Platform chemicals
Microbial arachidonic acid
The desaturase-elongase route in Mortierella alpina, the two-stage temperature regime that accumulates the lipid, and the reason single-cell oil won in infant nutrition specifically.
Arachidonic acid (20:4, omega-6) is a long-chain polyunsaturated fatty acid present in human milk and built into the membrane phospholipids of brain and retina. It is not made synthetically: four double bonds in fixed positions, all cis, put total synthesis far outside any plausible cost. The natural sources are animal tissue and egg lipid. The industrial source is a filamentous fungus, Mortierella alpina, grown by submerged fermentation.
The pathway, and why a fungus carries it
The fungus builds arachidonic acid from linoleic acid (18:2) by alternating two reaction types: a desaturase inserts a double bond, an elongase adds two carbons. The sequence runs Δ6-desaturation to 18:3, elongation to 20:3, Δ5-desaturation to 20:4. Mortierella alpina carries this enzyme set at high activity and — more important for process design — is oleaginous: when nitrogen runs out against a carbon excess, it switches from growth to storage and accumulates triacylglycerol to tens of percent of dry mass. Nitrogen limitation, not precursor feeding, is the industrial lever.
The constraint is temperature. Desaturases run more completely the colder the culture, because membrane fluidity demand rises as growth slows. Hence the standard two-stage regime: a warm biomass phase, then a temperature drop for accumulation, during which arachidonic acid’s share of the lipid rises. The optimum for biomass and the optimum for product fraction do not coincide, and the process lives in the compromise between them.
What actually caps the output
Three things. Viscosity first: a mycelial culture thickens the broth, oxygen transfer falls, and desaturases are oxygen-dependent enzymes, so aeration here is part of the chemistry rather than a utility. Mycelial morphology — pellets versus dispersed filaments — is set by inoculum and shear, and it decides the achievable cell density. Second, selectivity: the product is a fatty acid mixture, not one acid, so reaching a specified arachidonic fraction is a job for extraction and refining downstream, not for the fermenter. Third, oxidation: four double bonds mean high susceptibility to air, which is why the commercial form for powdered formula is microencapsulated oil rather than oil.
Specification won, not price
Single-cell oil is not cheaper than animal-derived lipid. It won because the infant-formula buyer does not state the requirement in money. That buyer needs a vegetarian source, free of the contaminants that travel with animal and marine lipids, reproducible in composition batch to batch, and documented back to the strain. Fermentation delivers exactly that: a constant fatty-acid profile, no seasonality, full traceability. Regulation locks it in — infant formula composition is prescribed, and changing the lipid source is a re-filing, which makes a qualified supplier durable.
That yields the general point worth carrying into the rest of this cluster: a bioprocess beats petrochemistry not when it is cheaper, but when the buyer needs a property the alternative physically cannot supply.