Protein-rich microalgae can grow at the high CO₂ levels in industrial exhaust

Our world’s population is growing fast—and with it, our demand for not only meat but also animal feed. Soy is a common protein supplement in animal feed, but growing soybeans requires fresh water, fertilizer, and vast swaths of land. Protein-rich microalgae need less of these resources, and a new study shows that they can grow at the carbon dioxide levels found in exhaust from coal-fired power plants, oil refineries, and other industrial processes (ACS Sustainable Chem. Eng. 2019, DOI: 10.1021/acssuschemeng.9b00656). The results show that microalgae have the potential to be a more sustainable alternative to soy in animal feed, the researchers say.

Earlier studies had reported that the CO₂ concentrations in industrial emissions were too high for microalgae to consume as an energy source and could inhibit their growth. Some researchers proposed that pH changes in the solution in which the microalgae grow, caused by increased CO₂ concentrations, are to blame. CO₂ dissolves in solution to become carbonic acid, which deprotonates, making the solution acidic and lowering pH levels. As a result, high CO₂ concentrations can make the solution too acidic for microalgae to grow. Researchers have tried to maintain constant pH levels through methods such as periodically adding ammonium salts or using higher concentrations of buffer, and then letting the pH gradually change.

Jerald L. Schnoor of the University of Iowa and Hannah R. Molitor, a graduate student in his laboratory, grew microalgae in bioreactors that enabled far more precise pH control. They chose the species Scenedesmus obliquus, since it’s highly nutritious, grows fast, and has qualities that make it more likely to thrive on wastewater, such as its football-like shape, which a previous study had suggested could help it resist shear forces in a wastewater stream. Continuous feedback from the bioreactor’s pH meter controlled the addition of a base to freshwater algae medium to maintain a constant pH of 6.8, which falls within the optimal pH range for S. obliquus growth.

The researchers grew S. obliquus at several different CO₂ concentrations, ranging from atmospheric levels to the higher levels found in industrial emissions. At each CO₂ concentration, they measured the optical density of S. obliquus samples taken over time and used mathematical modeling to calculate the maximum growth rate.

Previous studies had measured the highest maximum S. obliquus growth rate at 2.5% CO₂, but Schnoor and colleagues measured it at 4.1% CO₂, and their model predicted that maximum growth would occur at 4.5% CO₂ in the real world. In fact, S. obliquus didn’t show inhibited growth until 10% CO₂—higher than the levels in natural gas combustion and oil refining emissions—and grew well even at up to 35% CO₂—higher than the levels in cement manufacturing emissions. These results suggested that the CO₂ levels in industrial emissions are not a barrier to microalgae growth.

The researchers also compared the amino acid profiles of S. obliquus and soy. Farmers often have to supplement soy-containing cattle feed with methionine, but since the microalgae contained twice as much methionine as soy, they may not need to do that with microalgae-containing cattle feed, Molitor says.

Fengqi You, a chemical and biomolecular engineer at Cornell University, calls the findings “promising.” Growing microalgae for animal feed would not only remove planet-warming CO₂ from industrial emissions but also watershed-polluting nitrate from wastewater, which is also consumed by microalgae, he says.

But You also points out that the Iowa researchers conducted the study at benchtop scales under highly controlled laboratory conditions. Indeed, they didn’t grow S. obliquus on wastewater nor use actual industrial emissions samples, which would contain pollutants that may inhibit S. obliquus growth or have toxic effects on it. Although Molitor says their model’s prediction of the optimal CO₂ concentration for S. obliquus would likely hold up at industrial scales, they can’t say for certain until pilot studies validate their findings. The economic feasibility of growing microalgae near sources of industrial emissions also remains unclear, You adds.

Still, the study is “a nice proof of concept,” he says. If its findings do hold up at industrial scales, “it could lead to a huge change to nationwide or global food systems.”