Oil prices are soaring as supply chains in the Middle East become fragile. BP's quest for deep-water reserves in the Gulf of Mexico last year proved calamitous. Where will alternative energy supplies that are compatible with our oil infrastructure come from?

Algae, the plant-like creatures that make slime in ponds, hold promise. They can produce up to 100 times more combustible liquid energy than crop plants, such as wheat. Huge, water-filled, light-permeable plastic reactors allow algae to use the sun's energy to clip carbon-dioxide molecules together into oily chains that can be purified and release energy from combustion.

Depleting carbon dioxide and creating liquid energy at the same time sounds good. But can we do it on the scale that is needed? Billions have been invested in the technology, but this year Shell has abandoned its work with the algal energy firm Cellana. Others, too, are looking nervous. Genetic technology to optimise algal biochemistry towards fuel production is exciting but unrealised.

Engineering saltwater algae to reach production levels of freshwater species seems to be the most important step. Large volumes of water are needed to suspend the algae and water also contributes atoms to the fuel chemicals. But fresh water trumps fuel on the necessity ladder - and time is running out.