Newly Discovered Sexual Reproduction Process in Microalgae

by Research Organization of Information and Systems

Scientists at the National Institute of Genetics (NIG), in Japan, have discovered a sexual reproduction process in microalgae that helps them better understand algae and plant evolution. Their discovery could lead to new industrial applications for microalgae, ranging from wastewater treatment to production of food ingredients and pigments.

The research team published their findings in the Proceedings of the National Academy of Sciences the week of October 3, 2022.

The scientists conducted their studies on Galdieria, a unicellular species of red microalgae found in sulfuric acid hot springs around the world and expected to be an important nutrition source for humans. In recent years, researchers and businesses have been working to develop the quick-growing Galdieria as an industrial source of vitamins and pigments.

Galdiera, a super alga

Galdieria is quite versatile, capable of making its own energy through photosynthesis, using light and carbon dioxide. It uses a mix of different sources of energy and carbon and obtains its nutrients by consuming extracellular sugars or sugar alcohols. Galdieria is also able to tolerate higher levels of salt and heavy metals than many other microalgae. Yet their genome size is very small.

Because of Galdieria’s versatility, scientists consider it to be an emerging system for biotechnology applications. However, because it is surrounded by a thick and rigid cell wall, it requires energy-intensive physical processing to extract its cellular contents. The cell wall also hampers efforts at genetic modification. 

During their research, the team learned that the known cell-walled form of Galdieria is a diploid, a type of cell that contains two complete sets of chromosomes. However, when these diploids are exposed to a specific environment, a cell wall-less haploid is produced and the wall-less haploid only contains one set of chromosomes. 

To further their research, the team succeeded in stably propagating the cell wall-less haploid and in converting the haploid back to a diploid. “Thus, we discovered a sexual reproduction process in microalgae that emerged early in algal and plant evolution,” said Shunsuke Hirooka, a project assistant professor with the NIG.

Blue-colored algae

“We successfully developed a genetic modification technique for Galderia by using the cell-wall-less haploid. The procedure enables generating ‘self-cloning’ lines which do not contain any heterologous DNA sequence for industrial use. For example, we have succeeded in generating blue-colored algae (blue is the color of phycocyanin, which is used as a natural blue colorant in certain food products), which does not exist naturally,” said Shin-ya Miyagishima, a professor with the NIG.

The team’s findings are significant because although sexual reproduction is also found in some unicellular algae, the ancestors of plants, sexual reproduction has never been found in many unicellular algae that emerged early in evolution. “These unicellular algae have been assumed to proliferate only by cell division, or asexual reproduction, and the origin and evolutionary process of sexual reproduction in algae and plants have been unclear,” said Dr. Hirooka. 

Looking ahead, the researchers believe their findings will help further clarify the evolutionary process and origin of sexual reproduction in algae and plants. The team’s discoveries hold the potential to unlocking exciting future uses for the microalgae Galdieria.

“The haploid discovered in this study has the same growth potential as the diploid, and its contents can be easily extracted and genetically modified, which facilitates industrial use of Galdieria and is expected to create advanced forms of microalgae utilization, such as vaccine-containing feed using algae expressing viral proteins as antigens,” said Dr. Miyagishima.