Up to now, algae has provided invaluable nutrition in the health food sector but has struggled to be competitive against petroleum-derived chemical production. Algae is favorable to petroleum from an environmental standpoint but the production cost of culturing, collecting, extracting and refining adds up to make it too expensive for practical use. Thus, there is a need to improve production efficiency to reduce the cost of algae derived products in order for them to be a viable alternative to petroleum-derived products.

Now, a research team led by Alice Uchida and Masaki Ihara of Shinshu University has developed a method of cultivating microalgae that solves the issues of collection/recovery of compounds and extraction/purification of products.

• First, it was necessary not to kill the algal cells during extraction. By preserving the algae, there is no need to cultivate and multiply the algae.

• Second, the algae they chose (Tolypothrix sp. PCC7601) naturally gather together for ease of collection.

• Third, the compounds wanted for harvest—polysaccharides (carbohydrates) and phycobiliproteins—are released outside of the algae (extracellular) and bound to the cell surface. There is no need for a solvent for extraction or purification, significantly simplifying and decreasing the cost of processing.

Extracellular carbohydrate milking using a glass homogenizer. (A) Harvesting of Tolypothrix cells, (B) glass homogenizer treatment, (C) filtration of the homogenate to separate cells from the supernatant, (D) supernatant containing the extracellular matrix, and (E) alcohol precipitation of extracellular carbohydrate from the supernatant. Uchida et al.

One of the challenges in this field is to lower the cultivation and harvest costs and to achieve the steady productivity. To address this, new systems have been proposed, in which the products are extracted without killing the algal cells. These non-destructive extraction systems are called “milking.”

Some of the milking systems reported so far are continuous processes where culturing and milking occur simultaneously, and the others are a periodic process where cells are cyclically cultured and milked. These systems are based on the organic solvent extraction of non-polar products, such as lipids, terpenes, and carotenoids. However, a special facility required for handling organic solvents increases the costs and solvents lost during milking need to be recouped.

In this study, we examined a solvent-free method, based on mechanical milking using a shearing disperser (glass homogenizer). We cultured the N₂-fixing filamentous cyanobacteria, Tolypothrix sp. PCC7601 in non-sterile agricultural water and performed long-term (87 days) milking cycles to harvest the extracellular carbohydrates and phycobiliproteins. As a result, the productivity of extracellular carbohydrates and the cell densities remained constant throughout the milking cycle, yielding 90–140 mg/L of extracellular carbohydrates every 3 weeks. Our results demonstrated that mechanical milking is a practical and effective method that can be used to harvest products from algae steadily.

—Uchida et al.

In the beginning of the study, the researchers struggled to find a type of algae that could withstand mechanical shearing. They were not sure such an algae existed. However, after an extensive search, they were able to find the Tolypothrix filamentous cyanobacteria and were able to cultivate it continuously for 2 years with little cell damage despite mechanical shearing of the compounds bound to the cell surface.

The Ihara lab hopes to enable petroleum-based products to be replaced by algae-derived products that inflict less strain on the environment. In order to do so, algae production needs to happen on a much, much larger scale. He continues to look for tough algae that can survive in a variety of environments.

He hopes to be able to collaborate with researchers from a variety of fields including fermentation engineering, chemical engineering, polymer chemistry- specifically algal biomass conversion technology, environmental and forest conservation studies in order to study the effects of large-scale algae culture on the environment.

This research was supported by Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI) Grant Number JP17K07717.

Resources

• Alice Uchida, Yukiko Higashi, Shota Yamamoto, Jun Nakanishi, Naoki Kanayama, Kazuhiro Shibata, Masaki Ihara (2020) “Production of extracellular polysaccharides and phycobiliproteins from Tolypothrix sp. PCC7601 using mechanical milking systems,” Algal Research, Volume 48, 101929 doi: 10.1016/j.algal.2020.101929