Microalgae are promising biodiesel producers due to their high growth rate, high CO₂ fixation efficiency, high environmental stress tolerance, and the capacity to accumulate a large amount of lipid without competing for arable land and potable water. However, the commercial production of microalgal biodiesel currently faces the serious challenges of low lipid productivity and high production costs. Enhancing the economic feasibility of microalgal biodiesel will require critical engineering innovations in large-scale cultivation and breakthroughs for regulating lipid metabolism.

—Ho et al.

Focusing on marine microalgae, the researchers found that Chlamydomonas sp. JSC4, a new species of green alga harvested from brackish water, combines a high growth rate with high levels of lipids. The research team developed an analysis method called “dynamic metabolic profiling” and used this to analyze JSC4 and discover how this species produces oil within its cells.

The team incubated JSC4 with carbon dioxide as the sole carbon source. Four days after the start of incubation, more than 55% of cell weight consisted of carbohydrates (mainly starch). When saltwater comprised 1-2% of the incubation liquid, the team saw a decrease in carbohydrates and increase in oil, and 7 days after the start of incubation more than 45% of cell weight had become oil.

JSC4 has a high cell growth rate, and the lipid production rate in the culture solution achieved a speed that greatly surpassed previous experiments. At the start of the cultivation period starch particles were observed in the cells, but in saltwater these particles vanish and numerous oil droplets are seen.

Using dynamic metabolic profiling, the group found that the sugar biosynthesis pathway (activated when starch is produced) slows down, and the pathway is activated for synthesizing triacylglycerol, a constituent element of oil. In other words, the addition of seawater switched the pathway from starch to oil production. They also clarified that the activation of an enzyme that breaks down starch is increased in saltwater solution.

The discovery of this metabolic mechanism is not only an important biological finding, it could also be used to increase the production of biofuel by improving methods of algae cultivation. Based on these findings, the team will continue looking for ways to increase sustainable oil production by developing more efficient cultivation methods and through genetic engineering.

Resources

• Shih-Hsin Ho, Akihito Nakanishi, Yuichi Kato, Hiroaki Yamasaki, Jo-Shu Chang, Naomi Misawa, Yuu Hirose, Jun Minagawa, Tomohisa Hasunuma, Akihiko Kondo (2017) “Dynamic metabolic profiling together with transcription analysis reveals salinity-induced starch-to-lipid biosynthesis in alga Chlamydomonas sp. JSC4” Scientific Reports 7, Article number: 45471 doi: 10.1038/srep45471