High CO₂ levels drive the TCA cycle backwards towards autotrophy

Area of Science:

• Microbiology
• Biochemistry
• Geochemistry

Background:

• The tricarboxylic acid (TCA) cycle, typically involved in energy production, can be reversed in some anaerobic microorganisms for autotrophic carbon fixation.
• This reversed cycle utilizes specific enzymes like ferredoxin-dependent 2-oxoglutarate synthase and requires high concentrations of citrate synthase.

Purpose of the Study:

• To investigate the conditions driving the reversed oxidative TCA cycle in the thermophilic bacterium Hippea maritima.
• To elucidate the role of high carbon dioxide (CO2) partial pressures in this autotrophic pathway.

Main Methods:

• Utilized the thermophilic sulfur-reducing deltaproteobacterium Hippea maritima for experimental analysis.
• Investigated the biochemical pathway involving citrate synthase and pyruvate synthase under varying CO2 conditions.

Main Results:

• Demonstrated that high partial pressures of CO2 are essential for driving the reversed oxidative TCA cycle in Hippea maritima.
• Showed that acetyl coenzyme A (acetyl-CoA) removal via reductive carboxylation to pyruvate, catalyzed by pyruvate synthase, is crucial under high CO2.

Conclusions:

• The reversed oxidative TCA cycle is actively driven by high CO2 levels in Hippea maritima.
• This pathway offers a more energy-efficient alternative to the reductive TCA cycle for autotrophic carbon fixation.
• The findings suggest that this reversed TCA cycle could have been a significant mechanism for CO2 fixation in early Earth's primordial atmosphere.