November 25, 2020

I, Science

The science magazine of Imperial College

Open wide, son, you’re about to solve the energy crisis

Ol’ Daisy the cow doesn’t know it, but she might just be responsible for saving the world.

OK, not quite, but she does have a part to play in at least mitigating an ever-growing global energy crisis. Researchers have identified microbial genes that reside within a cow’s rumen that could potentially break down cellulose, the main structural component found in trees and plants, efficiently enough to be used for sustainable biofuel production.

Biofuels, i.e. bioethanol or biodiesel, have been deemed by many, including Barack Obama, as the solution to mitigating natural resource depletion.

Currently. 95% of the USA’s bioethanol production derives from corn, whilst Brazil is almost entirely dependent on sugar cane as its biofuel source. However, there is a third option tentatively being explored: the ultimate conversion of material from plant stems and more ‘woody’ vegetation into commercially usable bioethanol.

Although undoubtedly ‘greener’ than common gasoline, corn and other grain-derived ethanol has still come under intense scrutiny as an economically viable solution to relieving our dependence on oil as a transport fuel. They have been criticised, and often erroneously, for being economically and energetically inefficient, i.e. for the amount of money and energy put into production, what is gained in return isn’t particularly worth the initial outlay. There’s also the conflict of land use. In times of an ever-growing population, shouldn’t we be using the fields for food crops rather than for what could prove to be a cul-de-sac of a biofuel?

But this is where cellulosic bioethanol trumps the lot as it is exempt from the above problems. Additionally, cellulosic material can be attained from a wide range of sources beyond raw plant tissue. Cellulose is a large part of waste timber and wood pulp, or even scrap paper. Think about how much paper you throw away on a daily basis. Then multiply that by 7 billion (and rising!). This could all be turned into fuel for your family car.

Hence, cellulosic feedstocks are increasingly attracting the attention of academic researchers and commercial industries as a guilt-free alternative to grain-derived ethanol.

However, there are still hurdles to be cleared before cellulosic ethanol can become a mass-produced fuel. Due to the physical structure of cellulose microfibrils, as well as the surrounding lignin, which acts to further strengthen the cellulose, plant cell walls are difficult to break down by hydrolysis. Hydrolysis is part of the saccharification process that breaks down long cellulose chains into shorter soluble sugars. These sugars are the vital ingredient in the fermentation process that yields the all-important bioethanol.

Just as nature provides the problem, it also provides the solution. Cows are effectively living, breathing, walking cellulose-degrading machines. Within a bigger picture, their multiple stomachs are the result of a species’ insistence on eating pointless amounts of grass and the subsequent millennia of evolution.

That’s why researchers at the University of California, Berkley, funnelled switchgrass down a cow’s throat until it reached its rumen, where a multitude of microorganisms reside. These microorganisms are able to hydrolyse cellulosic material into digestible simple sugars such as glucose, and exactly the same type of sugars needed for bioethanol production.

After removal from the cow’s gut, the microogranisms that were able to assimilate the switchgrass within 72 hours were subject to metagenomic screening. Metagenomic techniques allow the direct analysis of DNA in the search for enzymes used by the microorganisms to break down cellulosic tissue from the switchgrass sample.

Analysis yielded nearly 30,000 genes implicated in carbohydrate metabolism. These genes were cloned into bacteria producing 90 potential proteins, 57% of which were active as cellulose-involved enzymes. 15 complete genomes of microbes (which haven’t been able to be cultured in a lab as of yet) were also assembled in this study.

This research, published in Science last week, is clearly a theoretical victory for environmentally viable transport fuels.

This is also another example of humans playing second fiddle and copycat to the greatest scientist of all: nature. Attempts in protein engineering to improve the efficiency of existing cellulose-degrading enzymes have proved largely underwhelming for now. It just so happens that the unassuming cow might have the answers we’ve been looking for in our mission to efficiently generate cellulosic bioethanol.

A couple of years ago, a story in the same vein originated from our very own academic home, Imperial College. A research group from the Biology department were examining the potential behind an ‘artificial leaf’ by mimicking naturally-occurring photosynthesis to produce the next generation of clean power fuels.

These findings from inside a cow’s stomach could, and hopefully will, kick-start a carbon-neutral revolution in the coming years. It’s more likely to be decades, and most likely to be centuries, but no doubt nature will dictate a solution through biofuel technology.