Researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have engineered a strain of bacteria that enables a "one-pot" method for producing advanced biofuels from a slurry of pre-treated plant material.
The Escherichia coli (E. coli) is able to tolerate the liquid salt used to break apart plant biomass into sugary polymers. Because the salt solvent, known as ionic liquids, interferes with later stages in biofuels production, it needs to be removed before proceeding, a process that takes time and money. Developing ionic-liquid-tolerant bacteria eliminates the need to wash away the residual ionic liquid.
The achievement, described in the journal Green Chemistry, is a critical step in making biofuels a viable competitor to fossil fuels because it helps streamline the production process.
"Being able to put everything together at one point, walk away, come back, and then get your fuel, is a necessary step in moving forward with a biofuel economy," said study principal investigator Aindrila Mukhopadhyay.
"The E. coli we've developed gets us closer to that goal. It is like a chassis that we build other things onto, like the chassis of a car. It can be used to integrate multiple recent technologies to convert a renewable carbon source like switchgrass to an advanced jet fuel."
RENEWABLE CHEMICALS DEFINITION
Renewable chemicals or bio-based chemicals like those developed in Berkeley Lab are obtained from renewable sources such as agricultural feedstock, agricultural waste, organic waste products, biomass, and microorganisms and are used to produce other chemicals. They are base chemicals for industrial manufacturing processes, according to BCC Research analyst Nikos Thomopoulos.
“Renewable chemicals are considered sustainable carbon sources, as their environmental impacts are lower when compared with conventional petroleum-based chemicals,” he explains. “Typical renewable chemicals include polymeric (lignin, hemicellulose, cellulose, starch, protein) and monomeric (carbohydrates, oils, plant extractives, amino acids) components. These chemicals are widely used as a direct substitute for conventional petroleum-based chemicals.”
The main types of renewable chemicals include bio-based alcohols, raw materials for renewable chemicals production, biopolymers, bio-based organic acids, ketones and aldehydes, as well as renewable platform chemicals.
RENEWABLE CHEMICALS PRODUCTION TECHNOLOGIES
Thomopolous says that chemical compounds produced from renewable resources could help to minimize fossil fuel burning and CO2 emissions. Bio-based chemicals produced from biomass, such as plants or organic waste, could help to reduce both the world’s dependence on oil as well as CO2 emissions. Renewable chemicals and bioproducts produced from plant biomass would mitigate global warming. Additionally, biofuel production along with bioproducts can provide new income and employment opportunities in rural areas, he adds.
Renewable chemicals manufacturing technologies are generally based on biological catalysis, chemical catalysis, thermochemical processing, wet chemistry and combinations of these techniques in biorefineries. Biological catalysis uses synthetic biology tools to alter microbial genetics for optimal enzyme activity. Chemical catalysis pathways make use of inorganic catalysts to facilitate conversion reactions to yield specific molecules.
Many renewable chemicals manufacturers employ high throughput screening methods to accelerate identification of optimal genetic mutations and also produce catalyst formulations.
There are two main routes available for producing renewable chemicals from biomass: thermochemical processing and biochemical processing. Thermochemical processing defines the conversion of biomass into a range of products, by thermal decay and chemical reformation. The method essentially involves heating biomass in the presence of different concentrations of oxygen. The clear advantage of thermochemical processing is that it can convert all of the biomass’ organic components, compared with biochemical processing, which mainly focuses on the polysaccharides.
BREAKING DOWN THE BIOFUEL PRODUCTION PROCESS
The basic steps of biofuel production start with deconstructing, or taking apart, the cellulose, hemicellulose and lignin that are bound together in the complex plant structure. Enzymes are then added to release the sugars from that gooey mixture of cellulose and hemicellulose, a step called saccharification. Bacteria can then take that sugar and churn out the desired biofuel. The multiple steps are all done in separate pots.
Researchers at JBEI pioneered the use of ionic liquids, salts that are liquid at room temperature, to tackle the deconstruction of plant material because of the efficiency with which the solvent works. But what makes ionic liquids great for deconstruction also makes it harmful for the downstream enzymes and bacteria used in biofuel production.
Marijke Frederix, a postdoctoral researcher in Mukhopadhyay's lab, established that an amino acid mutation in the gene rcdA, which helps regulate various genes, leads to an E. coli strain that is highly tolerant to ionic liquids, providing an important piece to the puzzle. They used this strain as the foundation to build on earlier work--including the ionic-liquid-tolerant enzymes--and take the steps further to the one-pot biofuel finishing line.
PUTTING THE PIECES TOGETHER
They proceeded to test the E. coli strain using ionic-liquid pretreated switchgrass provided by the DOE's Advanced Biofuels and Bioproducts Process Demonstration Unit (ABPDU), a biofuels facility at Berkeley Lab launched in 2011 to accelerate the commercialization of biofuels.
"Armed with the rcdA variant, we were able to engineer a strain of E. coli that could not only tolerate ionic liquid, but that could also produce ionic-liquid-tolerant enzymes that chew up the cellulose, make sugars, eat it and make biofuels," said Frederix. "E. coli remains the workhorse microbial host in synthetic biology, and in our study, using the ionic-liquid-tolerant E. coli strain, we can combine many earlier discoveries to create an advanced biofuel in a single pot."
While ethanol may be one of the more common products to emerge from this process, researchers have looked to more advanced biofuels that can pack more energy punch. In this case, they used production pathways also developed at JBEI previously, and produced d-limonene, a precursor to jet fuel.
"Ultimately, we at JBEI hope to develop processes that are robust and simple where one can directly convert any renewable plant material to a final fuel in a single pot," said Mukhopadhyay. "This study puts us one step closer to this moonshot."
Major chemical manufacturers are shifting focus towards using natural sources in order to produce various chemicals, in order to reduce dependence on fossil fuel resources.
Thomopoulos anticipates the global renewable chemicals market to grow from $51.7 billion in 2015 to $85.6 billion by 2020, demonstrating a five-year compound annual growth rate (CAGR) of 10.6%.