Intestinal germ as fuel producer
Researchers have found another way to use microbes to help produce more sustainable fuels. To do this, they equipped bacteria of the species Escherichia coli with the genes for four additional enzymes. This enabled the microbes to produce large quantities of so-called hydroxy fatty acids from sugar. These, in turn, can be converted by chemical catalysis into olefins — hydrocarbon chains found in plastics and fuels, among other things.
In a study published in Nature Chemistry, researchers report that they have harnessed the wonders of biology and chemistry to turn glucose (a type of sugar) into olefins (a type of hydrocarbon and one of several types of molecules that make up gasoline). The project was led by biochemists Zhen Q. Wang at the University of Buffalo and Michelle C. Y. Chang at the University of California, Berkeley. The work represents a step forward in efforts to develop sustainable biofuels.
When it comes to producing the most sustainable and climate-friendly fuels possible from biomass, microbial helpers are high on the list. This is because bacteria can decompose plant material with the help of enzymes, ideally producing hydrocarbons without requiring much energy. Examples include goat intestinal germs, petroleum-producing algae and a team of fungus and bacteria that can convert plant fibers into the fuel isobutanol.
“But many synthetic compounds, including mainly unsubstituted hydrocarbons, are still difficult to make using cells alone,” explain Zhen Wang of the University of California at Berkeley and her colleagues.
Alkene producers wanted
The research team has therefore been looking for a way to get the easily cultivated intestinal germ Escherichia coli to synthesize molecules that can then be readily converted into olefins. The term olefins covers hydrocarbons with at least one double bond. Such alkenes or cycloalkenes are important basic materials for plastic production and other chemical processes, but are also present in fuels such as gasoline.
The starting point for the study was normal Escherichia coli cultures. “These microbes are real sugar junkies,” says Wang. The bacteria were now to produce hydroxy fatty acids from the sugar as efficiently as possible and independently of their cell growth. These are medium-length hydrocarbon chains with special functional groups that can easily be catalytically converted into olefins. To this end, the researchers introduced the genes for four additional bacterial enzymes into the microbes.From sugar to olefin
The result is a microbial strain that can produce hydroxy fatty acids relatively efficiently from sugar in a four-step process. Depending on the variant, the yield was 780 to 1,600 milligrams per liter of microbial solution — corresponding to an efficiency of around 86 percent, as Wang and her team report. Chemical catalysts were then used to convert the compounds into alkenes of six to nine carbon atoms in length.
Olefins make up only a small percentage of the molecules in gasoline currently produced, but the process developed by the team could likely be adapted in the future to produce other types of hydrocarbons, including some of the other components of gasoline, Wang says. She also points out that olefins are used not only for fuels, but also for industrial lubricants and as precursors for making plastics.
“We have combined what biology does best with what can be done well chemically to develop this two-step process,” Wang says. “In principle, we can use it to make olefins directly from glucose.” Because plant sugars are produced through photosynthesis and thus with the absorption of CO2 from the air, fuels made from biomass are considered relatively climate-friendly — as long as their production does not require too much energy.
Yield can still be optimized significantly
“Producing biofuels from renewable raw materials such as plant sugars therefore has the potential to advance green technologies,” Wang points out. This is especially true if plant waste is used for this purpose. So far, however, the microbial olefin synthesis of Wang and her colleagues is still in its infancy. At just over eight percent, the alkene yield is still very low. It is also still unclear whether and how the process can be scaled up to industrial standards.