Producing hydrogen, not oxygen

The search for alternative energy sources is as wide-ranging as it is important. In a new chapter, scientists from the UK and China report that they have built tiny droplet-based microbial factories that produce hydrogen, rather than oxygen, when exposed to daylight in air.

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Electron microscopy image of a densely packed droplet of hydrogen-producing algal cells. Scale bar, 10 micrometres. Credit: Xin Huang, Harbin Institute of Technology

Hydrogen is potentially a climate-neutral fuel but making it usually involves a lot of energy, so a viable green alternative would be seen as an important step forward.

Normally, algal cells fix carbon dioxide and produce oxygen by photosynthesis. In this study, researchers from the University of Bristol and Harbin Institute of Technology used sugary droplets packed with living algal cells to generate hydrogen in the same way.

In a paper in the journal Nature Communications, they describe trapping ten thousand or so algal cells in each of the droplets, which were then crammed together by osmotic compression. 

By burying the cells deep inside the droplets, oxygen levels fell to a level that switched on special enzymes called hydrogenases that hijacked the normal photosynthetic pathway to produce hydrogen. 

In this way, around a quarter of a million microbial factories, typically only one-tenth of a millimetre in size, could be prepared in one millilitre of water.

To increase the level of hydrogen evolution, the team coated the living micro-reactors with a thin shell of bacteria, which were able to scavenge for oxygen and therefore increase the number of algal cells geared up for hydrogenase activity.

Although still at an early stage, the work provides a step towards photobiological green energy development under natural aerobic conditions, the researchers suggest.

“Our methodology is facile and should be capable of scale-up without impairing the viability of the living cells,” says Harbin’s Xin Huang. 

“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.”

Related reading: Cosmos Briefing: Hydrogen fuel

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