Both forward and reverse TCA cycles operate in green sulfur bacteria

Area of Science:

• Microbiology
• Biochemistry
• Metabolic pathways

Background:

• Anoxygenic green sulfur bacteria (GSBs) are autotrophs utilizing the reductive (reverse) tricarboxylic acid (RTCA) cycle for CO2 assimilation.
• GSBs can incorporate organic carbon sources like acetate and pyruvate during phototrophic growth, but the necessity of inorganic carbon for this process remains unclear.

Purpose of the Study:

• To investigate carbon flux during autotrophic and mixotrophic growth in the GSB Chlorobaculum tepidum.
• To elucidate the mechanisms of organic carbon assimilation and the role of inorganic carbon in GSB metabolism.

Main Methods:

• Analysis of carbon flux during different growth conditions (autotrophic and mixotrophic).
• Investigating the activity and contribution of the RTCA and oxidative (forward) TCA (OTCA) cycles.

Main Results:

• The RTCA cycle is active in both autotrophic and mixotrophic growth.
• Pyruvate is primarily assimilated via the RTCA cycle, with low flux from pyruvate to acetyl-CoA.
• Acetate assimilation occurs through both RTCA and OTCA cycles, with the OTCA cycle potentially enhancing growth.
• CO2 is essential for anaplerotic pathways, pyruvate synthesis, and the overall completion of the TCA cycle metabolites.

Conclusions:

• The RTCA cycle is fundamental for carbon assimilation in GSBs, even during mixotrophic growth.
• The oxidative TCA (OTCA) cycle contributes to energy generation during mixotrophic growth with acetate.
• Inorganic carbon (CO2) is indispensable for efficient organic carbon assimilation and the functioning of the TCA cycle in GSBs.