What do poplar trees, sharks, and biofuels have in common?
A team of Great Lakes Bioenergy Research Center scientists led by Michigan State University biochemists has reported exciting findings concerning all three in the quest for cleaner energy.
Appearing in the Plant Biotechnology Journal, the team’s latest paper explores how poplar trees can be engineered to produce a highly valuable chemical commonly obtained from shark livers.
Engineering poplars to produce this chemical would greatly boost their economic viability as an already-promising source of biofuels, as well as help cut back destructive shark harvesting.
“I think this project really highlights how we can use industrial crops in new ways,” said co-author Jake Bibik, a former doctoral student in the lab of Michigan State researcher Björn Hamberger. “Using engineered, non-food crops like poplar may provide a more sustainable alternative for generating chemicals typically derived from fossil fuels, or even new specialty chemicals altogether.”
More bang for the buck
On their own, poplars check several boxes needed to be a successful bioenergy feedstock: They grow quickly on non-agricultural land, and their biomass — the organic material where energy is stored — can be broken down and fermented into biofuel.
One of the biggest challenges comes down to simple economics.
“Biofuels are still not competitive against the cheap petrochemistry that’s out there,” said Hamberger, a professor of biochemistry and molecular biology and co-investigator with GLBRC, a U.S. Department of Energy-funded hub led by the University of Wisconsin–Madison.
GLBRC scientists have long sought ways to extract other high-value products from biomass. Research has shown that poplar can be a viable source of p-aminophenol, which is used to make dyes, adhesives, and other polymers, as well as paracetamol, the active ingredient in Tylenol.
When Hamberger joined the center in 2015, he suggested going after terpenes, a group of chemical compounds plants use in unique environmental interactions such as attracting pollinators or defending against pests.
“Terpenes are the oldest, largest class of specialized metabolites on the planet,” Hamberger said. “Since they’re important for all sorts of interactions, it’s driven their diversity to a spectacular point where the chemistry out there is just mind blowing.”
Used by humans for millennia, terpenes have been shown to possess anti-inflammatory, anti-microbial, anti-cancer, and anti-bacterial properties, and are key components in flavorings, cosmetics and perfumes just to name a few applications.
Bibik, Hamberger and their collaborators were targeting a terpene called squalene, an organic compound widely used in cosmetic products and a crucial component in vaccines.
Today, most squalene comes from shark liver — in fact, the name of the chemical even originates from the Latin word for shark, squalus.
During the project, the team engineered poplars to produce squalene along two distinct chemical pathways: One used the gel-like substance known as cytosol found in the center of cells, while another sought to produce squalene in chloroplasts, the organelles responsible for photosynthesis.
“By diverting carbon away from regular metabolism to make specialized chemicals in unique poplar tissues and droplets, Björn and his team are employing a highly innovative strategy to turn trees into biological factories,” said Shawn Mansfield, a professor of botany at the University of British Columbia and GLBRC co-investigator who collaborated on the paper. “It’s exciting to be part of this highly novel and forward-thinking project.”
While the cytosol pathway was discovered to interfere with poplar root formation, the chloroplast route resulted in the production of 0.63 milligrams per gram of squalene in leaves.
With this promising result, it was time for what Hamberger called a “reality check.”
Looking to upgrade
Working with GLBRC co-investigator Christos Maravelias, a professor of chemical and biological engineering at Princeton University, the team next performed an analysis to determine the minimum sales price their poplar-produced squalene would need to be sold at to break even.
The researchers found that number to be $144 per kilogram. Shark-derived squalene comes in at $40 per kilogram.
"If you want to sell a green product to a customer, it can’t only be green, it needs to be affordable,” Hamberger said. “Luckily, there are several ways to boost the value. One way is increasing overall production, and the other brings us to the cool world of perfumes and another marine animal product — ambergris.”
Produced in the digestive system of sperm whales, ambergris is a valuable fixative in perfumes that prolong scents.
Hamberger said it should be possible to “upgrade” squalene to ambrein, another high-value terpene that makes up ambergris. Scientists have already demonstrated that bacteria can be engineered to produce ambrein, paving the way for further investigation into how poplars could do the same.
Another MSU collaborator, Tom Sharkey, showed that poplars engineered to produce squalene emit less of a gas that indirectly contributes to the greenhouse effect.
“Jake, Björn and their colleagues are working on a method with perhaps the greatest promise to make specialty chemicals and fuel for when only liquid fuel will do, such as jet airplane travel,” said Sharkey, a former GLBRC co-investigator and professor of biochemistry and molecular biology.
With these collective findings, the researchers have broken new ground in the quest to transform poplars into an even more attractive source of biofuels and valuable compounds.
Bibik, now a senior scientist at the biotechnology company MelaTech, said the team’s findings are ultimately another step toward using our planet’s biochemical diversity to address some of its greatest challenges.
“I think this work contributes to a growing foundation that’s necessary to be able to translate plant engineering and terpenoid research into meaningful biotechnologies,” he said.