Reviewing: Sun, G.L., Reynolds, E.E. & Belcher, A.M. Using yeast to sustainably remediate and extract heavy metals from waste waters. Nat Sustain (2020).
DOI: https://doi.org/10.1038/s41893-020-0478-9
Toxic heavy metals in wastewater should be cleaned up
The outbreak of Minamata disease, a neurological disease caused by mercury poisoning, has alerted the public to be aware of the risks of mercury exposure. People in Minamata City, Japan, had been eating shellfish and fish with high levels of methylmercury absorbed from industrial wastewater for over 30 years, until the epidemic broke out in 1956. This still leads to intense debates over whether eating too much fish would expose people to too much mercury. Many people still wonder if eating a can of tuna every day is safe, or if pregnant women should be limiting or avoiding fish consumption.
Mercury is a type of heavy metal, along with other metals like copper, lead, cadmium, and zinc. They are called “heavy metals” because they are literally heavier than the “light” metals – a dime-sized disk of mercury would feel heavier on your palm than an identical piece of aluminum. Heavy metals naturally exist in Earth’s crust, and they enter the ocean as the rivers wash away the rocks and soils, or as wind blows dust particles into the ocean.
However, humans have been introducing additional heavy metals to the environment by burning oil and coal, producing cement, steel, electrical switches/lightings and batteries, and discharging industrial wastewater and sewage sludge into rivers. When these heavy metals get into the ocean, most of them accumulate in the sediments at the bottom of the ocean, but some are absorbed into marine plankton and move up the food chain, eventually accumulating in larger fish like tuna and mackerel that we consume. Exposure to high levels of mercury can cause a variety of symptoms such as nausea, respiratory problems, kidney disorders, tremors, paralysis and brain damage.
To reduce the health risks associated with heavy metal, we need to clean up heavy metals from our waste and from our oceans. One strategy that has been recently getting research attention is bioremediation: using natural living organisms like microbes and bacteria to treat contaminated waters.
Scientists have discovered microbes that produce hydrogen sulfide (H2S), a chemical compound that smells like rotten eggs. Hydrogen sulfide reacts with heavy metals dissolved in water to produce metal sulfides, which are solid and will separate from water (a process called “precipitation”). However, these organisms grow very slowly under specific living conditions that cannot be easily maintained, so they haven’t been thought of as a realistic solution to cleaning up wastewater. To find a solution to this problem, Sun et al. (2020) worked on genetically modifying yeast to make it produce hydrogen sulfide and remove heavy metals from wastewater.
How can yeast remove heavy metals from wastewater?
Yeast is the same ingredient you put into that loaf of bread in your oven, or into a beer fermenter in your basement. It is a fungus that is very common in the environment, and has been used by humans since the ancient Egyptians discovered its ability to raise bread. Nowadays, yeast manufacturing is a billion-dollar business, and there are many different types of yeast supplied to different industries, such as baking, beer and wine, bioethanol and pharmaceuticals.
Yeast produces amino acids, which are the building blocks of proteins essential for its growth. Some of these amino acids contain sulfur, and to produce these sulfur-containing amino acids, yeast first takes up sulfate (a form of salt with sulfur), converts it to hydrogen sulfide, then finally converts it to the amino acids. Scientists have identified a number of genes in yeast that control each step of these sulfur-conversion processes. Each of these genes were deleted to produce “knockouts”, so that these knockouts lose the ability to convert sulfate to amino acids. By testing each knockout for its efficiency of metal removal, specific genes that play the most important roles in converting hydrogen sulfide to sulfur-containing amino acids could be identified.
Sun et al. (2020) confirmed that the yeast knockouts can produce hydrogen sulfide, and that the hydrogen sulfide produced by yeast can successfully remove metals from water, by growing the knockouts in waters that had roughly a hundred times higher copper, lead, cadmium and mercury than drinkable water. Copper and lead were most quickly removed, followed by cadmium and mercury, but four rounds of precipitation removed >99% of all these metals, to an amount low enough to drink safely.
The research team also attempted to control the shape and structure of the heavy metals that are precipitated on the yeast surface. This was because the traditional heavy metal removal techniques by adding chemicals such as lime and sodium hydroxide produce precipitates with heavy metals and chemicals all mingled together – like different colors of Play-Doh all mashed up – and as they cannot be separated from each other easily, they all need to be dumped into landfills or burned. While the research team successfully managed to regulate the size and shape of metal precipitates on the yeast surface to some degree, additional work needs to be done to determine how to remove these metal particles from yeast and recycle them efficiently.
Why is the genetically engineered yeast promising for the future?
We are generating large quantities of industrial, agricultural and electronic wastes that contain toxic heavy metals. This waste, untreated or only partially treated, gets discharged into the environment, polluting rivers and oceans. When heavy metals get stored in living tissues and organs of marine organisms, they do not degrade over time, and eventually get transferred to larger predators. Large amounts of heavy metals can be detrimental to the growth, survival and reproduction of marine organisms, and consuming these heavy metal-contaminated organisms is a potential threat to public health.
Currently, industries use chemicals to treat wastewater, but these chemicals produce harmful sludge that needs to be buried or composted in landfills. The chemicals also need to be renewed each time after wastewater gets treated, so treatment costs build up quickly. But yeast may be a cheap, widely available alternative to the chemical treatments. More than one million tons of yeast were produced in 2015, and a global production and supply chain of yeast already exists. Genetically engineered yeast is very easy to grow, as it can survive a wide range of temperature, acidity and oxygen concentrations.
Yet additional work is necessary to successfully implement the yeast-bioremediation in the near future. As mentioned above, a cost- and time-efficient way to remove the metals precipitated on the yeast surface still needs to be developed, so that both heavy metals and yeast can be recycled. In addition, as some heavy metals like mercury and cadmium are particularly more threatening to human health than others like calcium and zinc, selectively or preferentially precipitating specific heavy metals can be much more practical.
Nevertheless, a wider application of genetically engineered yeast may significantly reduce the cost of wastewater treatment and sludge disposal, keep heavy metals out of our oceans and reduce environmental threats and health impacts from toxic waste to both humans and the marine ecosystem.
I am a PhD student in chemical oceanography at University of Washington. I am studying how different forms of metals in the ocean are shaping microbial communities in the North Pacific Ocean. When not working, I like going for a walk, visiting farmers’ markets and playing keyboard.
