Acetogenic bacteria (acid-forming bacteria)

Acetogenic bacteria are the microorganisms responsible for the third stage of anaerobic digestion — acetogenesis — in which volatile fatty acids longer than acetic (propionic, butyric, valeric) and alcohols produced upstream by acidogens are converted into acetic acid, hydrogen and carbon dioxide. These three products are the direct substrates that methanogens use to produce methane, so acetogens occupy a pivotal bridging role in the AD food chain.

Acetogens are a diverse group of strict anaerobes, including genera like Syntrophomonas, Syntrophobacter, Clostridium and Acetobacterium. Many of them perform syntrophic metabolism — meaning they depend on a partner methanogen to remove hydrogen from the local environment, because hydrogen build-up makes their reactions thermodynamically unfavourable. This obligate partnership is one reason AD systems are sensitive: hydrogen produced by acetogens must be consumed by hydrogenotrophic methanogens at a comparable rate, or acetogenesis stalls and intermediates accumulate.

The reactions catalysed are energetically marginal. Propionate oxidation to acetate, for example, is only thermodynamically favourable when partial pressure of hydrogen in the system is below about 10⁻⁴ atmospheres — a condition maintained only when methanogens are actively consuming hydrogen. This explains why VFA accumulation in stressed digesters often shows propionate at high levels even when acetate is being consumed: acetogens cannot keep up because their methanogen partners have slowed.

Operationally, acetogens are vulnerable to the same stressors as methanogens. They prefer pH 6.5-7.5, temperature stability at 35-40°C mesophilic or 50-55°C thermophilic, and abhor rapid changes in either. Their doubling time is slow — 1-2 days — which is why digesters recovering from upsets take 5-15 days to fully rebuild acetogenic populations and another similar period for methanogens. The practical implication is that the four-stage AD process is no faster than its slowest link. In high-fat feedstocks (slaughterhouse waste, used cooking oil) acetogens face long-chain fatty acid intermediates that are especially slow to oxidise, requiring HRT extensions of 30-50% and lower OLR to maintain balance.