Colin Ratledge

A comparison of the oleaginous yeast,Candida curvata, grown on different carbon sources in continuous and batch culture

The oleaginous yeast,Candida curvata D, was grown in both batch and continuous culture on 5 different carbon sources to compare the efficiency of fat production from the various substrates. Maximum lipid accumulation occurred in batch culture with xylose as the carbon source on nitrogenlimited medium reaching a level of 49% (w/w) of the biomass, but this was reduced to 37% at the optimum dilution rate (D=0.05/hr) in a chemostat. Both the highest biomass and lipid yields were attained in continuous culture with lactose as the sole carbon source at a dilution rate of D=0.04/hr, giving an efficiency of substrate conversion of 60 g of biomass and 18.6 g lipid per 100 g lactose utilized. The relative proportions of the major fatty acids (16∶0, 18∶0, 18∶1, 18∶2) in the lipid were found to vary considerably in batch culture and in continuous culture under carbon-limited conditions. However, on nitrogen-limited media in the chemostat, the fatty acid composition remained relatively constant over the whole range of dilution rates employed. Lipid from xylose-grown cells contained the greatest percentage of stearic acid (18∶0) 15% and the lowest linoleic acid (18∶2) 4%, whereas lipid from ethanol-grown cells contained elevated levels of oleic acid (18∶1) 51% and decreased palmitic acid (16∶0) 25%.

Single cell oils — have they a biotechnological future?

Although microorganisms have long been known as producers of edible oils, opportunities for their biotechnological exploitation are limited to the highest-value commodities. A recent attempt to develop a yeast oil cocoa-butter equivalent has not succeeded, not because of the inability to produce the correct formulation of fatty acids, but because of the falling price of cocoa butter on the world market. Better prospects appear to exist for producing polyunsaturated fatty acids (PUFAs) of both the n-6 and n-3 series, using either bacteria, fungi or algae. Many microbial PUFA-oils are characterized by the absence of other PUFAs, making purification of individual fatty acids an easier task than it is from other sources. Certain microorganisms may also produce prostaglandin precursors, or even prostaglandins themselves, as well as cerebroside lipids and other unusual lipids that are not normally regarded as being of microbial origin.

Yeasts, moulds, algae and bacteria as sources of lipids

Microorganisms, which range from simple bacteria to the more complex filamentous fungi and include the autotrophic, photosynthesizing organisms of cyanobacteria and algae, produce a greater range of lipid types than either plants or animals. The lipids range from simple fatty acyllipids, such as the triacylglycerols, found in the other kingdoms, to unusual polyisopropenoid ether lipids found in bacteria growing at the extremes of environmental conditions. Such lipids have been little explored for their applications but it is known that they contribute considerably to the physiology of the cells that may live at temperatures of over 80°C, at low or high pH values or in hyper-saline environments. Bacteria also produce a lipid-soluble, polyester material, poly-β-hydroxybutyrate, not usually found in higher microorganisms and which is now of considerable commercial interest as a biodegradable plastic. (This is referred to later in section 9.6.) The entire field of microbial lipids has been the topic of a recent two-volume monograph (Ratledge and Wilkinson, 1988, 1989) and details of the lipids of most organisms that have been examined are contained therein.

Production of docosahexaenoic acid by Crypthecodinium cohnii grown in a pH-auxostat culture with acetic acid as principal carbon source

Crypthecodinium cohnii, a marine alga used for the commercial production of docosahexaenoic acid (DHA), was cultivated in medium containing sodium acetate as principal carbon source; the pH was maintained at a constant value by addition of acetic acid, which also provided an additional carbon source in a controlled manner. The accumulation of lipid by C. cohnii in this pH-auxostat culture was significantly greater than previously reported for batch cultures using glucose as principal carbon source. Of six strains tested in pH-auxostat cultures, C. cohnii ATCC 30772 was the best, with the cells reaching 20 to 30 g dry weight per liter after 98 to 144 h and containing in excess of 40% (w/w) total lipid, with DHA representing approximately half of the total fatty acids in the triacylglycerol fraction. A productivity of 36 mg DHAL−1 h−1 was achieved during cultivation for 98 h using a 5% (vol/vol) inoculum, and DHA production was in excess of 3 g per liter of culture. Most of the DHA was present in neutral lipids.

Single cell oils for the 21st century.

Publisher Summary Single Cell Oils (SCOs) are defined as the edible oils obtainable from single-celled microorganisms that are primarily yeasts, fungi, and algae. This chapter provides an introductory overview to SCO and shows that current interest in their production and use has developed out of a long history of exploiting microorganisms as sources of oils and fats. Without these early endeavors, it is quite possible that none of the commercial SCO products currently on the market would have been developed as the basic understanding behind the exploitation of microbial oils would have delayed for several decades. The SCO for the 21st century deals with the quest for a Docosahexaenoic Acid-Rich SCO.