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 C5‐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.

Keywords: Archaea; cell structure; cell appendages; cell envelope; S‐layer; tetraether lipids; plasma membrane; flagella

Figure 1.

(a–c) Freeze‐etching of Metallosphaera sedula (Sulfolobales), depicting the hexagonal S‐layer with p3‐symmetry. (b) Freeze‐fractured cell of M. sedula, showing the cell of (c) at higher magnification. SL, S‐layer; PS, periplasmic space; CM, cytoplasmic membrane; C, cytoplasm. (d) Ultrathinsection of a high‐pressure frozen cell of Sulfolobus metallicus. Black arrows, S‐layer; grey arrows, cell appendages. (e) Negative‐staining of a rod‐shaped M. kandleri. Scale bars: 200 nm (a), 100 nm (b), 500 nm (c,d) and 1 µm (e).

Figure 2.

(a) Most common core lipids in archaeal membranes, R=polar headgroup; (b) 50 nm Epon section of a high pressure frozen Ignicoccus hospitalis cell contrasted with 0.5% uranyl acetate. CP, cytoplasm; CM, cytoplasmic membrane; IMC, inter membrane compartment; OCM, outer cellular membrane; V, vesicle. Scale bar: 0.5 µm.

Figure 3.

Flagella of Sulfolobus acidocaldarius MW156. (b) and (c) both show the cell appendages of the lobed coccoid cell in (a) at higher magnification. Especially in (c), the repetitive pattern of the protein subunits (flagellins) becomes visible. Negative staining with 2% uranyl acetate. Scale bars: 1 µm (a), 200 nm (b) and 100 nm (c).

Figure 4.

Archaeal cell appendages. (a) Micrograph of a negatively stained flagellum from M. hungatei showing the filament, hook and knob (Reproduced and modified from Archaeal cells by Terry J Beveridge [A383]). (b) Cannulae of the crenarchaeote P. abyssi. Courtesy of G. Rieger, Institute of Microbiology, University of Regensburg, Germany. (c) Fibers of I. hospitalis. Courtesy of C. Meyer, Institute of Groundwater Ecology, German Research Center for Environmental Health, Neuherberg, Germany. (d) Hami of SM1 euryarchaeon, exhibiting the grappling hook at the tip. Courtesy of C. Moissl‐Eichinger, Institute of Microbiology, University of Regensburg, Germany. (e) Mth60 fimbriae of Methanothermobacter thermautotrophicus. Courtesy of C. Sarbu, Institute of Microbiology, University of Regensburg, Germany. Negative staining with uranyl acetate. Scale bars: 100 nm.

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Further Reading

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Klingl, Andreas, Flechsler, Jennifer, Heimerl, Thomas, and Rachel, Reinhard(Sep 2013) Archaeal Cells. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000383.pub2]