Single cell oils--have they a biotechnological future?

Filamentous fungi for production of food additives and processing aids

Filamentous fungi are metabolically versatile organisms with a very wide distribution in nature. They exist in association with other species, e.g. as lichens or mycorrhiza, as pathogens of animals and plants or as free-living species. Many are regarded as nature's primary degraders because they secrete a wide variety of hydrolytic enzymes that degrade waste organic materials. Many species produce secondary metabolites such as polyketides or peptides and an increasing range of fungal species is exploited commercially as sources of enzymes and metabolites for food or pharmaceutical applications. The recent availability of fungal genome sequences has provided a major opportunity to explore and further exploit fungi as sources of enzymes and metabolites. In this review chapter we focus on the use of fungi in the production of food additives but take a largely pre-genomic, albeit a mainly molecular, view of the topic.

Biodiesel from microalgae

Continued use of petroleum sourced fuels is now widely recognized as unsustainable because of depleting supplies and the contribution of these fuels to the accumulation of carbon dioxide in the environment. Renewable, carbon neutral, transport fuels are necessary for environmental and economic sustainability. Biodiesel derived from oil crops is a potential renewable and carbon neutral alternative to petroleum fuels. Unfortunately, biodiesel from oil crops, waste cooking oil and animal fat cannot realistically satisfy even a small fraction of the existing demand for transport fuels. As demonstrated here, microalgae appear to be the only source of renewable biodiesel that is capable of meeting the global demand for transport fuels. Like plants, microalgae use sunlight to produce oils but they do so more efficiently than crop plants. Oil productivity of many microalgae greatly exceeds the oil productivity of the best producing oil crops. Approaches for making microalgal biodiesel economically competitive with petrodiesel are discussed.

Acclimation effect on fatty acids of the coral Montipora digitata and its symbiotic algae

Lipids play a key role in thermal and photo-acclimation processes, yet they are often neglected in stress studies. We investigated the influence of different light intensities and an increase of temperature on the fatty acid composition of the coral Montipora digitata and its symbiotic algae (i.e., zooxanthellae). Coral branches were subjected to 3 different light intensities (7, 30 and 95% sea surface photosynthetic active radiation) in filtered seawater for 35 days. Fatty acids as methyl esters were determined using gas chromatography (GC) and verified by GC-mass spectrometry. Different light intensities, but only in combination with increased temperature, significantly affected the fatty acid composition of the coral host and zooxanthellae. Temperature and light intensity increases caused reductions in the proportion of polyunsaturated fatty acids in both the host and symbionts. Most changes occurred in the host coral, which suggests that the host is more susceptible to environmental change than the symbiont, or that the host shields the symbionts from environmental change.

Fatty acids from lipids of marine organisms: molecular biodiversity, roles as biomarkers, biologically active compounds, and economical aspects

Because of their characteristic living environments, marine organisms produce a variety of lipids. Fatty acids constitute the essential part of triglycerides and wax esters, which are the major components of fats and oils. Nevertheless, phospholipids and glycolipids have considerable importance and will be taken into account, especially the latter compounds that excite increasing interest regarding their promising biological activities. Thus, in addition to the major polyunsaturated fatty acids (PUFA) such as eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids, a great number of various fatty acids occur in marine organisms, e.g. saturated, mono- and diunsaturated, branched, halogenated, hydroxylated, methoxylated, non-methylene-interrupted. Various unprecedented chemical structures of fatty acids, and lipid-containing fatty acids, have recently been discovered, especially from the most primitive animals such as sponges and gorgonians. This review of marine lipidology deals with recent advances in the field of fatty acids since the end of the 1990s. Different approaches will be followed, mainly developing biomarkers of trophic chains in marine ecosystems and of chemotaxonomic interest, reporting new structures, especially those with biological activities or biosynthetic interest. An important part of this review will be devoted to the major PUFA, their relevance to health and nutrition, their biosynthesis, their sources (usual and promising) and market.

Plants as 'chemical factories' for the production of polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFAs) are valuable products because of their involvement in several aspects of human health. Market demand for most PUFAs is growing continually and current sources are considered insufficient for satisfying this demand; alternative sources are actively sought after. Oilseed plants can be a potential source of PUFAs if they are appropriately gene engineered. Most of the basic tools for genetic engineering of oilseed plants for giving them the ability to produce PUFAs are already developed. Here we review the prospects of genetic engineering of oilseed plants for producing some valuable long-chain polyunsaturated fatty acids. Genetic transformation for GLA production seems to be a near-term possibility, but gene engineering seems considerably more difficult for the other long-chain PUFAs. Nevertheless, with the current rapid pace of biotechnological advancement, the remaining difficulties may be surmounted in the near future.

Identification and characterization of a novel Δ6 -fatty acid desaturase gene from Rhizopus arrhizus

A cDNA sequence putatively encoding a Delta(6)-fatty acid desaturase was isolated from Rhizopus arrhizus using reverse transcription polymerase chain reaction and rapid amplification of cDNA ends methods. Sequence analysis indicated that this cDNA sequence had an open reading frame of 1377 bp encoding 458 amino acids of 52 kDa. The deduced amino acid sequence showed high similarity to those of fungal Delta(6)-fatty acid desaturases which comprised the characteristics of membrane-bound desaturases, including three conserved histidine-rich motifs and hydropathy profile. A cytochrome b(5)-like domain was observed at the N-terminus. To elucidate the function of this novel putative desaturase, the coding sequence was expressed heterologously in Saccharomyces cerevisiae strain INVScl. The result demonstrated that the coding product of the sequence exhibited Delta(6)-fatty acid desaturase activity by the accumulation of gamma-linolenic acid.

