Estimated metabolic changes induced by astaxanthin production. Credit: Kobe University

Aresearch group led by Professor Hasunuma Tomohisa of Kobe University’s Engineering Biology Research Center have succeeded in synthesizing the natural pigment astaxanthin using the fast-growing marine cyanobacterium Synechococcus sp. PCC7002.

This process required light, water and CO2 to produce the valuable antioxidant astaxanthin from the cyanobacterium host at a faster rate and with lower contamination risks than previous methods of biologically synthesizing this useful substance. In addition, dynamic metabolic analysis revealed that astaxanthin production enhances the central metabolism of Synechococcus sp. PCC7002.

The results of this study were first published in the international journal “ACS Synthetic Biology” on October 25, 2019.

Astaxanthin (pink carotenoid) is the strongest antioxidant among known carotenoids. It is used as a natural coloring in aquaculture, cosmetic, nutrition and pharmaceutical industries among others, due to its enhancement of immune responses and anti-inflammatory properties.

Currently, the majority of commercial astaxanthin is chemically synthesized from petrochemicals. This enables large amounts to be produced in order to meet demand. However, there are concerns about the safety of consuming astaxanthin synthesized from petrochemicals, and as a result the demand for natural astaxanthin is increasing.

The freshwater alga Haematococcus pluvialis produces astaxanthin naturally. For commercial astaxanthin production, Haematococcus requires a complex 2-stage process. After the first growth stage, Haematococcus is placed under inductive stress conditions such as nitrogen starvation or high light irradiation. This induces the plant to form hematocysts and produce astaxanthin in the second stage.

The current study sped up the growth rate and reduced the contamination risks in biosynthesizing astaxanthin. The researchers succeeded in producing astaxanthin using the fast-growing marine blue-green algae species, or cyanobacterium, Synechococcus sp. PCC7002 as a host. This algae does not inherently produce astaxanthin, however by integrating β -carotene encoding genes into the Synechococcus, the expressed genes only require water, light and CO2 in order to produce astaxanthin. This single stage method does not require subjecting the cells to stress conditions and enabled astaxanthin to be produced in a shorter time period compared to the Haematococcus method. In addition, it is proposed that the rich salt concentration in Synechococcus could also lower the risk of contamination.

Overall, this study showed that the modified Synechococcus sp. PCC7002 is a promising host for producing astaxanthin biologically through photosynthesis. This could be investigated further by trying to synthesize various other useful substances utilizing Synechococcus sp. PCC7002.

In addition, it is hoped that the dynamic metabolic profiling method developed during this research could be utilized to improve understanding of metabolic processes in microorganisms, plants and animals.