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Progress of arachidonic acid biosynthesis in microorganisms

Single cell oils (SCO), produced by and extracted from some single-celled microorganisms, are featured with high levels of the major very long chain polyunsaturated fatty acids (PUFA). SCO are essential for health and potential sources of bio-diesel. There has been a long history of interest in the exploitation of microorganisms as oil and fat providers, due to the continuing diminution of arable land and climate change making animal and plant sourced oils even more limited. However, such endeavor has been progressing hardly, mostly because the efficiencies of oil synthesis by microorganisms are normally too low to be applied in industry at a reasonable price.

The article "Ion-beam-mutation breeding of an arachidonic acid biosynthesis microorganism and its industrial fermentation control" , by Z. L. Yu and Q. Huang et al., from the Hefei Institutes of Physical Science, Chinese Academy of Sciences, published in Chinese Science Bulletin (2012) 57: 883, has started to shed light on this issue. Cells of the oil-producing fungus Mortierella alpine were genetically modified by their patented technology, known as ion beam biotechnology, and screened using procedures tailored for oil yield. They obtained one strain: 50% of its biomass was fatty acids, of which 70% is arachidonic acid.

In their report "Fats and oils in human nutrition" released in 1993, the FAO and WHO suggested that diet for infants, especially preterm infants and infants with insufficient breast-feeding, should be supplemented with adequate amounts of fatty acids with composition corresponding to fatty acids contained in breast-milk. AA and docosahexaenoic acid (DHA) are particularly important for brain development but only found in breast milk naturally. Therefore it has become a long-term focus developing AA-rich oil in microorganisms.

Over two decades ago, the authors started the project by implanting energetic ions, generated by an ion source and accelerated to certain electrical potentials, into cells of Mortierella alpine, the fungus naturally synthesizing PUFA. Implanted ions resulted in genetic mutations. They firstly screened for mutant cells producing high levels of total fatty acids, presumably resulting from mutations in genes controlling palmitic acid synthesis from acetyl CoA. In the second stage, established mutant strains from the first stage were mutated again by ion implantation and strains accumulating AA, among various kinds of fatty acids, were screened for. Such two-stage protocol was repeated until a genetically stable, AA-rich strain was obtained. Its AA yield was nearly 20 times of that of the original non-mutated strain, showing prospect of industrial application. The strain was transferred to manufacturer and a production line was thereafter constructed in Wuhan, by which fermentation control study was initiated.

Fermentation control normally involves optimization of culture media and growth condition, to reach a comprehensive and balanced nutrition supply for cell metabolism. To be used as diet supplement for infants, safety is the most critical issue therefore a simple media formulation guaranteeing minimum substrate residue was desired. Researchers tested the possibility of recycling the fungi residue after oil extraction into culture media. The "waste utilization" simplified culture media from 7 ingredients to just 2, glucose and the fungi residue, therefore reduced manufacturing costs. Data from 11 batches of fermentation in 200 m3 reactor with the minimum substrates showed that, the new strain reached an average biomass of 35.8 g/L (dry fungi cells/fermentation liquid), oil and AA contents of 18.4 g/L and 8.97 g/L respectively. This result attracted attention from more researchers.

Z. Cohen and C. Ratledge commented in their compilation "Single Cell Oils" that, "Alternative microbial sources of AA are also being sought. Already it is known there is a process for AA production in China, operated by Wuhan Alking Bioengineering Co. Ltd, using a new strain of Mortierella alpine. This process appears to operate at the 50-100 ton level."

Fatty acids from most microbial sourced oils have a similar composition with plant oils. Developing microbial oils is a promising direction for both bio-diesel industry and biological economy. The work and its related technologies reported here, such as cell modification by ion beams and recycling of fungi residue as substrate, not only developed new AA resource, but also offered new thoughts and paved a novel route for future exploration of microbial oil industry. This study has been supported by the 8th, the 9th and the 10th Five-Year National Science and Technology Plans (Projects 85-722-22-01和2001BA302B-04).

Contact: Feng Huiyun
Science in China Press

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