🧪 1. Sulfate and Sulfur Reduction

• Sulfate (SO₄²⁻) is a common electron acceptor for anaerobic respiration.
• Two types:
  • Assimilative reduction → used to make amino acids like cysteine & methionine.
  • Dissimilative reduction → energy generation; produces H₂S (hydrogen sulfide).
• H₂S is toxic to animals but vital in ecosystems — can form metal sulfides (black sediments).
• Key players: Desulfovibrio and other sulfate-reducing bacteria.
• ATP sulfurylase activates sulfate → APS (adenosine phosphosulfate), then APS reductase and sulfite reductase finish the reduction to H₂S.
• Energy: low yield (~2 ATP per sulfate), driven by H₂ or organic acids (lactate, acetate) using cytochrome c₃ & hydrogenases.

⚙️ 2. Special Metabolisms of Sulfate Reducers

• Some can:
  • Reduce elemental sulfur (S⁰) to sulfide.
  • Perform disproportionation → one sulfur compound oxidized while another reduced (e.g., thiosulfate → H₂S + SO₄²⁻).
  • Oxidize phosphite → phosphate, coupled to sulfate reduction (chemolithotrophic).
• Highlights the versatility of anaerobes in using weird energy sources!

⚡ 3. Other Electron Acceptors (Anaerobic Respiration)

• Metals & metalloids (Fe³⁺, Mn⁴⁺, As⁵⁺, Cr⁶⁺, etc.) can be used as electron acceptors.
• Challenge: they’re insoluble, so bacteria must send electrons outside their cells!
• This leads to fascinating systems for extracellular electron transfer (you’ll see later with Geobacter and Shewanella).

🧫 4. Acetogenesis (CO₂ → Acetate)

• Acetogens (e.g., Acetobacterium woodii, Clostridium aceticum) reduce CO₂ to acetate:

4H₂ + 2CO₂ + H⁺ → CH₃COO⁻ + 4H₂O

• Uses the reductive acetyl-CoA pathway (aka Wood–Ljungdahl pathway).
• Two branches:
  • Methyl branch: CO₂ → CH₃ (via tetrahydrofolate, THF)
  • Carbonyl branch: CO₂ → CO (via CO dehydrogenase)
  • Combine → acetyl-CoA → acetate + ATP
• Energy conservation: via ion motive force (Na⁺ or H⁺) and Rnf complex.
• Yields very little ATP (~0.3 ATP per acetate) — these microbes are minimalists!

💨 5. Methanogenesis (CH₄ Production)

• Exclusive to Archaea — strict anaerobes found in wetlands, animal guts, and sediments.
• Converts CO₂ + H₂ → CH₄ (methane) 💡

CO₂ + 4H₂ → CH₄ + 2H₂O

• Coenzymes:
  • Methanofuran (MF) & Methanopterin (MP) → carry the carbon unit.
  • Coenzyme M (CoM) & Coenzyme B (CoB) → final CH₄ formation.
  • F₄₂₀ (fluorescent coenzyme!) → electron carrier.
  • F₄₃₀ (Ni-containing) → in methyl reductase enzyme.
• Two groups:
  • Without cytochromes → Na⁺ motive force (less ATP).
  • With cytochromes → H⁺ motive force (more ATP).
• Also metabolize:
  • Methyl compounds (e.g., methanol) → CH₄
  • Acetate (acetoclastic methanogenesis) → CO₂ + CH₄

🔁 6. Methanotrophy (CH₄ Oxidation)

• Methanotrophs = bacteria that eat methane.
• Aerobic pathway: CH₄ → CH₃OH → CH₂O → HCOOH → CO₂
  • Enzyme: methane monooxygenase (MMO) (either soluble or membrane-bound).
• Assimilation of C₁ units:
  • Serine pathway (needs energy & NADH).
  • Ribulose monophosphate (RuMP) pathway (more efficient).
• Anaerobic methane oxidation (AOM):
  • Partnership between methanotrophic Archaea (ANME) and sulfate-reducing bacteria (SRB).
  • Reverse methanogenesis!
  • Transfer electrons directly between partners using cytochromes & nanowires ⚡

🌋 7. Sulfur, Iron & Manganese Cycles (Ch. 21 Highlights)

Sulfur Cycle 🧄

• Interconversions among SO₄²⁻ ↔ S⁰ ↔ H₂S.
• Driven by:
  • Sulfate reducers (anaerobic, produce H₂S).
  • Sulfur oxidizers (aerobic or phototrophic).
  • Disproportionators (e.g., Desulfovibrio sulfodismutans).
• H₂S is toxic but essential for ecosystem sulfur turnover.

Iron & Manganese Cycles 🧲

• Fe and Mn cycle between soluble (Fe²⁺, Mn²⁺) and insoluble (Fe³⁺, Mn⁴⁺) forms.
• Metal reducers (Geobacter, Shewanella) send electrons outside the cell via:
  • Nanowires (pili) 🧵
  • Multiheme cytochromes (Mtr complex)
  • Electron shuttles (flavins, humic substances, pyocyanin)
• These mechanisms even inspire microbial fuel cells ⚡ — bacteria that generate electricity from waste!
• Metal oxidizers (e.g., Gallionella, Leptothrix, Mariprofundus) reverse the cycle — oxidizing Fe²⁺ → Fe³⁺.
• They secrete stalks or filaments to keep metal oxides from coating their cells.

💡 QUICK RECAP: