Researchers at the University of Bristol in the UK and Harbin Institute of Technology in China have built tiny droplet-based algal factories that produce hydrogen, instead of oxygen, when exposed to daylight in air. An open-access paper on their work is published in Nature Communications.

Normally, algal cells fix carbon dioxide and produce oxygen by photosynthesis. The study used sugary droplets packed with living algal cells to generate hydrogen, rather than oxygen, by photosynthesis.

The team, comprising Professor Stephen Mann and Dr Mei Li from Bristol’s School of Chemistry together with Professor Xin Huang and colleagues at Harbin Institute of Technology in China, trapped ten thousand or so algal cells in each droplet, 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 millimeter in size, could be prepared in one milliliter of water.

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

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.

Overall, our methodology provides a proof-of-principle for utilizing aqueous two-phase separated droplets as vectors for controlling algal cell organization and photosynthesis in synthetic micro-spaces. The procedure is facile and capable of high throughputs for modulating algal cell functionality towards hydrogen production without impairing the viability of the living cells. Moreover, it should be possible to combine our methodology with more complex bioengineering approaches involving sulfur deprivation, genetically modified oxygen-tolerant [FeFe]-hydrogenases or cellular surface modifications.

Compared with synthetic hydrogen-producing systems, the limited rates and yields in the multicellular spheroids remain challenging aspects of future work. In this regard, incorporating chemical-based hydrogen-generating machinery or antennae-reduced mutants into the algal cell spheroids could be promising strategies. More generally, our approach provides the possibility for modulating the functionality of other living cells; for example, the droplet-based microbial systems can be readily extended towards ethanol production via the programmed capture of large numbers of yeast cells within the multicellular spheroids.

—Xu et al.

Resources

• Xu, Z., Wang, S., Zhao, C. et al. (2020) “Photosynthetic hydrogen production by droplet-based microbial micro-reactors under aerobic conditions.” Nat Commun 11, 5985. doi: 10.1038/s41467-020-19823-5