Syntrophy is the mutually dependent relationship between acetogenic bacteria and methanogenic archaea that makes anaerobic digestion possible. The bacteria can only degrade fatty acids (such as propionic acid) if the archaea remove the hydrogen they produce; if H₂ builds up, the reaction becomes thermodynamically impossible and the process blocks. Propionic acid is the key indicator that this relationship has broken down. This article explains the thermodynamics of syntrophy, why propionic acid gives the earliest warning, how syntrophy breaks and what DIET is.
Syntrophy is one of the concepts that best explains why anaerobic digestion is so delicate. It is the obligate cooperation between two groups of microorganisms that, on their own, could not work.
Without syntrophy there is no stable biogas. Understanding it means understanding why a digester becomes unbalanced and why propionic acid is the most reliable warning signal.
This is the “what is” of syntrophy: its thermodynamics, the microorganisms involved, the role of hydrogen, propionic acid as an indicator and DIET, the modern alternative to classic syntrophy.
Definition of syntrophy
Syntrophy (literally “feeding together”) is a relationship of mutual metabolic dependence between several microorganisms: the reaction one of them carries out is only viable if the other consumes its product immediately.
In anaerobic digestion, the key syntrophy is the one between acetogenic bacteria (which degrade fatty acids and alcohols) and hydrogenotrophic methanogenic archaea (which consume the hydrogen). Neither works without the other, and it is that coupling that allows the phases of anaerobic digestion to move forward.
Thermodynamics of syntrophy: why it needs coupling
The degradation of acids such as propionic or butyric into acetate and hydrogen is thermodynamically unfavourable under standard conditions: on its own, the reaction releases no energy and does not happen.
It only becomes viable if the hydrogen concentration is kept extremely low. That is where the methanogens come in: by consuming H₂ as soon as it is produced, they “pull” the reaction and make it possible. It is an obligate thermodynamic coupling.
Syntrophic acetogenic bacteria: Syntrophomonas, Syntrophobacter
The characteristic syntrophic genera are Syntrophomonas (which degrades long chain fatty acids) and Syntrophobacter (specialised in propionic acid). They are slow microorganisms: their doubling rate ranges from 1 to 10 days.
That slowness, added to their dependence on the archaea, makes them very vulnerable: any disturbance affecting the methanogens drags them down as well.
The role of the H₂ partial pressure
Hydrogen is the currency of syntrophy. For propionic acid degradation to be viable, the H₂ partial pressure must stay below ~10⁻⁴ atm, an extraordinarily low value.
If the methanogens slow down (because of pH, temperature, inhibition and so on), H₂ builds up, its partial pressure rises and acid degradation stops. The first one to suffer is propionic acid, which starts to accumulate.
Syntrophy and propionic acid: the key indicator of decoupling
Propionic acid is the best telltale of syntrophy because its degradation is the most sensitive to H₂ pressure. When syntrophy, or the biological balance, breaks down, propionic acid accumulates before any other acid.
As an operational reference: an acetate/propionate ratio > 2 indicates a healthy process; below 1, there is a crisis. And a propionic acid concentration > 1,000 mg/L is a clear sign of decoupling. That is why it is monitored alongside FOS/TAC and the sentinel variables.
How syntrophy breaks: overload, inhibition, thermal disturbance
Syntrophy breaks whenever something slows the methanogens down and lets H₂ rise:
- Organic overload: more acid is fed than the archaea can process.
- Inhibition: free ammonia, sulphides or toxics that slow down methanogenesis (when to step in with chemistry is filtered by the additives protocol).
- Thermal disturbance: an abrupt temperature change stresses the archaea, which are more sensitive than the bacteria.
In every case the pattern is the same: H₂ rises, propionic acid accumulates and methane falls. Methanogenesis and syntrophy fall together.
DIET: direct electron transfer, the alternative to classic syntrophy
Classic syntrophy depends on hydrogen diffusing from one microorganism to another, which is slow. There is an alternative: Direct Interspecies Electron Transfer (DIET), in which bacteria and archaea exchange electrons directly, without hydrogen as an intermediary.
DIET can be up to a million (10⁶) times more efficient than H₂ mediated syntrophy. Conductive materials, such as the Smallops OPS nanoparticles, promote DIET and are therefore used to unblock digesters with syntrophy problems.
Frequently asked questions about syntrophy in biogas
What is syntrophy in anaerobic digestion?
It is the mutually dependent relationship between acetogenic bacteria (which degrade fatty acids and alcohols) and methanogenic archaea (which consume the hydrogen). The degradation of those acids is only thermodynamically viable if the archaea remove the hydrogen as it is produced. Without that cooperation, the process blocks. It is what makes anaerobic digestion a coupled system.
Why is propionic acid the key indicator of syntrophy?
Because propionic acid degradation is the reaction most sensitive to hydrogen pressure in the whole chain. As soon as syntrophy weakens and H₂ starts to build up, propionic acid is the first acid to rise. That is why its concentration (above 1,000 mg/L is a warning sign) and the acetate/propionate ratio (healthy above 2, in crisis below 1) are the best early indicators of imbalance.
What is DIET and how does it differ from classic syntrophy?
DIET stands for Direct Interspecies Electron Transfer. In classic syntrophy, bacteria and archaea exchange energy through hydrogen, which diffuses from one to the other (a slow and fragile process). In DIET they exchange electrons directly, with no hydrogen in between, which can be up to a million times more efficient. Conductive materials (such as the Smallops OPS iron nanoparticles) promote DIET and help to stabilise and speed up digestion.
How is the loss of syntrophy detected in a biogas plant?
Through propionic acid accumulation and falling methane. In practice you monitor the propionic acid concentration (alarm above 1,000 mg/L), the acetate/propionate ratio (crisis below 1) and FOS/TAC. When propionic acid rises while methane falls, that is the signature of syntrophy having decoupled: the methanogens are not consuming hydrogen fast enough and the syntrophic bacteria stop.
Smallops and the monitoring of syntrophy
Syntrophy is invisible in day to day operation, but when it breaks it is expensive. At Smallops we monitor it through its indicators (propionic acid, FOS/TAC) with an Operational Excellence Diagnosis.
Do you suspect your digester is losing syntrophy?
Request a Smallops Operational Excellence Diagnosis: we analyse propionic acid, VFA ratios and FOS/TAC to detect the decoupling before the plant stops.
References and standards
McInerney, M.J. et al. (2009). Syntrophy in anaerobic global carbon cycles. Current Opinion in Biotechnology, 20 (6), 623-632. → doi.org/10.1016/j.copbio.2009.10.001
Stams, A.J.M. & Plugge, C.M. (2009). Electron transfer in syntrophic communities of anaerobic bacteria and archaea. Nature Reviews Microbiology, 7, 568-577. → doi.org/10.1038/nrmicro2166
Lovley, D.R. (2017). Syntrophy goes electric: direct interspecies electron transfer. Annual Review of Microbiology, 71, 643-664. → doi.org/10.1146/annurev-micro-030117-020420
Schink, B. (1997). Energetics of syntrophic cooperation in methanogenic degradation. Microbiology and Molecular Biology Reviews, 61 (2), 262-280.