaped cells and are equally sized as Bacteria. Together they are referred to as ‘Prokaryotes’ because neither Archaea nor Bacteria have a nucleus. Although this term indeed might be helpful in habitual language use, it does not refer to a phylogenetic group. In fact, transcription and translation machineries of Archaea have much more in common with eukaryotic cells than with Bacteria. In addition, there are many features that remain characteristic for Archaea, given by the fact that many representatives live and thrive under extreme environmental conditions. In the review, the authors give a comprehensive overview about the stunning diversity of structural features in archaeal cell envelopes, membranes and cell appendages. Key Concepts: By application of recently developed PCR techniques, Archaea can be found in almost every habitat and sometimes are even more abundant than bacteria. Archaea show special adaptations to their sometimes extreme environments, like caldarchaeols, which are more stable at high temperatures. Like Bacteria, Archaea are also surrounded by a lipid bilayer, but in the latter case the lipid moiety consists of C 5 ‐isoprenoid units that are coupled to glycerol via ether bonds at (sn)‐2,3 positions of the glycerol. Cell walls can be as simple as a proteinaceous surface layer or as complicated as in some methanogens with multiple layers, additional sheats enclosing several cells and even more complex cell wall compounds. Archaea exhibit a broad variety in cell appendages that are different to bacterial ones in fine structure, composition, biosynthesis and anchorage in the cell.

A New Addition to the Cell Plan of Anammox Bacteria: "Candidatus Kuenenia stuttgartiensis" Has a Protein Surface Layer as the Outermost Layer of the Cell

“…The latter function seems especially interesting in the case of anammox bacteria, since they are proposed to lack peptidoglycan and might therefore be in need of a structure that maintains cell shape and integrity. In the case of Archaea, which also lack peptidoglycan (23,72), S-layers are indeed often assumed to have a function in maintaining cellular integrity, which is also substantiated by the fact that S-layer-deficient mutants in Archaea have not been found (64). Loss of S-layers in laboratory strains is a common feature of Bacteria, which probably occurs when the cells do not need their S-layers under culturing conditions, in which case S-layer-deficient mutants might outgrow S-layer-containing cells (58,73).…”

“…S-layers constitute a two-dimensional (2D) crystalline array of (usually) identical protein subunits covering the entire cell surface (23,24). The regular pattern formed by the S-layer can exist in oblique (p1 and p2), square (p4), or hexagonal (p3 and p6) symmetry, which is dictated by the arrangement and number of protein subunits (indicated by the number behind the p) that form the single morphological unit.…”

New Mode of Energy Metabolism in the Seventh Order of Methanogens as Revealed by Comparative Genome Analysis of “Candidatus Methanoplasma termitum”

“…Methanoplasma termitum" and Mmc. luminyensis do not consist of a single lipid membrane covered by a proteinaceous S-layer, as in most other archaea (75,76), but that the cells have a two-membrane system. Dual membranes in archaea have so far been restricted to Ignicoccus species (77) and the ultrasmall ARMAN cells (78).…”

A Complex Endomembrane System in the Archaeon Ignicoccus hospitalis Tapped by Nanoarchaeum equitans

“…A taxonomic characteristic that distinguishes Ignicoccus cells from all other known Archaea is their unusual cell anatomy (for a review on archaeal cell structures see Klingl et al, 2013 ). Unlike most other Archaea, Ignicoccus cells lack a proteinaceous cell envelope (S-layer) and are instead surrounded by a double membrane system: an outer cellular membrane (OCM) and an inner (“cytoplasmic”) membrane (IM) (Näther and Rachel, 2004 ; Huber et al, 2012 ).…”