Microbial Metabolism: Chemolithoautotrophs, Methanogens, Acetogens, and the Sulfur Cycle

Chemolithoautotrophs

  • Use inorganic electron donors.

  • Can function in the presence or absence of oxygen.

  • Carbon source.

Reverse Electron Flow

  • A portion of the proton motive force (PMF) generated through respiration is used for NADH + H^+ synthesis.

  • NADH is exchanged for NADPH via the Pyridine nucleotide transhydrogenase.

  • Both NADPH and ATP are required for CO_2 fixation.

  • This process reduces the amount of energy available for other cellular functions.

Methanogens – Obligate Anaerobes

  • Archaea (Euryarchaeota)

  • Reaction: 4H2 + CO2 \rightarrow CH4 + 2H2O

  • The process requires unique coenzymes.

  • Involves C1 carriers.

  • Involves electron carriers.

C1 Carbon Carriers

  • Methanofuran (MF)

  • Tetrahydromethanopterin (H4MPT)

  • Coenzyme M (COM)

Methylreductase Complex Cofactors

  • Coenzyme F420

  • Coenzyme F430

  • Coenzyme B (CoB)

Process Overview

  1. CO_2 is converted to Formate.

  2. Formate is then converted to Formyl-MF.

  3. Formyl-MF is converted to Formyl-H4MPT.

  4. Formyl-H4MPT is converted to CH2-H4MPT.

  5. CH2-H4MPT is converted to CH3-H4MPT.

  6. CH3-H4MPT is converted to Methyl-COM.

  7. Methyl-COM is converted to Methane (CH_4).

Electron Flow and Energy Conservation

  • Reduced ferredoxin (Fd_{red}) is involved in the reduction steps.

  • Coenzyme F420 acts as an electron carrier.

  • Electron bifurcating hydrogenase is used.

  • Sodium ion (Na^+) gradient is generated, which drives ATP synthesis.

Methyl Transferase and Methyl Reductase

  • Various methyl substrates, such as methylamines, methyl sulfides, and methanol, can be converted to methane.

  • Methyl reductase complex is utilized with cofactors.

Methane Metabolism

  • Methanotrophs metabolize C1 compounds, including methane.

  • Step 1: Methane monooxygenase is used.

  • Steps 2-4: Dehydrogenases produce NADH + H^+, generating energy.

Biosynthesis in Methanogens

  • Formaldehyde is channeled into the Ribulose monophosphate cycle (RuMP) or Serine cycles.

  • Some methanogens can use CO_2 via the Calvin Cycle.

Acetogens

  • C1 metabolism leads to the production of acetate.

  • Tetrahydrofolate (THF) is a key cofactor.

Process

  1. CO_2 is converted to Formate-THF.

  2. Formate-THF is converted to CH2-THF.

  3. CH2-THF is converted to CH3-THF.

  4. CH3-THF is converted to CH3-CoFeSP.

Energy Generation

  • Rnf complex is involved in electron transfer and sodium ion (Na^+) gradient generation.

  • Electron bifurcating hydrogenase is used.

  • Acetyl-CoA synthase/CODH is used to produce Acetyl-CoA.

Methanogens vs. Acetogens

  • Methanogens reduce CO2 to methane (CH4).

  • Acetogens reduce CO_2 to acetate.

  • Both involve unique coenzymes and electron carriers.

The Sulfur Cycle

  • Sulfur is essential for all living cells.

  • Found in cofactors and some amino acids.

  • Exists in various oxidation states, from sulfate (SO4^{2-}) to hydrogen sulfide (H2S).

Sulfate Reducers

  • Form of anaerobic respiration.

  • Sulfate (or other oxidized sulfur species) is the terminal electron acceptor.

  • Often referred to as SRB (Sulfate-Reducing Bacteria).

  • Electron donor can be a reduced organic compound or H_2.

  • Adenosine phosphosulfate (APS) is involved.

  • Periplasmic hydrogenase is used.

Sulfide Oxidation

  • Reduced sulfur compounds can act as electron donors for chemolithoautotrophs.

  • Sulfur oxidation system (SOX) bypasses the upstream portion of the ETC.

  • Fewer protons are pumped across the membrane.

  • Sulfate reduction in reverse generates energy using the ETC with oxygen as the terminal electron acceptor (TEA).

Elemental Sulfur Storage

  • Some organisms store elemental sulfur in external granules.

  • Sulfur transformations can result in the production of sulfuric acid.

Syntrophy

  • Conversion of ethanol to acetate is energetically unfavorable alone, but viable through syntrophy.

  • Syntrophy is a metabolic dependence where two or more organisms cooperate to carry out a metabolic process that they cannot perform individually.

  • Example: Anaerobic Oxidation of Methane

    • Archaea and SRB work together.

    • CH4 + 3H2O \rightarrow HCO3^- + 4H2 + H^+ (\Delta G = +132kJ)

    • SO4^{2-} + 4H2 + H^+ \rightarrow HS^- + 4H_2O (\Delta G = -150kJ)

Anaerobic Food Web

  • The anaerobic food web involves a cycle of carbon and sulfur.

  • Sulfate reducers reduce sulfate (SO4^{2-}) to hydrogen sulfide (H2S).

  • Organic polymers are broken down by exoenzyme producers into monomers and oligomers.

  • Fermenters convert these into organic acids and alcohols.

  • Secondary fermenters/syntrophs convert organic acids and alcohols into acetate, H2, and CO2.

  • Acetogens convert H2 and CO2 into acetate.

  • Methanogens convert acetate into methane (CH4) or CO2.

  • ANME/syntrophs oxidize methane in syntrophic relationships.