Biotechnological production and applications of the omega-3 polyunsaturated fatty acid docosahexaenoic acid.

Docosahexaenoic acid (DHA) is a polyunsaturated fatty acid composed of 22 carbon atoms and six double bonds. Because the first double bond, as counted from the methyl terminus, is at position three, DHA belongs to the so-called omega-3 group. In recent years, DHA has attracted much attention because of its beneficial effect on human health. At present, fish oil is the major source of DHA, but alternatively it may be produced by use of microorganisms. Marine microorganisms may contain large quantities of DHA and are considered a potential source of this important fatty acid. Some of these organisms can be grown heterotrophically on organic substrates without light. These processes can be well controlled and DHA with constant quality can be produced all year round. This paper reviews recent advances in the biotechnological production of DHA by marine microorganisms.

Fed-batch cultivation of the docosahexaenoic-acid-producing marine alga Crypthecodinium cohnii on ethanol.

The heterotrophic marine microalga Crypthecodinium cohnii produces docosahexaenoic acid (DHA), a polyunsaturated fatty acid with food and pharmaceutical applications. So far, DHA production has been studied with glucose and acetic acid as carbon sources. This study investigates the potential of ethanol as an alternative carbon source for DHA production by C. cohnii. In shake-flask cultures, the alga was able to grow on ethanol. The specific growth rate was optimal with 5 g l(-1) ethanol and growth did not occur at 0 g l(-1) and above 15 g l(-1). By contrast, in fed-batch cultivations with a controlled feed of pure ethanol, cumulative ethanol addition could be much higher than 15 g l(-1), thus enabling a high final cell density and DHA production. In a representative fed-batch cultivation of C. cohnii with pure ethanol as feed, 83 g dry biomass l(-1), 35 g total lipid l(-1) and 11.7 g DHA l(-1) were produced in 220 h. The overall volumetric productivity of DHA was 53 mg l(-1 )h(-1), which is the highest value reported so far for this alga.

Characterisation of extracellular polysaccharides produced by Crypthecodinium cohnii.

The valuable polyunsaturated fatty acid, docosahexaenoic acid, can be produced by cultivation of the heterotrophic microalga, Crypthecodinium cohnii. During batch growth of C. cohnii on glucose, sea salt and yeast extract for 5 days, so far unreported extracellular polysaccharides were produced. These caused an increased viscosity and a strong drop in the maximum oxygen transfer. The viscosity increased most markedly as cells entered the stationary phase. The polysaccharides varied in size (from 6 kDa to >1,660 kDa) and monomer distribution. A high molecular mass fraction (from 100 kDa to >1,660 kDa) and a medium molecular mass fraction (6-48 kDa) were prepared. The high molecular mass fraction contained (on a molar basis) 71.7% glucose, 13.1% galactose and 3.8% mannose, whereas the medium molecular mass fraction contained 37.7% glucose, 19.8% galactose and 28.1% mannose. Other monomers present in both fractions were fucose, uronic acid and xylose. Monomers were coupled mainly via alpha-(1-3) links. Increased viscosity due to polysaccharide production complicates the development of commercial, high cell-density processes for the production of docosahexaenoic acid.

The specificity of glycolipid-preferredoxin interaction: requirements for membrane binding.

Preferredoxin (prefd) is a precursor protein that is imported into chloroplasts. Monolayer experiments have shown that prefd has a high affinity for monogalactosyldiglyceride (MGaIDG) isolated from chloroplasts, which contains polyunsaturated fatty acid constituents and is therefore in a liquid-expanded state, but has been found to interact also with MGaIDG with long-chain saturated fatty acids, which exist in a gel state. For an optimal interaction, the fatty acid chain length and the extent of unsaturation are also important parameters, whereas the conformation of the sugar moiety, the sugar-glycerol or glycerol-hydrocarbon chain linkages are of little influence on the pressure changes measured in monomolecular layers. Conversely, steric hindrance of a methyl group at position 3 of the sugar largely inhibits the interaction. Quantification of the interaction with radiolabelled prefd shows that only a small part of the molecule is able to penetrate MGaIDG in the gel state, whereas a nearly four-times larger part is able to penetrate MGaIDG isolated from chloroplasts. It is likely that interactions of the transit sequence of prefd with the glycolytic head group MGaIDG are involved in targeting and binding to the chloroplast membrane.