By Justin Bauer
The Flint Water Crisis began in 2014 and has still not ended, drawing worldwide attention to the effect of metal pollutants in water. Agricultural runoff, oil leaks, and burning of fossil fuels all contribute to contaminated waters. This can have many negative effects both on humans and sea life, ranging from diseases such as cancer to wiping out entire species.
Many companies already have solutions in place, however, these are often very complex and too expensive. They use chemical precipitation techniques, adding a reagent to water that triggers a chemical reaction in which the dissolved metals form solid particles. After that, the particles are filtered out and the water is clean. A problem with this process is that the variety of metals makes it difficult to find a perfect reagent that will work on all of them. However, nature already has its own solution in place: A family of organisms called Sulfate-reducing microorganisms (SRMs) that can naturally precipitate metals. Through a series of reactions, they generate the chemical compound Hydrogen Sulfide which leads nearby metals to precipitate. However, they are very sensitive and it would be nearly impossible to engineer them to fix the water pollution problem on a larger level. Yet scientist George Sun and his colleagues decided to engineer another organism to produce this metal precipitating reaction, but at a more appropriate rate. They found their perfect solution with this common organism already used in baking and brewing: Yeast.
Yeast offers several advantages: it is stable at a range of both temperature and pH; it is easy to manipulate; and it is cheap to produce allowing mass production of different strains. Immediately they began transforming yeasts’ metabolic pathways to produce hydrogen sulfide at similar levels as the SRMs. An unsuspected group of people, namely winemakers, were able to greatly assist them during this process. It turns out that the egg-like odor wine produces when it is over fermented is due to the build-up of the compound hydrogen sulfide; the same compound the scientists were trying to produce. For years winemakers had been engineering yeasts to produce less hydrogen sulfide. The scientists simply reversed those steps and through gene analysis and were able to discover the specific genes in yeast responsible for producing hydrogen sulfide. Through gene manipulation, they were able to manipulate the pathways and engineer yeast to produce hydrogen sulfide at the levels needed.
To put their work to test outside of the laboratory, they applied it to water from the Athabasca oil sands in Canada. This industrial area is rich with crude oil and fossil fuels and has been subject to heavy pollution due to human mining and drilling. The engineered yeasts managed to precipitate more than 85% of copper mercury and lead and 30-50% of Cadmium and zinc from the polluted water.
In the future, George and his team hope to modify the yeast further so that it will remove the most toxic metals such as cadmium and mercury at a higher rate. More thought needs to be put into large scale applications on actual polluted waters, but this research shows that yeast can be used in the future to reduce metal contamination in the water and help improve the life quality of not only humans but also sea life.
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