Microbial electrosynthesis

Microbial electrosynthesis fuses electrolysis with fermentation: a cathode held at a negative potential replaces sugar as the electron donor, and acetogenic bacteria use the current to reduce CO2 into acetate, alcohols and biomass. The appeal is a direct pipe from renewable electricity into chemistry, with no plants and no photosynthesis in between. Everything hard about the idea concentrates in two places: getting electrons into the cell, and getting the product out of the broth. The mirror process — microbes handing electrons to an anode instead of taking them from a cathode — is covered under microbial fuel cells.

How electrons cross into the cell

Two routes exist. In direct uptake the microbe docks on the cathode and draws electrons through redox proteins on its surface. In the mediated route the cathode first electrolyses water, and what the cell actually consumes is the emerging hydrogen — split by hydrogenases — or formate dissolved off the electrode. Which route dominates is genuinely contested. Even in experiments designed for direct uptake, abiotic hydrogen evolution at the working potential can carry most of the current, and many acetogens are Gram-positive organisms without the outer membrane that makes direct transfer easy in metal-respiring bacteria such as Geobacter. Designing an electrode for “the microbe” when the microbe may merely be eating electrolysed hydrogen is the field’s persistent ambiguity.

Wood–Ljungdahl: fixation on an energy budget

The chemistry runs through the Wood–Ljungdahl pathway. A methyl branch reduces CO2 to a methyl group via formate and a folate carrier; a carbonyl branch reduces a second CO2 to enzyme-bound carbon monoxide; acetyl-CoA synthase stitches the two together into acetyl-CoA, the crossroads metabolite from which acetate, ethanol, butanol and biomass all descend. The pathway’s distinction is economy: acetogenesis nets roughly one ATP per acetate, squeezed out by flavin-based electron bifurcation. It is the cheapest known route from CO2 to organic carbon, which is why acetogens can live on hydrogen alone. The cheapness cuts both ways: with almost no energy to spend, there is nothing to invest in fast growth, in thick biofilms, or in driving products to high concentration against a gradient.

Where the electrons are lost

The honest currency of the field is coulombic efficiency — the share of cathode current that ends up in product. Electrons leak into hydrogen bubbles that escape unused; methanogens invading the reactor convert hydrogen and acetate to methane, silently redirecting the energy stream; corrosion and side reactions take their cut, and ohmic resistance through a thin biofilm wastes the rest. Reported figures swing from tens of percent to near unity depending on electrode, potential and community. That spread is itself the finding: the process sits at the edge of what a biofilm can control, and every claimed gain has to be read alongside the inventory of leaks.

Why titres stay low

Even good runs deliver acetate in the grams per litre range; industrial fermentations run in the tens and beyond a hundred. The causes stack. Current density per electrode area sits orders of magnitude below industrial electrolysis, because only a micrometre-thin biofilm does the work. Hydrogen and CO2 are poorly soluble in water, so gas transfer throttles the whole reactor — hollow-fibre membranes that feed gas bubble-free exist precisely for this. Acetate itself stresses the strict anaerobes as it accumulates. And a dilute small acid is exactly the product class that resists cheap recovery: it cannot be distilled off, and neutralisation hands you a salt that must be undone. The route therefore lives or dies on cheap electrons and on whether anyone wants dilute acetate at that price.