Algae that normally use sunlight to make oxygen can be persuaded to produce hydrogen instead. Fortunately, there is no danger of this becoming universal and cutting off our air supply. Instead it could offer a way to produce a pollution-free power source, suited to storage and transportation.

Unlike other fuels, hydrogen produces just water and heat when burned. Already heavily used for industry, hydrogen could become the basis of much of the transportation system and a store of energy for windless nights or cloudy days in a world running on clean power. Most efforts to produce hydrogen cleanly involve making electricity from renewable sources and finding catalysts that split water into hydrogen and oxygen more rapidly and cheaply than existing versions.

These ideas are getting close enough to feasibility that financing is being sought for megaprojects where solar and wind power are abundant. Nevertheless, it's not clear if the technology will be cost-effective soon enough to save us from climate destruction without subsidies that can't be relied on. Professor Xin Huang of China's Harbin Institute of Technology has found a vastly different path.

Huang and colleagues mixed cells of the alga Chlorella pyrenoidosa that normally photosynthesize oxygen from carbon dioxide and water into a solution of polysaccharide dextran. The dextran was then mixed into a solution of polyethylene glycol (PEG) to create an emulsion with around 9,000 algal cells packed into each sugary droplet. The PEG put pressure on the droplets, causing them to shrink, and acted as a barrier between them and the air. The combination deprived the algal cells of oxygen, which might have been expected to make them produce more of their own to compensate. Instead, they underwent a shift, turning on enzymes known as hydrogenases.

As the name suggests, hydrogenases produce hydrogen, something they achieve by taking over the pathway the Chlorella usually use to photosynthesize oxygen.

So long as the cells could be kept away from oxygen, the team found they would keep on harnessing sunlight to produce hydrogen that could be drawn off for use. In Nature Communications, they report that coating the algae-containing droplets with bacteria ensured that oxygen, whether from the atmosphere or from unconverted cells, was quickly scavenged to avoid disrupting the hydrogen production.

The droplets are tiny – their diameter similar to the width of a human hair – so the hydrogen produced from each is minuscule. However, millions can be created with ease.

"Our methodology is facile and should be capable of scale-up without impairing the viability of the living cells. Prof. Huang said in a statement. “It also seems flexible; for example, we recently captured large numbers of yeast cells in the droplets and used the microbial reactors for ethanol production." Fuel and alcohol, who could ask for more?

Whether the method will prove more cost-effective than turning sunlight into electricity and then using this for water electrolysis remains to be seen, but when the future of the world is at stake, it's good to have a back-up plan.