What Is Microbiologically Influenced Corrosion (MIC)?

Microbiologically influenced corrosion (MIC) describes corrosion where microbiological processes significantly change the electrochemistry or chemistry at the metal surface. MIC does not replace traditional corrosion mechanisms; it modifies and often accelerates them.

Typical MIC hot spots include:

• Low-flow or stagnant sections of pipelines and firewater systems,
• Under-deposit and under-scale regions, including deadlegs and low points,
• Tank bottoms, separators and other equipment where water, solids and nutrients accumulate.

In many of these environments, sulfate-reducing microorganisms (SRM) are a key part of the microbial community and often linked to severe, highly localized pitting.

How Sulfate-Reducing Bacteria Drive MIC

Sulfate-reducing bacteria (SRB) and other SRM use sulfate (SO₄²⁻) as a terminal electron acceptor and reduce it to sulfide (H₂S/HS⁻). When this metabolism is coupled to electrons coming from the metal, it forms a complete corrosion cell.

In simplified form:

• Anodic reaction (metal dissolution): Fe⁰ → Fe²⁺ + 2 e⁻
• Cathodic reaction (microbially catalysed sulfate reduction): SO₄²⁻ + 9 H⁺ + 8 e⁻ → HS⁻ + 4 H₂O

SRM can access electrons in several ways:

• Direct extracellular electron transfer (EET): cells or their cytochromes take up electrons directly from steel or conductive FeS layers.
• Hydrogen-mediated: H₂ is produced chemically or enzymatically at the surface and rapidly consumed by SRM.
• Enzyme-mediated EET: excreted enzymes bound to steel catalyse electron transfer reactions.

The produced sulfide reacts with iron to form FeS/FeS₂ scales, which can be electrically conductive. These scales help set up galvanic microcells and maintain localized anoxic conditions, promoting deep pitting characteristic of SRB-related MIC.

The dsrAB Gene: Signature Marker for Sulfate Reduction

To track SRM in field samples, labs rarely rely on classical culturing alone. Instead, they use functional genes that encode enzymes in the sulfate reduction pathway. Two key genes are:

• aprA – encodes the alpha subunit of APS reductase, a mid-step in the pathway,
• dsrAB – encodes the alpha and beta subunits of dissimilatory sulfite reductase, the terminal enzyme.

While both are important biologically, the dsrAB gene has become the core functional marker for several reasons:

• dsrAB is tightly linked to dissimilatory sulfate/sulfite respiration, the energy metabolism that directly supports MIC-relevant processes.
• dsrAB phylogeny aligns well with 16S rRNA phylogeny, so dsrAB sequences can be mapped to known SRM lineages (e.g. Desulfovibrio, Desulfomicrobium, Desulfotomaculum, Archaeoglobus).
• In many SRM genomes, dsrAB is present in single copy, which helps translate gene copy numbers into approximate cell numbers in qPCR-based monitoring.

For MIC monitoring, dsrAB-based qPCR and sequencing therefore provide a more direct view of the sulfate-reducing guild than aprA alone, which is also present in many sulfur-oxidizing organisms.

Why Designing a Good dsrAB qPCR Target Is Challenging

If dsrAB is so useful, why aren’t we all using one universal, perfect dsrAB assay? In reality, designing robust dsrAB primers and probes is complex, for multiple reasons.

1. Extreme Sequence Diversity

dsrAB is an ancient enzyme family spread across many bacterial and archaeal phyla. Key motifs in the protein are conserved, but the underlying DNA sequence shows high variability. To capture this diversity, primers often need to be highly degenerate, which:

• Reduces effective primer concentration per variant,
• Can lower qPCR efficiency,
• Increases the risk of off-target amplification.

2. One Gene, Multiple Functional Guilds

The same dsrAB framework (in reverse) is used by some sulfur-oxidizing microorganisms (rDsr). On full-length sequence trees, reductive and oxidative clades are distinguishable. But at the scale of 18–22 bp primer binding sites, it is much harder to separate them cleanly.

As a result, broad dsrAB assays may detect:

• The dissimilatory sulfate reducers relevant to MIC, and
• At least some sulfur oxidizers carrying rDsr.

3. Archaeal dsrAB and Emerging Diversity

Modern metagenomic studies keep adding new dsrAB-carrying lineages, including divergent archaeal sequences and previously unknown bacterial clades. Primer sets designed years ago against a limited culture collection may under-represent this new diversity.

For MIC-focused monitoring, that means dsrAB assays should be periodically re-evaluated in silico against updated dsrAB reference databases, especially for high-temperature or high-salinity assets where unusual SRM are common.

4. Quantitative Bias in Mixed Communities

Even a single mismatch in a primer binding site can reduce amplification efficiency for certain dsrAB variants. In a mixed biofilm sample, that leads to:

• Over-amplification of “easy” templates,
• Under-amplification of “difficult” templates,
• Bias in both relative abundance and absolute counts.

For any dsrAB assay used in MIC decision-making, validation with mock communities and synthetic standards is therefore critical.

From dsrAB Numbers to MIC Risk Management

dsrAB data are most powerful when they are integrated into a broader MIC monitoring programme. A practical workflow often includes:

• Baseline dsrAB qPCR: track SRM potential over time, across systems and under different treatment regimes (e.g. biocide or nitrate dosing).
• 16S rRNA profiling: place SRM in context with fermenters, nitrate reducers, methanogens and sulfur oxidizers to understand whole-community interactions.
• Corrosion data: combine dsrAB with pit depth measurements, coupon data, FeS scaling, and visual inspection.
• Chemistry: monitor sulfate, sulfide, nitrate, organic acids and iron to link genetic potential to actual process conditions.

Interpreted together, these datasets help answer practical questions:

• Is SRM-related MIC likely or mainly a theoretical risk?
• Is our mitigation strategy (biocide, nitrate, pigging) actually controlling SRM activity?
• Where should we focus inspection resources to catch MIC damage early?

FAQ: dsrAB, Sulfate-Reducing Bacteria and MIC

Does a high dsrAB signal always mean severe MIC?

Not necessarily. A strong dsrAB signal indicates a high potential for dissimilatory sulfate/sulfite reduction. Whether this translates into aggressive MIC depends on local conditions: redox state, availability of electron donors, presence of deposits, and whether SRM are located directly on the metal surface.

Can we rely on sulfate and sulfide measurements alone?

Chemistry is crucial, but it mainly reflects what has already happened. dsrAB and other functional gene data show the capacity of the community to drive MIC under the right conditions. Combining both viewpoints gives a more robust risk assessment.

Is aprA still useful if dsrAB is the main marker?

Yes. aprA is highly valuable when you want to understand the full sulfur cycle, including sulfur oxidizers. For MIC-focused work, aprA is best interpreted alongside dsrAB, 16S and corrosion data, rather than as a stand-alone indicator of SRM-driven MIC.