Microbial communities clean toxic waste and generate useful chemicals
Microbial communities clean toxic waste and generate useful chemicals
July 17, 2017
July 17, 2017
Microbes are Nature’s great multitaskers. They are ubiquitous inhabitants of the environment, flourishing in landscapes ranging from undersea vents to frozen Nordic tundra, where they are involved in the decomposition of organic matter and the cycling of chemicals essential to life, such as carbon and nitrogen.
In new research, Sofia Esquivel-Elizondo, Rosa Krajmalnik-Brown and Anca Delgado at the Biodesign Institute at ASU demonstrate that microbes can also be harnessed to clean up recalcitrant pollutants and generate chemicals of widespread benefit to industry.
The team belongs to the Swette Center for Environmental Biotechnology at Biodesign, a group whose primary mission is to leverage the unique capabilities of microbes for the benefit of society. Their work is also integral part of the ASU’s Center for Bio-mediated & Bio-inspired Geotechnics.
In the new study, the authors demonstrate that communities of microbes enriched from a wastewater treatment plant are capable of consuming off gasses containing carbon monoxide (CO) and converting them into useful industrial chemicals, including ethanol (a clean biofuel) and acetate, a precursor chemical with a broad range of uses, particularly for plastics manufacturing and biotechnology systems.
Microbes are unusually tolerant to carbon monoxide, a chemical notoriously hazardous to most life forms. Such specialized organisms, known as carboxidotrophs, digest CO in the course of their normal respiration, converting it to beneficial byproducts, including acetate and butyrate, as well as biofuels, including ethanol, butanol, hydrogen, and methane.
By adjusting the level of CO, under anaerobic conditions (i.e., in the absence of oxygen), the researchers were able to provide a suitable environment for the growth of beneficial microbes capable of CO metabolism, editing out competing microbes unable to withstand CO toxicity. The selected microbes were then able to transform CO into useful chemicals.
Research findings appear in the current issue of the journal FEMS Microbiology Ecology.
Genesis of research
As professor Krajmalnik-Brown recalls, research that seeded some of the ideas for the current project began over a decade ago, when she became interested in the clean up or bioremediation of a particularly dangerous and stubborn class of pollutants, known as chlorinated solvents. Certain microbes are able to use and transform these chlorinated chemicals, converting them to benign byproducts in the course of their respiration, through a process called dechlorination.
It turns out that these dechlorinating microbes, which are able to purify water contaminated with chlorinated chemicals, have competitors in the environment. Some of the microorganisms that dechlorinating bacteria compete with are archaea or methanogens. “I was looking for ways to decrease the amount of methanogens around these superstar microbes. One of the ideas was using carbon monoxide to reduce the amount of competitors and maybe help dechlorinators get their own way. By adding CO, we enriched for special microorganisms known as carboxidotrophs. These microbes happen to be great partners for dechlorinators because they reduce CO concentrations in the environment and produce hydrogen and acetate, the primary electron donor and carbon source, respectively, for the best dechlorinators.”
In addition to their role in bioremediation, microbes can be used to produce useable energy. In a device known as a microbial fuel cell, microorganisms known as anode-respiring bacteria are able to strip electrons from organic waste, transferring them to the anode of a battery in order to generate electricity. Such bacteria face similar competition from methanogens. Perhaps, Krajmalnik-Brown reasoned, carbon monoxide fermentation could again act to limit competition for the energy-producing bacteria.
Esquivel-Elizondo, a fifth year PhD student in environmental engineering and lead author of the new study became intrigued with the behavior of CO-consuming microbes and their plausible applications. “I was fascinated to see how something so toxic was actually an energy source for microorganisms.” An answer to the puzzle lies in the fact that such microbes evolved some 4 billion years ago, at a time when the earth’s atmosphere was much richer in CO and genes allowing respiration of the chemical provided a survival advantage to these bacteria.
Today, CO in the atmosphere is limited to trace amounts. Inducing microbes to consume it as a fuel source in the presence of many preferable chemicals is tricky and time-consuming. “Carbon monoxide levels on Earth now are so low,” Esquivel-Elizondo says, “that microorganisms think ‘why should I bother expressing genes to consume carbon monoxide? I prefer to eat other organic carbon sources, like sugar.’”
But if the microbes are cultured on a diet containing increasing concentrations of CO over an extended period, ranging from weeks to months, they begin to make use of it as a carbon and electron source, engaging the repressed, ancient genes permitting CO respiration once other nutrient sources like sugar have been expended. “At the beginning, they didn’t need CO, but after a while, when they didn’t see any other food, the microbes thought ‘this feels like 4 billion years ago, so maybe we have to express the genes (for consuming CO) again,’” Esquivel-Elizondo says.
Green energy from waste
Certain forms of waste are highly resistant to normal degradation in wastewater treatment or landfill operations. Such recalcitrant waste may be incinerated under conditions of high temperature and pressure. The gasified waste is known as syngas, and consists of carbon monoxide, carbon dioxide, and hydrogen. If syngas is digested by microorganisms, fuel for cars and homes in the form of ethanol could be generated in place of hazardous, CO-liberating waste. Additionally, acetate and in some cases, methane or butanol (both valuable biofuels) can be produced in this way.
“These are very peculiar and interesting living things because they can consume something very toxic and produce something very useful to society,” Esquivel-Elizondo says. Additionally, the bacteria are able to produce acetate, which is essentially, vinegar. Acetate is a critical precursor for the chemical industry, used for the production of adhesives, paints, and a broad range of plastic products, including glasses and water bottles. The global demand for acetate is in the millions of tons annually. Currently, it is produced chemically, though the new study suggests that large quantities of acetate could be generated by microbes from digested pollutants.
The new study explores the threshold of CO consumption microbes can withstand and still remain viable. While competing methanogen bacteria display a low tolerance for CO, carboxidotrophic bacteria can be induced over time to consume CO in significant quantities. Results showed that an input gas containing about 40 percent CO induces the microbial colony to produce acetate, and not methane, as the main byproduct of respiration. Increasing the amount of CO fed to the microbes causes them to yield ethanol. Experiments demonstrated that certain hearty and highly resistant carboxidotrophic bacteria can actually survive on atmospheres with 100 percent CO.
The process of converting syngas from waste incineration to ethanol is already big business, though it involves the use of pure cultures of genetically engineered microbes grown under sterile conditions. The new technique would allow the normal microbial communities present in wastewater streams to perform the conversion, making the process simpler, cheaper, and more efficient.
As the authors note, acetate is the preferred food source for bacteria able to carry out bioremediation of chlorinated chemicals, which are ranked high in the list of groundwater pollutants of concern in the United States. Further, acetate is the preferred carbon source for the anode-respiring bacteria found in microbial fuel cells and used to produce electricity. Hence, beneficial microbes could potentially perform double duty, cleaning up highly toxic contaminants while liberating industrially useful chemicals and producing clean energy.
Written by: Richard Harth