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Researchers from the University of Bristol in the UK and the Harbin Institute of Technology in China have built tiny droplet-based algae factories that produce hydrogen, instead of oxygen, when exposed to daylight in the 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 sugar droplets filled with living algal cells to generate hydrogen, rather than oxygen, via photosynthesis.
The team, made up of Professor Stephen Mann and Dr Mei Li of the Bristol School of Chemistry together with Professor Xin Huang and colleagues from the Harbin Institute of Technology in China, trapped around ten thousand algal cells in each drop, which were then massed together by osmotic compression. By burying the cells deep within the droplets, the oxygen levels dropped to a level that activated special enzymes called hydrogenases that hijacked the normal photosynthetic pathway to produce hydrogen. In this way, about a quarter of a million microbial factories, typically only a tenth of a millimeter, could be prepared in a milliliter of water.
Electron microscope 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 clean the oxygen and thus increase the number of algal cells predisposed for the activity of the ‘hydrogenase.
Although still in an early stage, the work represents a step towards the development of photobiological green energy under natural aerobic conditions.
Overall, our methodology provides a proof of principle for using separate aqueous two-phase droplets as vectors for controlling algal cell organization and photosynthesis in synthetic micro-spaces. The procedure is simple and capable of providing high performance to modulate the functionality of algal cells towards the production of hydrogen without compromising the viability of living cells. Furthermore, it should be possible to combine our methodology with more complex bioengineering approaches involving the deprivation of sulfur, which is tolerant to genetically modified oxygen. [FeFe]-hydrogenase or cell surface modifications.
Compared to synthetic hydrogen production systems, the limited rates and yields in multicellular spheroids remain challenging aspects of future work. In this regard, the incorporation of chemical-based hydrogen generation machinery or antenna-reducing mutants into algal cell spheroids could be promising strategies. More generally, our approach envisages the possibility of modulating the functionality of other living cells; for example, droplet-based microbial systems can be readily extended to ethanol production through the programmed capture of large numbers of yeast cells within multicellular spheroids.
-Currency et al.
Resources
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Xu, Z., Wang, S., Zhao, C. et al. (2020) “Production of photosynthetic hydrogen from droplet-based microbial micro-reactors under aerobic conditions.” Nat Common 11, 5985. doi: 10.1038 / s41467-020-19823-5
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