Blue Light-induced Oil Production

P​rof. XU Jian and his research team from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) at the Chinese Academy of Sciences (CAS), have discovered a genetic sensor of blue light that regulates oil synthesis in an industrial microalga. Exploring this discovery further led to doubling the microalgal productivity of oils.

The researchers have proposed a new technology called Blue-Light Induced Oil synthesis (BLIO), which they say has major implications in microalgae-based conversion of carbon dioxide to biofuels. Their findings were published on March 29 in Nature Communications.

In the oil-producing algae, called oleaginous microalgae, environmental stresses such as nutrient deprivation, high light or heat typically cause the oil to accumulate. These high energy density oils are triacylglycerols (TAGs), precursors for biodiesels. Microalgae are a promising feedstock for TAG production due to their rapid growth and high oil contents.

Scientists have long known that oil production is part of the response of microalgal cells to environmental stresses but exploiting this knowledge for greater oil productivity is hard because they lack a full understanding of how the process works.

This experiment was conducted to present the advantage of BL (blue light) plus NobZIP77-KO-approach, as compared to continuous WL (white light) plus WT (wild type — the conventional way of microalgal cultivation). A Experimental design of BLIO: the NobZIP77ko-1 line of N. oceanica was cultured under WL for 9 days and then under BL for 7 days. WT grown under WL for 16 days (WT-WL) was used as the control. B, C Comparison of TAG yield (mg/L) (B) and average TAG productivity (mg/L·d) (C) between the two approaches. Data are represented as mean ± SD (n = 3 biologically independent samples)

The QIBEBT research team has long been looking for a better way to induce the oil productivity in microalgae. “New environmental stimuli that allow efficient and precise control of cellular TAG assembly yet without compromising biomass productivity are highly desirable,” said ZHANG Peng, a postdoctoral researcher at Single-Cell Center of QIBEBT.

The team has been studying the industrial oleaginous microalga Nannochloropsis oceanica — a type of marine microalga that can produce high-value oils from sea water and CO₂ — for over a decade. Their long journey for a new stimulus that they can control more precisely for oil production eventually led them to blue light. The research team discovered a previously unknown “BlueLight-NobZIP77-NoDGAT2B” pathway.

When nutrients such as nitrogen are abundant, a blue-light-sensing regulator called NobZIP77 would turn off TAG production in the microalga, by inhibiting the expression of oil-producing enzymes such as NoDGAT2B. “However, when nitrogen is depleted, chlorophyll, which normally absorbs blue light, is reduced, resulting in more blue light entering the nucleus where NobZIP77 resides. The increased exposure of NobZIP77 to blue light unlocks its inhibiting effect on TAG-synthetic enzymes and unleashes NoDGAT2B to produce more TAGs,” ZHANG Peng explained.

“Such a concise mechanism that links light sensing to TAG synthesis was not previously known, thus is quite exciting,” said XIN Yi, an associate professor at Single-Cell Center.

Based on these findings, the team invented BLIO technology, in which the NobZIP77-removed microalga is first exposed to white light and then to blue light. This results in a level of peak productivity of TAG that is double the unmodified microalga under constant white light.

“Light quality is a highly desirable tool of control. Thus, our discovery in this study points to a new direction in feedstock development, photobioreactor design, or bioprocess control,” said XU Jian, Head of Single-Cell Center, and the senior author of the study.

The researchers believe this genetic mechanism is widely present in microalgae and higher plants and envision a future when the BLIO technology and its variants contribute to circumstances where highly efficient conversion of CO₂ to oils or other macromolecules is required.