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Sustainability

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

Tapping into carbon dioxide emissions from industrial processes could make microalgae a sustainable protein source for animal feed

by Melissa Pandika, special to C&EN
May 21, 2019

20190521lnp2-microalgae.jpg
Credit: Hannah R. Molitor
The microalgal species Scenedesmus obliquus (400x magnification) can grow at the CO2 levels found in the exhaust from various industrial processes.

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.

20190521lnp2-bioreactor.jpg
Credit: Hannah R. Molitor
A bioreactor, illuminated by red and blue LED light, maintains the optimal pH level for growing the microalgal species Scenedesmus obliquus.

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.”

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Comments
FA Agblevor (May 22, 2019 1:35 PM)
It is distressing to read that researchers want to grow algae on industrial wastes (CO2 and wastewater), then feed the proteins to animals which will then serve as food for humans. This research shows that we have not learnt any lessons from our past mistakes. Not long ago, there was always news about mad cow disease, which was traced to feeding of cheap proteins from rendered dead animals to cows and other animals. We all know the outcome, humans eventually developed the same disease that the cows acquired from the proteins derived from dead mad cow disease infected sheep. Now we want to feed algae grown on wastewater and polluted CO2 from coal burning plants to algae and then feed these algae to cows which will be use for human food. This sounds like the mad cow disease all over again and some people will have to fall victim before we start correcting our mistakes. Soybean maybe expensive, but at least it is clean and does not cause mad cow disease and other diseases associated with polluted animal feed. Using algae for fuel production makes sense because it does not enter the food chain, but using it as animal feed that will enter the human food chain is very troubling indeed. I hope we do not repeat the mad cow disease story again.
Brian J. B. Wood (May 23, 2019 1:41 PM)
Some cyanobacteria grow in nature at pH values corresponding to fairly high mineral acid concentrations, for example those found in acid pools and streams near to sites where volcanic activity is present. Even some eukaryotes, for example Cyanidium caldarum (I think that its name may have changed since I last cultivated it), as the name suggests, grows in pools associated with volcanic vents I did not have cause to try it in high carbon dioxide concentrations but it is a real tough critter and I can well believe that it would relish such carbon dioxide levels as are quoted here. Sampling at sites in the hills of spent oil shale near Edinburgh I encountered an Euglena, tentatively identified as E. mutabilis, that only grew where there was so much acidification through bacterial oxidation of sulfur compounds in the spent shale that the deposits of iron carbonates and hydroxides that normally clouded the entire environment were dissolved. I will bet that this alga will accept higher carbon dioxide levels and, like all euglenoids it will have a very favourable chemical composition for use as a cattle food. We are only now beginning to appreciate the range of environments that algae and cyanobacteria can colonize and it would be fun to challenge mixed populations from these harsh environments with high carbon dioxide concentrations.

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