With the world continuing on its course toward severe climate instability in the future, the desire to reduce the use of fossil fuels and lower humanity’s production of greenhouse gasses like carbon dioxide has become an issue of ever greater importance as the years go on. But there are a number of highly beneficial chemicals required for use in basic hygiene and industrial production and research that unfortunately are not free from the weight of carbon dioxide.. Acetone and isopropanol are two such examples that are used in every biological, chemical, and medical research lab around the world to ensure proper sanitation and reduce the spread of harmful pathogens.
The production of them, however, is invariably tied to the use of fossil fuels as a byproduct of the chemical synthesis steps for making either chemical. This is a major sticking point due to the ubiquity and versatility of both chemicals, along with them being used as components for making further products such as acrylic glass and even as greener fuel substitutes to once again reduce our reliance on fossil fuel sources. So, how do we get around this required fossil fuel creation in order to make them in the first place?
Finding A New Pathway
There have been some alternatives discovered over the years, particularly the process of biofermentation using solventogenic species (bacteria that can make solvents like the above chemicals) and merely supplying them with sugars for their growth and development. The use of these bacteria was a major deal throughout the latter half of the 20th century, seeing production reach even 500 metric tons a year, but the side costs and requirement of each production line focusing only on a single bacteria and chemical producer made the process unsustainable in the long term. By the 1980’s, only a few countries were still bothering with using it.
Many research attempts have been made since then to improve the bacterial process, focus it on other chemicals, and to even get E. coli to be able to use the fermentation pathways. But the same issues of low efficiency plagued all such efforts. For the subject of acetone and isopropanol, the actual biochemical processes physically can’t get above 50% efficiency due to how carbon dioxide is produced from sugar fermentation. That isn’t the only chemical process available though.
If we instead move to autotrophs that use gas fermentation, we get into syngas territory and methods that can recycle any produced carbon byproducts. Which is all well and good, except that there aren’t any autotrophs that have the pathways for producing acetone and isopropanol. Not at any detectable amounts, anyways, though some past studies have managed to manipulate the bacterial genome to instead allow them to do so. Still very low results, however.
The anaerobic acetogen bacteria have seen significant interest among these autotrophs, as they don’t require light for their reactions and so don’t have that as a bottleneck for scaling up production and they utilize the most efficient known carbon dioxide fixation pathways among the autotrophs. Acetogen genetic engineering has been in use for over a decade at this point and has helped in making dozens of chemical compounds. But, actual scaled production remains a problem and only ethanol has seen true industrial scale systems put in place using these acetogens, with some other compounds seeing general industrial production.
From Ethanol To Acetone
In fact, this ethanol accomplishment was done by the very research lab we’re discussing today out of Northwestern University. This time, they took on the challenge of using Clostridium autoethanogenum, a species that they themselves developed that they had previously made for ethanol production and went about altering it for acetone and isopropanol instead. The first step was figuring out what molecular pathways to use to have the right enzymes be involved to then have the correct final biosynthesis product. While the data of genes from a bunch of autotroph bacteria had already been sequenced, their uses hadn’t been looked at to determine their function and so the research team was able to find 41 new enzymes involved in acetone biosynthesis after doing so.
Narrowing down the list, they selected some of the enzymes and assembled them into an intermediate E. coli host to check if they functioned as desired. Since C. autoethanogenum is already capable of reducing acetone to isopropanol thanks to an existing alcohol dehydrogenase enzyme, a knockout mutant was used to test the engineered acetone production pathways, as they wouldn’t be measurable otherwise if they were being converted to isopropanol. The use of appropriate promoter sequences resulted in acetone production that was an order of magnitude higher than any previous efforts. With the added in isopropanol pathways left functioning for a different set of the bacteria, the team was able to see improvements from 3.8 milligrams per liter per hour of bacterial culture production of the two chemicals to around 3 whole grams and allowed for sustained production from a single culture for over 3 weeks.
They additionally did a pilot test involving a 120 liter scaled up version that, if used in an industrial platform, would be able to reach the level of thousands of metric tons of production per year. Thus proving that their engineered bacteria can be sustainable on a larger scale. Furthermore, the gas fermentation process uses up carbon dioxide throughout its steps, resulting in a net negative carbon footprint for each production path of acetone and isopropanol of over a kilogram of carbon dioxide used when producing a kilogram of the chemical.
Reducing CO2 And Taking Names
So, not only did the research team find a way to create two massively important chemical substances without using the regular biochemical synthesis methods that produce greenhouse gasses, but they were able to actively reduce carbon dioxide from the process. If this can be properly scaled up and used to replace all industrial methods for making acetone and isopropanol, the improvements would be a tremendous boon to the world, while still allowing us to keep making the critical disinfectants and solvents required for the modern world.
While this may only be one small part of the problem of greenhouse gas production leading to climate change, it is yet another important accomplishment and shows us that there are always alternative options. We can only continue pressing forward and find all the other options that are out there in our quest to progress science and make this planet a better place at the same time.
Photo CCs: Escherichia-coli-bacterium(1) from Wikimedia Commons