As gasoline prices climb and the U.S. Energy Information Administration predicts the global consumption of liquid fuels to increase in the coming years, many scientists explore alternatives to petroleum. Researchers ponder using bacteria as a renewable way to create biofuel by considering what the ideal bacterium for the job should look like.
What are Bacterially Produced Biofuels?
Biofuels are fuels that are made from living matter. Unlike gasoline, biofuels can be produced from renewable resources, meaning that they could provide an indefinite fuel source (5). Using bacteria to create biofuels such as bioethanol is an idea that inspires many of a renewable future.
Of course, bacteria can’t create something out of nothing. The process involves converting lignocellulosic biomass from plants into biofuel. Lignocellulose is plant matter that contains three defining molecules: cellulose, hemicellulose, and lignin (3). It makes a promising substrate to create biofuel because it is abundant in nature, found globally, and doesn’t involve wasting food products. That’s why cheeseburger-powered cars never took off as an idea.
Bioconversion of lignocellulose to biofuel involves four processes:
- Pretreatment is the process of disrupting the carbohydrate-lignin shield that prevents the bacterial enzymes from reaching the cellulose and hemicellulose.
- The enzymatic process is when the bacteria break down complex sugars into simple sugars by using enzyme proteins.
- The fermentation process is when organisms such as yeast use metabolic processes to convert sugars to bioethanol.
- The distillation process separates the bioethanol from the water and residual solids.
Why Don’t We Use Biofuels Now?
The current issue we face is that natural strains of bacteria don’t seem up to the task of producing biofuel on an industrial scale. Many regulatory factors keep bacteria from behaving as the cellular factories that we want them to be. Fortunately, scientists are exploring the possibility of designing more economic bacterial strains through genetic and metabolic engineering.
Designing the Right Bacterium for the Job
To design the ideal bacterium for biofuel production, we need to know what it should look like. What can it do that other bacteria can’t? There are several conditions that the bacterium should meet to ensure efficient productivity (6).
Can work with a large amount of substrate
Producing bioethanol one drop at a time won’t be enough to keep up with the industrial demands for fuel. That’s why the right bacterium should be able to use large amounts of substrate to create large amounts of product. To do this, it should multiply and form productive communities in a reasonably short length of time. The general idea here is the age-old concept that more workers = more productivity.
Use pathways that are fast and unregulated
Normally, metabolic pathways and processes are regulated by a variety of complex conditions. This regulation can involve turning off the pathway, therefore decreasing productivity. Such restrictions usually serve practical purposes in nature, but they only get in the way when bacteria act as biofuel factories. That is why the ideal bacterium should use metabolic pathways that work quickly and are deregulated.
Resist inhibitory compounds
During the process of biofuel production, a number of inhibitory compounds can be produced. These inhibitory compounds can hurt the growth and activities of the bacteria, ultimately reducing their productivity. Since the production of inhibitory compounds can’t always be avoided, the ideal bacterium should be resistant to these compounds.
High environmental resilience
Bacteria can experience many changes in their environment. These changes include fewer nutrients, higher concentrations of end products, and higher cell density. Such changes can affect the productivity of the bacteria. Recent developments in cell engineering have sought to overcome this obstacle and create more efficient cell factories by overwriting the ways that bacteria respond to their environment. Metabolite-responsive transcription factors are one such solution that rewires the cell’s regulatory pathways and helps the cell respond to its changing environment in new ways (4). Ultimately, these may be used to help the bacterium maintain high productivity, even when it finds itself facing stressful environmental conditions.
A solution in the form of renewable biofuels is more than welcome. Understanding what the ideal bacterial strain looks like for biofuel production is an important step in developing that strain. Advances in bioengineering technologies have paved the way for designing the ideal microbe. The road ahead involves identifying key enzymes and mapping out relevant bacterial pathways.
Sources
1.
U.S. Energy Information Administration – EIA – Independent Statistics and Analysis, (available at https://www.eia.gov/pressroom/releases/press525.php).
2.
B. Yang, C. E. Wyman, Pretreatment: the key to unlocking low-cost cellulosic ethanol. Biofuels, Bioprod. Bioref. 2, 26–40 (2008).
3.
Lignocellulose – an overview | ScienceDirect Topics, (available at https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/lignocellulose).
4.
X. Wan, M. Marsafari, P. Xu, Engineering metabolite-responsive transcriptional factors to sense small molecules in eukaryotes: current state and perspectives. Microbial Cell Factories. 18, 61 (2019).
5.
O. US EPA, Economics of Biofuels (2014), (available at https://www.epa.gov/environmental-economics/economics-biofuels).
6.
M. F. Adegboye, O. B. Ojuederie, P. M. Talia, O. O. Babalola, Bioprospecting of microbial strains for biofuel production: metabolic engineering, applications, and challenges. Biotechnology for Biofuels. 14, 5 (2021).