Acetogenesis (acetogenic stage)

Acetogenesis is the third of four stages of anaerobic digestion, in which syntrophic acetogenic bacteria convert the longer-chain volatile fatty acids and alcohols produced during acidogenesis — propionate, butyrate, valerate, ethanol, lactate — into acetate, hydrogen, and carbon dioxide. These three end products are the direct substrates for the methanogenic archaea in the final stage. Acetogenesis is a critical biochemical bridge: without it, only acetate-producing acidogenesis would feed methanogens, and 60-70% of the methane potential in lipid-rich and propionate-rich feedstocks would be lost.

Key acetogenic genera include Syntrophobacter, Syntrophomonas, Smithella, and Pelotomaculum. The defining feature of these bacteria is that their conversion reactions are thermodynamically unfavourable under standard conditions — they only proceed when hydrogen partial pressure is kept extremely low (below 10 to the minus 4 atmospheres). This is achieved through obligate syntrophy with hydrogen-consuming methanogens: the acetogens produce hydrogen, the methanogens consume it immediately, and the resulting low partial pressure pulls the acetogenic reactions forward.

This tight biochemical coupling has several operational implications:

• Spatial proximity matters: methanogens and acetogens must remain physically close, often as mixed-species granules or biofilms. Excessive mechanical mixing can disrupt these clusters and reduce yield.
• Hydrogen accumulation is toxic: any disturbance that suppresses methanogenesis (low pH, ammonia inhibition, temperature shock) causes hydrogen to build up, which halts acetogenesis within hours and triggers VFA accumulation.
• Propionate is a sentinel metabolite: rising propionate concentration in digestate samples is the earliest signal of acetogenic-methanogenic imbalance, often appearing days before pH drops or biogas methane content falls.

Acetogenesis is slower than acidogenesis but faster than methanogenesis. In healthy digesters, propionate stays below 1,000 mg/L and acetate below 2,000 mg/L. When propionate exceeds 3,000 mg/L, recovery becomes difficult — propionate degradation has the highest hydrogen-sensitivity in the consortium. The trade-off in process management is that operators cannot directly control acetogenesis through dosing or feeding; they can only protect it by maintaining the conditions methanogenesis needs, since healthy methanogenesis is what keeps acetogenesis viable.