Engineered yeast strain could revive interest in non-food bio-feedstocks

CHEMICAL engineers at MIT have genetically engineered yeast that are tolerant to toxic biorefining conditions, a development that offers a route for converting tricky non-food feedstocks into fuel.

Lignocellulosic biomass is a hugely abundant feedstock available from agricultural waste streams that do not compete with foods. These include grasses and the non-edible leftovers from food crops such as the leaves, stalks and cobs from corn. The amount of bio-based feedstock that is available could help displace significant volumes of fossil feedstock in the production of fuels, but the severe pre-treatment needed to produce fermentable sugars from lignocellulosic biomass results in conditions toxic to the microorganisms required to convert it to ethanol.

MIT researchers have overcome this issue by genetically engineering a strain of yeast so that it converts the toxic byproducts resulting from pre-treatment – aldehydes – into alcohols. In earlier research, the team showed that adding buffers including potassium chloride to the bioreactor strengthened the membranes of the yeast, providing tolerance to alcohols. By pairing both techniques the team showed the yeast can withstand toxic conditions and produce industrially-relevant concentrations of ethanol – around 100 g/L – from a range of lignocellulosic feedstocks including corn stover, wheat grass, and switchgrass.

The MIT team hopes that its research will help turn around industry’s diminishing interest in producing fuels from lignocellulosic feedstocks. Writing in the journal Science Advances, the team notes that technical production challenges have seen the US cellulosic ethanol industry dwindle sharply with just a single preproduction plant remaining, operated by POET-DSM in Iowa.

Asked about next steps, MIT researcher Felix Lam said toxic conditions have prevented the investigation of more aggressive processing conditions using more acid and higher solids loading which could reduce costs and increase sugar output for fuels production.

“Our results remove that major constraint, but now process development needs to happen to explore the benefits,” said Lam, who is a postdoc at MIT’s Metabolic Engineering Laboratory run by chemical engineering professor Gregory Stephanopoulos.

“Our hope is that our results re-energise interest in such a vast, conflict-free feedstock,” Lam said.

Stephanopoulos said: “The implications are very significant for biorefining because simpler methods using weak acids will be possible to use for the hydrolysis of biomass and release of sugars; enzyme use may no longer be required for the hydrolysis step, which would reduce the cost significantly; and the whole biomass pre-treatment step, a real bottleneck in biomass utilisation, is now simplified and made cheaper.”

Lam added that another important consequence is the diversity of supplies that the tolerant yeast opens up for refiners to process: “Any supply chain bedrock at the national scale needs low supply uncertainty to forestall volatility. The ability to diversify across crop types and pre-treatments is a major boon.”

Stephanopoulos said that the next step will be to demonstrate the results at larger scale, though cautioned that this is an expensive proposition that may be difficult to pursue while oil prices are relatively low.

“Another path might be to apply the same concepts to our oil-producing or alkane-producing yeasts in pursuit of a cellulosic oil process,” Stephanopoulos said, referring to research published in Nature Communications last year.

As part of their latest research, Lam and colleagues also engineered their aldehyde-to-ethanol enzyme into a strain of yeast that produces lactic acid – a precursor to bioplastics. The strain produced a similar yield of lactic acid from cellulosic feedstocks as it did from simple sugars. This shows that the genetic modification technique has the promise of being dropped into other strains, making them tolerant to harsh pre-treatment conditions and opening up production of other products including plastics, solvents and diesel from lignocellulosic biomass.

“We hope this will also aid in revitalising cellulosic interest,” Lam said.

Science Advances: https://doi.org/gmdnk3