What a tonne of algal CO2 actually requires

Algae are sold as carbon capture with a product attached, and the biology genuinely works: dissolved CO2 is fixed into biomass at rates terrestrial crops cannot match per hectare. But the process runs on the same photosynthesis as every other plant, which means it inherits every ceiling photosynthesis has — and the ceilings, not the biology, decide whether a tonne of carbon is a business.

The efficiency ceiling

Counting photons gives the theoretical bound: the photosynthetic apparatus spends roughly eight to ten photons to fix one molecule of CO2, which sets a thermodynamic ceiling of a few percent of incoming usable light. Sunlight then breaks the assumption the machinery was built for. Photosynthesis saturates — leaves and algal cells reach maximum rate at a fraction of full sun — so half of every day’s light arrives faster than the cell can spend it, and the surplus is not stored but dissipated as heat or, at worst, damages the photosystems. A dense culture worsens the arithmetic: the surface layer saturates while the depth below starves, so reactors are mixed and kept thin to spread the photons. And at night the ledger runs backwards: dark respiration burns part of the day’s assimilation to keep the cell alive, a tax collected every 24 hours. Real annual conversion of total sunlight into algal biomass lands near the low single digits of percent, and that is before harvesting.

The ledger behind a kilogram of biomass

Fixing a kilogram of dry algal biomass takes roughly two kilograms of CO2 — the arithmetic of carbon content — but carbon is the cheapest input on the page. The same biomass demands nitrogen and phosphorus at crop-like rates; biomass that is several percent nitrogen makes every kilogram of algal carbon a fertilizer purchase. The gas itself is awkward: flue gas arrives dilute, transfer into water is the slow step, and a growing culture drives pH up, which pushes CO2 back out as bicarbonate — capture efficiency below unity is a structural property, not an engineering detail. Then the water problem: cultures hold grams of biomass per litre, so collecting a kilogram means processing a tonne or more of liquid, and dewatering is typically the single largest energy line. All three inputs — light, nutrients, dewatering — scale with tonnage.

Why the economics lean on molecules, not tonnes

Because those ceilings fix a floor under the cost of a kilogram of biomass, commodity outputs struggle to clear it: an algal fuel competes against pumped oil, an algal pigment competes against extraction from rare sources. That is why the sector’s revenue in practice comes from astaxanthin, omega-3 and feed additives — molecules priced per gram, able to pay for the photobioreactor — while carbon itself is the by-product. Carbon-removal claims inherit the same ledger honestly stated: the energy consumed in mixing, aerating and dewatering subtracts from the captured tonnage, and the captured carbon only counts if its fate is long-lived; biomass that decays or is burned within a season has merely cycled the gas. Within these bounds the field keeps its place in marine biotechnology as the biological branch of carbon utilization — real, but bounded by photons the way mining is bounded by ore grade.