Sun-powered ‘green oil’: UC Berkeley review says engineered cyanobacteria can pump out

The search for a carbon-negative way to make liquid fuels has a new favourite candidate: the humble blue-green algae that once oxygenated Earth’s atmosphere. A wide-ranging review published this week in the Journal of Bioresources and Bioproducts concludes that modern synthetic-biology tools have turned cyanobacteria into “mini-refineries” capable of secreting everything from ethanol to aviation-grade alkanes in a single step, using only sunlight, seawater and atmospheric CO₂ as feed-stocks.

By mining more than 300 recent papers, author Bharat Majhi shows that inserting plant or bacterial genes into model strains such as Synechocystis sp. PCC 6803 reroutes up to 70 % of fixed carbon away from glycogen storage and into target molecules. The most successful constructs express pyruvate decarboxylase and alcohol dehydrogenase for ethanol; a 2-keto-acid pathway for isobutanol; and acyl-ACP reductase plus aldehyde decarbonylase for C13–C17 alkanes that mimic kerosene. Titres have climbed steadily: ethanol has reached 3.8 g L⁻¹ in 10 days, isobutanol 1.24 g L⁻¹ in 58 days, and the alkane fraction now equals 0.5 % of dry cell weight—still below the 2 % needed for economic extraction but already double the yield of the best algal lipid programmes.

What makes the platform attractive to investors is the photosynthetic efficiency. Because cyanobacteria harvest light at only 8 % efficiency compared with 3–4 % for terrestrial crops, the theoretical yield of jet fuel per hectare is 15 times higher than jatropha and 30 times higher than soy. Moreover, the organisms grow in brackish or waste water, avoiding the food-versus-fuel conflict that plagues corn ethanol.

Yet the review is frank about bottlenecks. The fastest laboratory strain doubles every 2 h under ideal LED light, but outdoor pilots average 8–12 h, allowing cheaper competitors such as E. coli to overtake the process. Accumulation of alcohols above 10 g L⁻¹ or monoterpenes such as limonene above 24 mg g⁻¹ dry weight stalls growth and triggers membrane leakage. Harvesting 0.5-mm-long cells still demands centrifugation or magnetic nanoparticle flocculation that can consume 30 % of the energy contained in the final fuel. And while CRISPR-Cas9 has produced alcohol-tolerant mutants that survive 4 % v/v ethanol, the trade-off is a 25 % drop in photosystem-II efficiency, eroding the carbon advantage.

“No single breakthrough will flip the economics,” Majhi told reporters. “What we need is a stacked solution—ultrafast-growing chassis like the newly isolated Synechococcus UTEX 2973, inducible promoters that switch off toxic pathways during night-time, and cheap internally-lit photobioreactors that keep cells at the optimum 32 °C without evaporating tonnes of water.”

Market signals are nevertheless aligning. The International Energy Agency puts global biofuel output at 160 billion litres in 2023, but only 0.2 % came from micro-organisms. Analysts Grand View Research forecast the algal and cyanobacterial segment to expand at 10 % per year, driven by airline off-take agreements seeking sustainable aviation fuel. If titres reach 15 g L⁻¹ and harvesting energy falls below 0.5 kWh kg⁻¹ biomass, the review calculates that cyanobacterial kerosene could be produced for USD 1.90 per gallon—within the USD 2.00 ceiling set by the U.S. Department of Energy’s SAF Grand Challenge.

Until then, the paper recommends focusing on high-value, low-volume products such as cosmetic fatty alcohols or fragrance-grade limonene that sell for USD 50–200 kg⁻¹, allowing producers to finance the scale-up of infrastructure that will ultimately deliver commodity fuels. “We are one robust pilot plant away from proving that sunlight alone can power a post-fossil world,” Majhi concludes.