Methanol can be synthesized from carbon dioxide and water using renewable energy. When this green methanol is metabolized by specialized bacteria, a variety of chemical substances can be biotechnologically produced. The application could allow the chemical industry to convert the greenhouse gas carbon dioxide into valuable climate-neutral chemicals – and significantly reduce its ecological footprint.
The chemical industry currently relies on fossil raw materials such as crude oil to produce various chemicals such as plastics, dyes and artificial flavors. “Worldwide, it consumes 500 million tons per year, i.e. more than one million tons per day,” says Julia Vorholt, Professor at the Institute of Microbiology at ETH Zurich. “As the chemical conversions are very energy-intensive, the chemical industry’s carbon footprint is even six to ten times larger: it amounts to around five percent of total global emissions.” She and her team are therefore looking for ways to reduce the chemical industry’s dependence on fossil fuels.
Green methanol
The focus here is on bacteria that feed on methanol, which in technical jargon means they are methylotrophic. Methanol has a single carbon atom and is therefore one of the simplest organic molecules. It can be produced from the greenhouse gas carbon dioxide and water. If the energy for this synthesis reaction comes from renewable sources, the methanol is described as green. “There are natural methylotrophs, but using them industrially remains difficult despite a great deal of research,” says Michael Reiter, postdoctoral researcher in Vorholt’s research group, which instead works with the model bacterium Escherichia coli, which is well known in biotechnology. Vorholt’s team has been pursuing the idea of equipping the model bacterium that grows on sugar with the ability to biochemically utilize methanol for several years.
Complete restructuring of the metabolism
“This is a major challenge, because it requires a complete restructuring of the metabolism,” says Vorholt. The researchers initially simulated this change using computer models. They then specifically removed two genes and inserted three additional genes in their place. “This allowed the bacteria to absorb the methanol, albeit only in small quantities,” says Reiter. They then continued to cultivate the bacteria for more than a year under special conditions in the laboratory until the conversion was successful – and the microbes were able to produce all cell components from methanol. Over the course of around 1000 further generations, these so-called synthetic methylotrophs became more and more efficient, so that they finally doubled in size every four hours when fed exclusively with methanol. “The improved growth rate makes the bacteria economically interesting,” says Vorholt.
Optimization through loss of function
As Vorholt’s team explains in the recently published article, several random mutations are responsible for the increased efficiency of methanol utilization. Most of these mutations led to the loss of function of various genes. This is surprising at first glance, but a closer look reveals that the cells can save energy thanks to the loss of function of the genes. For example, some mutations cause the reverse reactions of important biochemical reactions to fail. “This avoids superfluous cycles – and optimizes the metabolic flows in the cells,” the researchers write. In order to explore the potential of synthetic methylotrophs for the biotechnological production of industrially relevant bulk chemicals, Vorholt and her team have equipped the bacteria with additional genes for four different biosynthetic pathways. In their study, they have now shown that the bacteria actually produced the desired compounds in all cases.
Versatile production platform
For the researchers, this is clear evidence that their highly cultivated bacteria are living up to their original expectations: The microbes are a kind of highly versatile production platform into which biosynthesis modules can be installed according to the “plug-and-play” principle, causing the bacteria to convert the methanol into any biochemical substance. However, the researchers still need to significantly increase the yield and productivity to enable the bacteria to be used in an economically viable way. Vorholt and her team recently received innovation funding “to further develop the plans towards application and figure out which products to focus on first,” says Vorholt.
When Reiter talks about how the cultivation of bacteria in bioreactors can be optimized, he is bursting with energy. “In view of global warming, it is clear that we need alternatives to fossil fuels,” he emphasizes. “We are developing a technology that does not release any additional CO2 into the atmosphere,” says Reiter. And since synthetic methylotrophs do not require any carbon sources other than green methanol for their growth and products, they make it possible to produce “renewable chemicals that do not pollute the environment”.
Picture above: Bacteria that feed on methanol can produce sustainable chemicals. Illustration: Sean Kilian
