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: $4H<em>2 + CO</em>2 \rightarrow CH<em>4 + 2H</em>2O$

• 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 $CO<em>2$ to methane ($CH</em>4$).

• 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 ($SO<em>4^{2-}$) to hydrogen sulfide ($H</em>2S$).

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.

  • $CH<em>4 + 3H</em>2O \rightarrow HCO<em>3^- + 4H</em>2 + H^+$ ($\Delta G = +132kJ$)

  • $SO<em>4^{2-} + 4H</em>2 + H^+ \rightarrow HS^- + 4H_2O$ ($\Delta G = -150kJ$)

Anaerobic Food Web

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

• Sulfate reducers reduce sulfate ($SO<em>4^{2-}$) to hydrogen sulfide ($H</em>2S$).

• 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, $H<em>2$, and $CO</em>2$.

• Acetogens convert $H<em>2$ and $CO</em>2$ into acetate.

• Methanogens convert acetate into methane ($CH<em>4$) or $CO</em>2$.

• ANME/syntrophs oxidize methane in syntrophic relationships.