MICROBIAL LIPIDS - PRODUCTION OF LIPIDS FROM MICROBIAL SOURCES

What are microbial lipids?

Microorganisms such as bacteria, algae, yeast, etc. possess the ability for lipogenesis. These lipids are rich in unsaturated fatty acids, and can even be used as hydrocarbons. 1 The cell wall of Gram negative bacteria is a rich source of lipids.

Single Cell Oils (SCOs) are defined as those oils which are synthesized by microorganisms present at the bottom of the food chain. These lipids are synthesized by the organisms for the purpose of their own survival and metabolic activities.2

These microbial lipids are similar to lipid components generated by plants and microorganisms.2

Need for microbial lipids

Microorganisms display a high growth rate in simple media. Microbial enzymes can participate in a wide range of transformational reactions, thus producing lipids. Lipids sourced from microorganisms can be used for dietary purposes; biofuel, animal feed, etc. 2,3 They can be sourced at a much lower cost and higher yield, as well as resulting in lesser damage to the environment. The feed obtained from such microorganisms can be regarded to be safe for human consumption. 2

Microbial sources of lipids –

Microorganisms possessing the ability to synthesize lipids are known as oleaginous species. The property of oleaginicity may be defined as the presence of the enzyme ATP:citrate lyase (3:4). This enzyme helps in synthesis of Acetyl CoA for effective lipid accumulation in microorganisms.4

Under natural conditions, lipid accumulation in microorganisms depends upon genetic constituents, as well as environmental conditions such as pH, temperature, exposure to natural light, etc. The main nitrogen source for the microorganism should be depleted. Additionally, there needs to be an excess of carbon available which can then be used for lipid synthesis. During this process, cell division is temporarily halted. However, in non-oleaginous microorganisms, under conditions of nitrogen depletion, glucose is utilized to be converted to other biomolecules such as polysaccharides.5,6

The vast majority of microorganisms producing lipids are yeasts and moulds, having 20-25% of their biomass comprising of lipids. 2 However, fungal species such as Cryptococcus curvatus, Rhodosporidium, Rhodotorula, and Lipomyces can contain 60% to 70% lipids as per biomass weight. 6

Bacteria such as Arthrobacter as well as Acinetobacter calcoaceticus possess high lipid content of around 40%. Actinomycetes convert glucose to fatty acids (70%) under highly restricted conditions. 6

Algal lipids contain a high content of polyunsaturated fatty acids possessing dietary value such as gamma-linoleic acid, dihomo-gamma linoleic acid, etc. 2 Examples of oleaginous algae include Chlorella, Cylindrotheca, Tetraselmis, etc. The average lipid content in oleaginous algae ranges from 1% to 70%, and the maximum amount of lipid content is 90%. 6

Oleaginous microorganisms possess the following properties on account of which they can be used for synthesis of lipids – 7,8,9

1. These microorganisms utilize natural substrates for growth hence lower cultivation costs

2. Ability to produce a diverse range of lipids and lipid derivatives

3. These microorganisms can be subject to genetic manipulation, and the most productive microorganisms can thus be chosen

Examples of microbial lipids – 3,10,11,12

  • Microbial fatty acids – Single Cell Oils are regarded as sources of arachidonic acid and docosahexaenoic acid. Production of polyunsaturated fatty acids in most microorganisms requires the availability of the enzyme Fatty Acid Synthetase (FAS), alongwith saturases and elongases, seen in Schizochytrium. The most common fatty acid found in bacteria is cis-vaccenic acid. Cocoa butter produced by the yeast Candida curvata contains 30% stearic acid due to the presence of sterculec acid.
  • Microbial cholesterol – Sterols are found in algae, fungi, and yeasts. Hopanoids are lipid compounds present in the bacterial cell membrane. They have been shown to possess sterol-like properties. Such compounds have been found in the gram negative bacterium Methylobacterium extorquens.
  • Microbial lipid compounds – Phosphatidylglycerol is widely found in bacteria, and is accompanied by cardiolipin. Glycosyldiacylglycerols are ubiquitously present in algae and cyanobacteria. Some bacteroides contain ceramides. Mycobacterial cell wall contains lipid polymers, such as phenolic lipids, and peptidolipids.
Future Research

The current situation for synthesis of microbial lipids is not economically feasible. In order to reduce the high costs involved in production of microbial lipids, three things need to be taken into consideration – 6

1. Selection of oleaginous microorganisms

2. Viability of available products

3. Mechanisms of recombinant conversion of microorganisms

In the future, genetic engineering would be the most economically viable option to obtain specific strains of required microorganisms which produce lipids and lipid components at lower costs.

References
1. Liang M-H, Jiang J-G. Advancing oleaginous microorganisms to produce lipid via metabolic engineering technology. Progress in Lipid Research. 2013;52(4):395-408. doi:10.1016/j.plipres.2013.05.002
2. Rao G. Single Cell Oil Production. Available from http://www.slideshare.net/guruict/single-cell-oil-production (Accessed on June 7, 2016)
3. Pujari S. Why Micro-Organisms are considered as Promising Lipid Sources? Available from http://www.yourarticlelibrary.com/micro-biology/why-microorganisms-are-considered-as-promising-lipid-sources/33700/ (Accessed on June 7, 2016)
4. Ratledge C. Microbial Oils and Fats: An Overview. In: Ed. Ratledge C, Dawson PSS, Rattray J. Biotechnology for the Oils and Fats Industry. American Oil Chemists' Society, 1984. p:119-128
5. Lemmer KC, Dohnalkova AC, Noguera DR, Donohue TJ. Oxygen-Dependent Regulation of Bacterial Lipid Production. O'Toole GA, ed. Journal of Bacteriology. 2015;197(9):1649-1658. doi:10.1128/JB.02510-14
6. Uprety BK. Biodiesel production from oleaginous microorganisms. Available from http://www.slideshare.net/bijayauprety/assig-2-edited (Accessed on June 7, 2016)
7. Schulze I, Hansen S, GroBhans S, et al. Characterization of newly isolated oleaginous yeasts - Cryptococcus podzolicus, Trichosporon porosum, and Pichia segobiensis. AMB Express. 2014;4:24. doi:10.1186/s13568-014-0024-0
8. Wang G, Xiong X, Ghoghare R, et al. Exploring fatty-alcohol producing capability of Yarrowia lipolytica. Biotechnology for Biofuels. 2016;9(107):1-10. doi: 10.1186/s13068-016-0512-3
9. Lin H, Wang Q, Shen Q, Zhan J, Zhao Y. Genetic engineering of microorganisms for biodiesel production. Bioengineered. 2013;4(5):292-304. doi:10.4161/bioe.23114
10. Christie WW. Lipid Compositions in Plants and Microorganisms. Available from http://lipidlibrary.aocs.org/Primer/content.cfm?ItemNumber=39308 (Accessed on June 8, 2016)
11. Monroig O, Tocher DR, Navarro JC. Biosynthesis of Polyunsaturated Fatty Acids in Marine Invertebrates: Recent Advances in Molecular Mechanisms. Marine Drugs. 2013;11(10):3998-4018. doi:10.3390/md11103998
12. Saenz JP, Grosser D, Bradley AS, et al. Hopanoids as functional analogues of cholesterol in bacterial membranes. Proceedings of the National Academy of Sciences of the United States of America. 2015;112(38):11971-11976. doi:10.1073/pnas.1515607112


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I am a post-graduate in Biochemistry from the University of Mumbai