Different colors and wavelengths of light increase the growth of a common type of microalgae, which could significantly lower the cost of producing biofuel and biodiesel on a large scale, researchers have found.

A combination of red and blue LED light enhances the lipid content of microscopic algae, according to a new study published Wednesday in the Journal of Renewable and Sustainable Energy. The study is one of the first to look at the relationship between the type of light and lipid growth in a popular strain of microalgae named Dunaliella salina.  

Microscopic algae are tiny aquatic organisms, invisible to the naked eye, that produce about half of the oxygen we breathe. These microalgae are often cultivated for biofuels, as they have high levels of lipids, or fatty acids, that can be turned into natural fuel. D. salina is especially well suited for biofuels, with its high growth rate, high lipid buildup and resistance to environmental stressors. It also lacks a cell wall, which "is very helpful to biodiesel production," as the lipids are then easier to extract, said Xiaojian Zhou, lead author of the study and a professor at Yangzhou University. Zhou thinks that D. salina holds the key to reducing costs and boosting yields in the biofuel industry.

Microalgae is greatly influenced by light, as both insufficient light and overexposure can limit its growth. D. salina, which uses light as a source of energy for metabolism, is also affected by the wavelength of light. Zhou chose LED for microalgae cultivation "because of its accuracy and stability in the emission of light at a specific wavelength." LED also has a high energy efficiency over a long period of time, especially when compared to fluorescent lamps. 

Zhou and his colleagues tested nine different LED illuminators that emitted white light, monochromatic red light, monochromatic blue light and a combination of red and blue light. The white light was used as a control to measure a base level of lipid production, growth and cell density. 

The researchers grew D. salina in culture flasks placed in containers covered with aluminum foil. The foil served to "eliminate light leakage and protect the sample from any external light," Zhou said. The samples were exposed to illumination four times, in a cycle of 16 hours of light and 8 hours of darkness. After 22 days of incubation, the D. salina samples were harvested and measured. 

Researchers found that a combination of red and blue light increased lipid productivity by 35.33% compared to the white light control. The optimal ratio of 4:3 (red to blue light) led to higher levels of lipids "without the cost of reduced cell density, which is a very exciting result for biofuel production," Zhou told The Academic Times. He expects this illumination "strategy [to] work on other species of microalgae," including Chlorella vulgaris, a green microalga often used as a dietary supplement in Japan.

The combination of red and blue light in this study is unique. Zhou was inspired by "vegetable and flower greenhouses in China," where red light is "often used to supplement specific wavelengths of light to promote crop growth."  He thought of using a similar "light spectrum to promote lipid production" in microalgae.

Previous research on microalgae has shown that red light alone leads to reduced growth in D. salina, while blue light is favorable for photosynthesis. However, "None of these published reports conclude a relationship between the wavelength of the light source and the growth of D. salina," Zhou said. This lighting control has a huge potential to lower the cost of biofuels.

Biofuels are a sustainable and viable option to decrease reliance on petroleum products such as oil and gas, but the cost of producing biofuel from microalgae is still quite high. If biofuels are to be commercially available on a scale that rivals petroleum, their production will have to be significantly cheaper — which can be possible with efficient inputs such as microalgae grown under LED lighting. Zhou and his team are next working on the composition of fatty acids in microalgae as a continuation of their biofuel research. 

The study, "Impact of combined monochromatic light on the bio-component productivity of Dunaliella salina," published March 31 in the Journal of Renewable and Sustainable Energy, was authored by Cuili Jin, Binqi Yu, Shouyuan Qian, Qing Liu and Xiaojian Zhou, Yangzhou University.