Archaea Study Guide

High‑Yield Archaea Study Guide (Designed for rapid review before an exam) --- 📖 Core Concepts Domain Archaea – One of the three cellular domains (Archaea, Bacteria, Eukarya). Prokaryotic but phylogenetically distinct; includes the eukaryotes in some analyses. Ether‑linked membrane lipids – Glycerol‑ether lipids (sn‑glycerol‑1‑phosphate) form either bilayers or monolayers; opposite stereochemistry to bacterial/eukaryotic ester‑linked lipids. Methanogenesis – Unique anaerobic metabolism that reduces CO₂ (or other C‑compounds) with H₂ to CH₄, using coenzymes M, F430, methanofuran. Archaellum – The archaeal motility structure; a proton‑driven filament assembled at its base, not at the tip like bacterial flagella. Cell‑wall S‑layer – Protein‑based crystalline sheet; most archaea lack peptidoglycan, some have pseudo‑peptidoglycan (no D‑amino acids, no N‑acetylmuramic acid). Phylogenetic placement – Archaea are a paraphyletic group that includes eukaryotes; “Asgard” (Loki‑, Thor‑, Odin‑, Heimdallarchaeota) are closest to eukaryotes. Transcription/Translation similarity – Archaeal RNA polymerase ≈ eukaryotic RNAP II; transcription factors and many ribosomal proteins are more eukaryote‑like than bacterial. Cell‑division machineries – FtsZ ring (bacterial‑like) in Methanobacteria; Cdv/ESCRT‑III system (eukaryote‑like) in Crenarchaea/Thaumarchaea. --- 📌 Must Remember Membrane backbone: sn‑glycerol‑1‑phosphate (archaea) vs sn‑glycerol‑3‑phosphate (bacteria/eukaryotes). Key methanogenesis reaction: $$\mathrm{CO2 + 4\,H2 \;\longrightarrow\; CH4 + 2\,H2O}$$ Three‑Domain System (Woese, Kandler, Wheelis): Bacteria ‑ Archaea ‑ Eukarya. Asgard superphylum = kingdom Promethearchaeota, proposed sister to eukaryotes. Archaellum power source: proton gradient (not sodium). Major carbon‑fixation pathways: Modified Calvin, 3‑hydroxypropionate/4‑hydroxybutyrate, reverse Krebs, reductive acetyl‑CoA. Halophilic light‑driven pumps: bacteriorhodopsin (proton pump) & halorhodopsin (Cl⁻ pump). Archaeal RNA polymerase: single multisubunit enzyme; no σ‑factor, uses TBP & TFB (eukaryote‑like). Ecological impact: ≈ 30 % of global methane originates from archaeal methanogenesis; ammonia‑oxidizing archaea dominate nitrification in many soils & oceans. --- 🔄 Key Processes Methanogenesis (CO₂‑reduction route) H₂ → reduced electron carriers → coenzyme M → reduction of CO₂ → CH₄ + H₂O. Archaeal cell‑division (Cdv pathway) CdvA binds DNA → CdvB polymerizes → CdvC (AAA‑ATPase) constricts membrane → scission (ESCRT‑III‑like). Phototrophic ATP generation in halophiles Light → bacteriorhodopsin pumps H⁺ out → proton gradient → ATP synthase synthesizes ATP. RNA transcription initiation TBP binds TATA‑box → TFB recruits RNAP → open complex formation → elongation. S‑layer assembly Secreted S‑layer proteins self‑assemble into a 2‑D crystal covering the cell surface. --- 🔍 Key Comparisons Archaea vs Bacteria (cell envelope) Archaea: ether‑linked lipids, S‑layer, no peptidoglycan. Bacteria: ester‑linked lipids, peptidoglycan (Gram‑positive thick, Gram‑negative thin). Archaellum vs Bacterial flagellum Archaellum: assembled at base, powered by proton gradient, no hook. Flagellum: tip‑assembly, powered by proton or sodium motive force, has hook. FtsZ division vs Cdv/ESCRT‑III division FtsZ: tubulin‑like ring, bacterial style. Cdv: ESCRT‑III homologs, membrane scission similar to eukaryotic cytokinesis. Methanogenesis vs Aerobic respiration Methanogenesis: anaerobic, produces CH₄, uses unique coenzymes. Respiration: uses O₂ as terminal electron acceptor, yields H₂O. --- ⚠️ Common Misunderstandings “All archaea are extremophiles.” Reality: Many live in moderate soils, oceans, and the human gut. “Archaea have nuclei.” Reality: They lack membrane‑bound organelles, just like bacteria. “Methanogenesis occurs in all anaerobes.” Reality: Only specific methanogenic archaea possess the required coenzymes. “Archaeal viruses behave like bacteriophages.” Reality: They often show mixed lytic/lysogenic strategies and unusual morphologies. --- 🧠 Mental Models / Intuition “Ether‑mirror” model: Imagine bacterial lipids as a right‑handed screw; archaeal lipids are the left‑handed mirror—this explains resistance to heat and extreme pH. “Two‑engine car” analogy for metabolism: One engine (methanogenesis) runs on H₂ + CO₂ → CH₄; another (phototrophy) runs on light → proton pump → ATP. “Bridge to eukaryotes” – Think of Asgard archaea as the “construction site” where eukaryotic features (actin, ESCRT, transcription factors) were first assembled. --- 🚩 Exceptions & Edge Cases Pseudopeptidoglycan: Some archaea (e.g., Methanothermus spp.) have a cell wall that mimics peptidoglycan but lacks D‑amino acids and N‑acetylmuramic acid. Monolayer membranes: Acidophilic and hyperthermophilic archaea often use tetraether lipids that span the entire membrane, forming a single‑layer “film” for extra stability. Archaea lacking archaella: Many marine planktonic archaea are non‑motile; they rely on passive diffusion. --- 📍 When to Use Which Identify a methanogen? Look for: (a) habitat = anaerobic, (b) presence of coenzyme M/F430 genes, (c) CH₄ production in culture. Choose a DNA polymerase for PCR: Use thermostable archaeal polymerases (e.g., Pyrococcus furiosus Pfu) for high‑fidelity, high‑temperature runs. Determine cell‑wall type: If Gram‑staining is ambiguous and 16S indicates archaeal lineage → assume S‑layer or pseudopeptidoglycan, not peptidoglycan. Select a phylogenetic marker: 16S rRNA works for broad domain placement; for fine resolution among Asgard lineages, use concatenated ribosomal proteins or specific ESCRT genes. --- 👀 Patterns to Recognize Methane ⇢ CO₂ + H₂ in anaerobic gut or sediment samples → suspect methanogenic archaea. High salinity + light absorption peak at 560 nm → bacteriorhodopsin‑containing halophiles. Co‑occurrence of archaea with protozoa in ruminant gut → syntrophic H₂ consumption → efficient cellulose digestion. Presence of TBP/TFB genes in a genome → archaeal (or eukaryotic) transcription system, not bacterial. --- 🗂️ Exam Traps “All archaeal membranes are monolayers.” Trap: Only some (acidophiles, hyperthermophiles) have tetraether monolayers; many have bilayers. “Archaea are always Gram‑positive.” Trap: Gram‑stain does not apply; they lack peptidoglycan altogether. “Methanogenesis can use nitrate as electron acceptor.” Trap: Methanogenesis is strictly anaerobic; nitrate reduction is a bacterial process. “Archaellum = flagellum.” Trap: Different assembly, genetics, and energy source; treat as distinct structures. --- Use this guide to skim each topic, test yourself on the bullet points, and flag any “trap” statements you encounter on practice exams.