Much of the discussion over biofuel production has focused on higher plants and the problems associated with their use, such as the loss of ecosystems or the driving up of food prices, as discussed in your Editorial 'Kill king corn' and News Feature 'The little shrub that could — maybe' (Nature 449, 637 and 652–655 2007). The use of microalgae, which need not displace food crops, has tended to be overlooked.

Mass algal cultivation has been used to produce pharmaceuticals for more than a decade, so the technology is well developed. Annual yields per unit area are an order of magnitude greater than those for higher plants. Conservative estimates of 30,000–50,000 litres of lipid per hectare per year compare with 1,300–2,400 litres per hectare per year reported for plants with a high oil yield, such as oil palm or jatropha.

Also, biomass production need not compete with food production for either land or water. Both marine and freshwater algae may be used, so either sea water or fresh water can serve as the basis of the culture medium. And as land-based algal culture systems don't depend on soil fertility, barren land can be used.

Spin-offs in the form of aquaculture would seem feasible, so local communities could benefit from the activity. And in the case of freshwater algae, energy can be recovered from anaerobic digestion of the non-lipid waste: methane produced by this process could supply the needs of the algal cultivation processes and desalination of sea water, or be used for ammonia synthesis.

The cost of biofuel production from algae has been estimated at $50 or less per barrel (M. E. Huntley and D. G. Redalje Mit. Adapt. Strat. Glob. Ch. 12, 573–608; 2007), making it more than economic at current oil prices, without subsidies.