MicH vs mcrA: Which qPCR Target Identifies Methanogenic MIC Risk?
A high methanogen or mcrA result does not automatically indicate corrosive methanogens. MicH was developed to ask a narrower question: is selected DNA associated with a special extracellular hydrogenase mechanism represented in the sample?
Use mcrA for broader methanogenic context and MicH for a narrower corrosion mechanism
mcrA encodes the alpha subunit of methyl-coenzyme M reductase, central to methanogenesis and related MCR-based pathways. It is widely used to detect methanogenic potential within an assay’s coverage.
micH targets a specialised [NiFe] hydrogenase gene associated with corrosive methanogenic biofilms. A paired result can show whether broad methanogenic potential and the selected corrosion mechanism occur together, but neither DNA target proves current activity or corrosion.
MicH and mcrA: the difference at a glance
| Feature | mcrA | MicH |
|---|---|---|
| Primary question | Is selected genetic potential for methyl-coenzyme M reductase-based metabolism present? | Is the selected extracellular [NiFe] hydrogenase marker associated with corrosive methanogens present? |
| Biological breadth | Broad across many methanogenic Archaea; wider MCR families can include reverse methane and other anaerobic alkane pathways. | Narrower assay coverage among methanogenic Archaea carrying sufficiently matching micH sequences. |
| Closest operational use | Methanogen surveillance, methane-cycle context and population trending. | Mechanism-oriented methanogenic MIC investigation and surveillance. |
| Does it prove MIC? | No. | No. |
| Evidence maturity | Established functional marker with multiple assay designs. | Emerging corrosion biomarker supported by pure-culture, laboratory-reactor and field studies. |
What does mcrA measure?
mcrA encodes the alpha subunit of methyl-coenzyme M reductase. In methanogenesis, the enzyme catalyses the final methane-forming step. Because mcrA is functionally important and phylogenetically informative, it has become a common molecular marker for methanogenic communities.
“mcrA qPCR” is not one universal assay. Primer and probe sets cover different branches of a diverse gene family. Some may be optimised for conventional methanogens; broader sequence searches also encounter MCR-like systems involved in reverse methanogenesis or anaerobic alkane metabolism. Reports should therefore describe validated coverage rather than simply state “all methanogens”.
Best use: interpret mcrA with archaeal context, methane and substrate data, redox, salinity, temperature, flow path and a defined sample matrix.
What does MicH measure?
micH encodes the large subunit of a special extracellular [NiFe] hydrogenase associated with corrosive methanogenic biofilms. The proposed mechanism is acceleration of hydrogen formation at metallic iron, which improves access to metal-derived reducing power for selected hydrogenotrophic methanogens.
MicH is not a methanogen enumeration target. It is intended to distinguish a narrower mechanism-associated subgroup from the much broader methanogenic community. Standard MicH DNA qPCR still does not show that the hydrogenase was expressed or active at sampling time.
For more detail, see what the MicH biomarker measures.
How should combined mcrA and MicH patterns be interpreted?
| Pattern | What it may support | What to check next |
|---|---|---|
| mcrA positive MicH non-detect | Methanogenic or related MCR-based potential is represented, but the selected MicH mechanism was not detected in this sample. | Assay coverage, sample proximity, external hydrogen or substrates, other methanogenic corrosion mechanisms and corrosion evidence. |
| mcrA positive MicH positive | Both broader methanogenic context and the selected hydrogenase-associated marker are represented. | Surface association, repeated trend, methane/substrate context, metal loss and morphology. |
| mcrA non-detect MicH positive | Possible differences in assay coverage, abundance near limits, recovery or analytical performance. | Repeat extraction/qPCR, controls, reporting limits and the exact sequence scope of each assay. |
| Both non-detect | Neither selected target was detected above the reporting limit in this sample. | Sample representativeness, DNA recovery, other locations, different methanogen lineages and non-methanogenic MIC mechanisms. |
A MicH/mcrA ratio can look attractive as a “corrosive fraction”, but it is not automatically valid. Target copy number per genome, assay coverage, efficiency, quantification limits and recovery can differ. Any ratio needs explicit validation for the defined methods and asset.
Why methane production does not prove methanogenic MIC
Methanogens can produce methane using hydrogen, acetate, methylated compounds or other substrates, depending on the lineage. Hydrogen can come from fermentation or process chemistry rather than the steel. Methane can also be transported from another location. A methane result therefore provides process context but does not identify the electron source or prove material damage.
Methane present, MicH non-detect
Methanogenesis may be supported by another lineage, substrate or mechanism, or the target may be outside assay coverage. Corrosion cannot be inferred.
MicH positive, methane not measured
The DNA target is present, but the metabolic and electrochemical state remains unresolved. Add relevant process and surface evidence.
Likewise, a general methanogen count is not a corrosion-rate proxy. The 2021 oilfield study found broad methanogen abundance to be a poor discriminator of perceived MIC severity, which is precisely the diagnostic gap MicH was designed to address.
