Chapter 4: Archaeal Cell Structure & Extreme Environments

Chapter 4 details the complex and often unique cellular architecture of organisms belonging to Domain Archaea, a distinct taxon whose members exhibit a chimeric nature with characteristics similar to both bacteria and eukaryotes. While their general cell structure resembles canonical prokaryotes, their molecular components are unique. Archaeal morphology varies widely, commonly including rods and cocci, but also encompasses unique forms like branched cells and flat, postage-stamp shapes (such as Haloquadratum walsbyi). The archaeal cell envelope, consisting of the plasma membrane and external layers, is structurally diverse. Crucially, the plasma membranes are distinguished by their lipids, which are derived from isoprene units (making them branched) and are linked to glycerol via ether bonds, enhancing stability against extreme conditions. These lipids form either typical bilayers (using glycerol diethers) or highly rigid monolayers (using diglycerol tetraethers), often found in extreme thermophiles. Archaeal cell walls universally lack peptidoglycan; the most prevalent wall type is an S-layer composed of protein or glycoprotein. Some species utilize a peptidoglycan-like polymer called pseudomurein, which differs chemically in its cross-links and glycosidic bonds, making it resistant to antibiotics like penicillin. Internally, the cytoplasm is similar to bacteria, containing 70S ribosomes; however, archaeal ribosomes have more proteins, many of which share homology with eukaryotic proteins, rendering them immune to antibiotics that target bacterial ribosomes. The nucleoid contains circular, double-stranded DNA organized by nucleoid-associated proteins (NAPs). Notably, many Euryarchaeota are polyploid and package their DNA using histones that form nucleosomes similar to those found in eukaryotes. Externally, archaea utilize pili for adhesion and DNA transfer, including specialized structures like cannulae (tubes connecting daughter cells) and hami (grappling hook appendages). Motility is achieved via archaella (archaeal flagella), which are thinner than bacterial flagella, are assembled in a manner akin to bacterial type IV pili, and utilize ATP hydrolysis for rotation, enabling extremely fast "relocate-and-seek" swimming behaviors and allowing for taxis responses like chemotaxis and phototaxis. This diversity in structure supports the existence of physiological groups such as methanogens, which are environmentally significant producers of methane gas.