High value harvests from designer algae

Engineered photosynthetic algae could be used within sunlight-powered sustainable chemical biofactories using a new circular production process developed at KAUST^[1]. The scalable process uses bespoke functionalized microparticles — rather than flammable organic solvents — for the critical step of harvesting the valuable chemicals that the algae produce. The microparticles are robust, reusable, and can be tailored to capture a range of chemical products.

Algae, whose metabolism has been reprogrammed to biosynthesize target chemicals in high volume, could offer a green and sustainable way to make essential products. “Our lab has several metabolically engineered algal strains that produce chemicals called terpenoids. These have potential applications ranging from cosmetics ingredients to biofuels,” says Sebastian Overmans, a postdoc in the lab of Kyle Lauersen, who led the research.

Overmans explains that upscaling the process has been hampered by challenges with extracting the chemicals from the watery fluid that the algae are grown in. “Traditionally, the terpenoids are extracted using a layer of alkane solvent on top of the algal culture,” he says. As the algae circulate in the medium, the chemicals that they generate can dissolve into the solvent layer, where they can be collected.

Other challenges limit the scalability of this process. Firstly, the solvents can be toxic and flammable.

Secondly, as air and carbon dioxide are bubbled through the reactor to support algal growth, the solvent layer can mix into the culture layer. “This mixing causes foam formation and emulsions, which limit productivity and hamper extraction,” Lauersen says.

A cross-campus collaboration with Himanshu Mishra and his team at KAUST could finally have solved the problem. “They recommended that we chemically link the solvent onto the surface of low-cost silica particles to create particles that capture our target terpenoids like a solvent but are physically easier to work with,” Lauersen adds.

The new particles work harmoniously with the gas stream bubbled up through the reactor. “When we tested this method in commercial hanging-bag reactors, we confirmed that the uplift of the air-CO₂ gas mix is sufficient to keep the particles suspended,” Overmans explains. “Once we stop the bubbling, the microparticles settle quickly, which makes them easy to separate from the algal culture for product recovery.”

The valuable chemicals captured in the particles’ solvent coating can be collected by washing them with a small volume of ethanol. “The efficiency of the particles’ extraction varied depending on the compound that we were trying to extract from the algae — but even in cases where the microparticles extracted less well than traditional solvents, the advantages of our method far outweighed the slightly lower performance,” Overmans says.

The team is currently optimizing parameters such as the mixing rate and particle separation procedure, which could further improve efficiency.

“Also, one distinct advantage is that the microparticles can be tailored with coatings optimized for different compounds of interest,” Overmans concludes. “The next step is to show that the technology can be upscaled further before benchmarking its performance in real-world bioproduction.”