Overcoming oscillations in gas-to-liquid fuel fermentations

Environment and sustainability 10. aug 2020 4 min Scientific Director Lars Keld Nielsen Written by Kristian Sjøgren

Bacteria have the potential to convert synthesis gas – a mixture of carbon monoxide (CO), hydrogen (H2) and often carbon dioxide (CO2) – into liquid fuel for passenger and commercial transport ranging from cars to aircraft. But first, researchers must overcome an obstacle. Danish researchers have helped to identify the mechanism behind this.

Being able to convert the residual energy from various waste streams into transport fuels is a long-held dream. Most waste streams are recalcitrant, and one option is to gasify the waste into syngas and convert this gas into fuel using one-carbon (C1) chemistry.

Chemical gas-to-liquid conversion technology, such as the Fischer–Tropsch process, has been pursued for almost 100 years, but it is relatively sensitive to gas composition and requires a very large scale. In contrast, biological conversion using gas fermentation is less sensitive to contaminants and production scale.

“We want to be able to use bacteria to convert fuel gas to liquid fuel. In this context, we have observed that the productivity of bacteria oscillates when they convert carbon monoxide and hydrogen to ethanol. We have now identified why their productivity oscillates,” explains Lars Keld Nielsen, Scientific Director, Novo Nordisk Foundation Center for Biosustainability, Technical University of Denmark, Kongens Lyngby.

The research has been published in the Proceedings of the National Academy of Sciences of the United States of America.

Acetogens convert gases to fuel

The research specifically focuses on the acetogen microorganism Clostridium autoethanogenum.

Acetogens are capable of converting many substrates, including various gases, into acetate, which plays an important nutritional role not only for the bacteria themselves but also for the host organisms in which the acetogens often settle.

In addition to acetate, the bacteria can also make the commercially useful ethanol, which can be further refined into aviation fuel or gasoline for cars. Researchers have evolved acetogens to make more ethanol than acetate, so they produce more of what we want and less of what we cannot really use for anything.

“Much of our research has focused on understanding the processes involved when the acetogens make ethanol. What proteins are involved? What does their metabolism look like?” explains Lars Keld Nielsen.

Bacteria have a preference

Acetogens can grow on many types of feedstocks but have some preferences. One is synthesis gas, a mixture of carbon monoxide (CO), hydrogen (H2) and often also carbon dioxide (CO2).

The acetogens can convert synthesis gas to ethanol, but long ago researchers identified an annoying obstacle to achieving commercial success.

The researchers can mix more and more synthesis gas with acetogens in their bioreactors, causing the acetogens to produce increasing amounts of ethanol, but at some point the process begins to oscillate, and instead of increasing, productivity begins to fall again.

After it declines, productivity rises again, and the process continues to vary in fixed cycles, no matter how much more synthesis gas the researchers feed into the mix.

One oscillation lasts for about 180 hours, and the difference in output from the peak to the trough may be up to twofold.

This needs to be changed to make the process commercially attractive.

“We thought we would reach a level at which production would not increase anymore but would stabilize when the amount of gas in the liquid with acetogens became the limiting factor. However, until recently, nobody knew why these oscillations occur when the system is pushed to its limit to maximize production,” says Lars Keld Nielsen.

Lars Keld Nielsen says that the same thing happens with baking yeast when you push it to exceed its metabolic robustness.

Halting development

Since the obvious problem is that the system needs to be more efficient to be a commercial success, there are considerable rewards for the researchers who can crack the code to stabilize the production of ethanol by acetogens.

Lars Keld Nielsen explains that researchers need to produce more ethanol from every litre of the bioreactor mix to become an attractive alternative to extracting fossil fuels.

“This is the limiting factor, because we cannot move forward if we cannot solve this,” says Lars Keld Nielsen.

Explosive growth slams on the brakes

As always, the first step in solving a problem is to identify and analyse the problem, and Lars Keld Nielsen has done exactly that with colleagues in Australia.

Lars Keld Nielsen’s many experiments with acetogens in the laboratory show that bacteria start to grow by using carbon monoxide as a growth source.

When the concentration of acetogens is sufficiently high and that of carbon monoxide is low, the acetogens also start to use hydrogen to grow and then their growth increases explosively, which is actually very good.

Nevertheless, when the concentration of hydrogen declines and the acetogens have to return to growing on carbon monoxide alone, things start to go downhill because the acetogens take time to switch to solely using carbon monoxide as a growth source.

This results in decreasing metabolic robustness, which leads to lower production of ethanol.

“The way the system works is that the acetogens can accelerate their growth and metabolism quickly when they can grow on hydrogen, but then it takes a long time to return to the starting-point,” explains Lars Keld Nielsen.

Discovering new enzymes to add to the acetogens

Lars Keld Nielsen says that the problem of the oscillating acetogens has several possible solutions.

Researchers can try to manipulate the system by using synthesis gas with different ratios of carbon monoxide to hydrogen. However, this solution is not optimal, since a major attraction of acetogens is that they can grow on different kinds of synthesis gas, so growth does not require special conditions.

Another more attractive option is to manipulate the genes of the acetogens so that they cope better with switching between carbon monoxide and hydrogen.

An enzyme called hydrogenase plays an important role in this because excess carbon monoxide inhibits its activity. The whole process slows down when hydrogenase is inhibited and cannot convert hydrogen into ethanol.

“We need to find other hydrogenases that will ensure that acetogens are not inhibited by excess carbon monoxide but can continue to use hydrogen throughout the fermentation. This could overcome the obstacle and take the possibility of using acetogens a step further towards achieving greater commercial potential,” says Lars Keld Nielsen.

Redox controls metabolic robustness in the gas-fermenting acetogen Clostridium autoethanogenum” has been published in the Proceedings of the National Academy of Sciences of the United States of America. Lars Keld Nielsen is the Scientific Director of the Novo Nordisk Foundation Center for Biosustainability.

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