Microbial Carbon Cycle

What enzyme catalyses CO₂ fixation in the Calvin cycle?

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)

Name one anaerobic carbon fixation pathway and a microbe that uses it.

• Reductive Acetyl-CoA pathway

• Methanobacterium

What is the overall reaction for aerobic respiration of glucose?

C₆​H₁₂​O₆​ + 6O_2​ → 6CO₂​ + 6H₂​O + ATP

Which microbial group oxidises methane under aerobic conditions?

Methanotrophs like Methylococcus capsulatus

What is the reaction for hydrogenotrophic methanogenesis?

CO₂ + 4H₂​ → CH₄​ + 2H₂​O

Which microbes perform anaerobic methane oxidation (AOM)?

• ANME archaea

• Sulphate-reducing bacteria (e.g. Desulfosarcina)

What environmental condition suppresses methanogenesis?

Presence of alternative electron acceptors (e.g. SO₄²⁻, NO₃⁻)

What is a key product of fermentation in anaerobic environments?

• Ethanol

• Acetate

• H₂

• CO₂

• Lactate

What controls carbonate precipitation in microbial mats?

CO₂ degassing and pH increase due to respiration/photosynthesis

What process links the carbon and sulphur cycles in marine sediments?

Anaerobic oxidation of methane (AOM) coupled to sulphate reduction

Which carbon fixation pathway is considered the most energy-efficient, and where is it found?

• Reductive Acetyl-CoA pathway

• Found in acetogens and methanogens in deep anoxic environments.

What is the function of the reverse TCA (rTCA) cycle, and which microbes use it?

• Fixes CO₂ in anaerobic, low-light environments

• Used by Chlorobium (green sulphur bacteria).

Name a microbial process in the carbon cycle that does not require an electron transport chain.

Fermentation – redox reactions occur internally without external electron acceptors.

What is the significance of δ¹³C-depleted lipids in sediment studies?

Indicate anaerobic methane oxidation (AOM) by ANME archaea.

What type of carbon fixation occurs in Nitrosomonas and Nitrobacter?

Chemolithoautotrophic carbon fixation via the Calvin cycle, powered by nitrification.

Which methanogen performs both acetoclastic and hydrogenotrophic methanogenesis?

Methanosarcina

What is the terminal electron acceptor in iron-reducing carbon respiration?

Ferric iron (Fe³⁺) - reduced to Fe²⁺ by Geobacter and Shewanella.

How do pressure and temperature affect microbial carbon cycling in deep crust environments?

High pressure and low nutrient flux select for slow-growing, syntrophic, or hydrogen-dependent microbes.

What microbial process dominates in peat bogs and rice paddies for carbon release?

Methanogenesis (mainly hydrogenotrophic), due to waterlogged, anoxic conditions.

Why does AOM-SR represent a syntrophic process?

Because ANME archaea oxidise methane and transfer electrons to SRBs which reduce sulphate—neither can perform the full reaction independently.