Match the sample to methanogenic MIC—not only methanogenesis
Water and filters are useful for mapping methanogenic DNA through a production system. However, localized MIC occurs at a material interface. A defined swab, coupon biofilm, deposit, corrosion product or pig-debris fraction has a stronger spatial connection to that process.
| Question | Preferred sample emphasis | Supporting information |
|---|---|---|
| Where are methanogens transported? | Repeated produced-water or filtered-water samples along the flow path. | Methane, substrates, flow, temperature, salinity and operating state. |
| Is MicH associated with a corrosion location? | Feature-associated swab, deposit, corrosion product or removed specimen. | Morphology, metallurgy, products, local chemistry and alternative causes. |
| Did a treatment or pigging event change the pattern? | Matched pre/post water samples plus surface or pig-debris samples where possible. | Dose, contact time, residual, neutralisation, mobilisation and regrowth timing. |
Build a layered methanogenic MIC panel
- Taxonomic context: archaeal 16S or defined lineage assays where broader community information is needed.
- Functional context: a validated mcrA assay to track selected methanogenic potential.
- Mechanism-oriented evidence: MicH to detect the selected extracellular hydrogenase gene.
- Process context: methane, electron donors, redox, temperature, salinity, water wetting and treatment.
- Material evidence: surface samples, deposits, corrosion products, coupon/probe trends, inspection and morphology.
This structure makes the interpretation transparent: mcrA does not become “methanogenic corrosion”, and MicH does not become a stand-alone verdict.
Do you need to separate general methanogens from a narrower methanogenic MIC mechanism?
MICBUSTERS can help select archaeal, mcrA and MicH targets, choose representative water and surface samples, and build a controlled field qPCR trend around the operational decision.
Frequently asked questions
What is the main difference between MicH and mcrA?
mcrA is a broad functional marker for methanogenic and related MCR-based metabolism. MicH is a narrower marker for a specialised extracellular hydrogenase associated with corrosive methanogenic biofilms.
Does mcrA prove methanogenic MIC?
No. It supports selected methanogenic or MCR-related potential within assay coverage. It does not identify the electron source, activity or corrosion effect.
Does MicH detect all methanogens?
No. MicH targets a narrower hydrogenase-associated subgroup and should not be used as a total methanogen assay.
Should MicH replace mcrA?
Usually not. They answer different questions and are often most informative together: mcrA for broader methanogenic context and MicH for the selected corrosion mechanism.
What does mcrA-positive and MicH-negative mean?
Methanogenic or related MCR-based potential was detected, while the selected MicH sequence was not detected in that sample above the reporting limit. It does not establish whether the population is corrosive or harmless.
Can methane production prove that methanogens corrode steel?
No. Methane may be produced from hydrogen or other substrates unrelated to metal oxidation and can be transported from elsewhere.
Can I calculate a corrosive-methanogen fraction from MicH/mcrA?
Not without method- and asset-specific validation. Copy number, assay coverage, efficiency, detection limits and DNA recovery can differ between targets.
Which sample is most useful for MicH and mcrA?
Water filters are useful for repeatable process trends; surface-associated samples are more relevant to localized corrosion. The best design commonly uses both for different questions.
References and standards
- Steinberg LM, Regan JM. mcrA-Targeted Real-Time Quantitative PCR Method To Examine Methanogen Communities. Applied and Environmental Microbiology. 2009;75(13):4435–4442. doi:10.1128/AEM.02858-08.
- Dziewit L, Pyzik A, Matlakowska R, et al. Novel Molecular Markers for the Detection of Methanogens and Phylogenetic Analyses of Methanogenic Communities. Frontiers in Microbiology. 2015;6:694. doi:10.3389/fmicb.2015.00694.
- Deutzmann JS, Sahin M, Spormann AM. Extracellular Enzymes Facilitate Electron Uptake in Biocorrosion and Bioelectrosynthesis. mBio. 2015;6(2):e00496-15. doi:10.1128/mBio.00496-15.
- Tsurumaru H, Ito N, Mori K, et al. An Extracellular [NiFe] Hydrogenase Mediating Iron Corrosion Is Encoded in a Genetically Unstable Genomic Island in Methanococcus maripaludis. Scientific Reports. 2018;8:15149. doi:10.1038/s41598-018-33541-5.
- Lahme S, Mand J, Longwell J, Smith R, Enning D. Severe Corrosion of Carbon Steel in Oil Field Produced Water Can Be Linked to Methanogenic Archaea Containing a Special Type of [NiFe] Hydrogenase. Applied and Environmental Microbiology. 2021;87(3):e01819-20. doi:10.1128/AEM.01819-20.
- Lahme S, Mand J, Oparaodu K, Enning D. Detection and Monitoring of Corrosive Oilfield Microorganisms via Novel Biomarker Technologies. SPE-211238-MS, ADIPEC. 2022. doi:10.2118/211238-MS.
- Knisz J, Eckert R, Gieg LM, et al. Microbiologically Influenced Corrosion—More Than Just Microorganisms. FEMS Microbiology Reviews. 2023;47(5):fuad041. doi:10.1093/femsre/fuad041.
- AMPP. TM0212-2018: Detection, Testing, and Evaluation of Microbiologically Influenced Corrosion on Internal Surfaces of Pipelines. Consult the current official edition for normative requirements.
- AMPP. TM21465-2024: Molecular Microbiological Methods—Sample Handling and Laboratory Processing. Consult the current official edition for normative requirements.