Heterotrophic production of eicosapentaenoic acid by microalgae

Eicosapentaenoic acid (EPA) is an omega-3 polyunsaturated fatty acid that plays an important role in the regulation of biological functions and prevention and treatment of a number of human diseases such as heart and inflammatory diseases. As fish oil fails to meet the increasing demand for purified EPA, alternative sources are being sought. Microalgae contain large quantities of high-quality EPA and they are considered a potential source of this important fatty acid. Some microalgae can be grown heterotrophically on cheap organic substrate without light. This mode of cultivation can be well controlled and provides the possibility to maximize EPA production on a large scale. Numerous strategies have been investigated for commercial production of EPA by microalgae. These include screening of high EPA-yielding microalgal strains, improvement of strains by genetic manipulation, optimization of culture conditions, and development of efficient cultivation systems. This paper reviews recent advances in heterotrophic production of EPA by microalgae with an emphasis on the use of diatoms as producing organisms.

Production of gamma-linolenic acid by disrupted mycelia of Mortierella isabellina

AIMS

To optimize the production of linolenic acid by disrupted mycelia of Mortierella isabellina.

METHODS AND RESULTS

Effects of incubation conditions such as incubation time, pH of reaction mixture, concentration of Mg2+ or malate and incubation temperature on production of linolenic acid were studied. The production of gamma-linolenic acid reached 224 mg g-1 dry cells when the reaction mixture was composed of 1.0 g (dry mycelial mass) of disrupted mycelia of M. isabellina, 50 ml (50 mmol l(-1)) potassium phosphate buffer supplemented with 0.312 mmol l(-1) of Mg2+ and 10 mmol l(-1) of malate, pH 7.0 and incubated at 5 degrees C for 1 day.

CONCLUSIONS

Incubation temperature, concentration of Mg2+ and malate showed major effects on the increased linolenic acid production.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights conditions for increasing gamma-linolenic acid production by cell-free mycelia of M. isabellina and an insight into rapidly gaining high production of polyunsaturated fatty acids.

A third fatty acid delta9-desaturase from Mortierella alpina with a different substrate specificity to ole1p and ole2p

A third gene (Delta9-3) encoding a fatty acid Delta9-desaturase was isolated from the oil-producing fungus Mortierella alpina. The predicted protein of 512 aa shared 53% sequence identity with the two fatty acid Delta9-desaturases, ole1p and ole2p, already described in this organism and contained three histidine boxes, four putative transmembrane domains and a C-terminal cytochrome b(5) fusion that are typical of most fungal membrane-bound fatty acid desaturases. However, unlike the M. alpina ole1 and ole2 genes, the Delta9-3 ORF failed to complement the Saccharomyces cerevisiae ole1 mutation. GC-MS analysis of fatty-acid-supplemented ole1 yeast transformants containing the Delta9-3 gene indicated that this enzyme had negligible activity with endogenous palmitic acid (16:0) as substrate and moderate activity (30-65% desaturation) with endogenous stearic acid (18:0). Yeast transformants overexpressing any one of the three M. alpina fatty acid Delta9-desaturase genes or the S. cerevisiae OLE1 gene produced low amounts of hexacosenoic acid [26:1(n-9)], a fatty acid that is not normally present in yeast cells. It follows that these Delta9-desaturases may also display low n-9 desaturation activity with very long-chain saturated fatty acid substrates. Conversely, high levels of desaturase in the endoplasmic reticulum membrane of these yeast transformants may increase the availability of suitable monounsaturated substrates for fatty acid elongation.

Production of gamma-linolenic acid by Mortierella isabellina grown on hexadecanol

AIMS

To optimize the production of linolenic acid by Mortierella isabellina grown on hexadecanol.

METHODS AND RESULTS

Effects of culture conditions such as culture time, pH of medium, hexadecanol concentration, incubation temperature and ageing of mycelia on production of linolenic acid were studied. The production of gamma-linolenic acid reached 2.44 mg ml-1 (271 mg g-1 dry cells) when Mortierella isabellina was cultivated in a medium consisting of 2% hexadecanol and 1% yeast extract at 23 degrees C for 120 h and then the mycelia, after removal of medium by suction filtration, were allowed to stand for a further 15 d at 5 degrees C.

CONCLUSION

Ageing of mycelia and incubation temperature showed predominant effects on the increased linolenic acid production.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights effective conditions for increasing linolenic acid production by Mortierella isabellina grown on hexadecanol.

gamma-Linolenic acid production by solid-state fermentation of Mucorales strains on cereals

Oleaginous fungi of the genus Mucorales were screened for gamma-linolenic acid (GLA) production on solid substrates containing moistened cereals. Cunninghamella elegans CCF 1318 produced the highest yields of GLA when cultivated on barley. Substrate moisture and cultivation temperature proved critical for effective GLA production. Vegetable oil supplied to the cultures improved GLA production. Rotating bottles and plastic bags were used as cultivation vessels to reproduce the conditions found in rotating drums and tray bioreactors, respectively. After 11 days of cultivation at 21 degrees C, C. elegans produced 14.2 mg of GLA per gram of dry substrate, composed of a mixture of barley, spent malt grains (SMG) and peanut oil. GLA represented 15.6% of the total fatty acids in the lipid extract.

Evaluation of extraction methods for recovery of fatty acids from lipid-producing microheterotrophs

The effect of different extraction techniques on the recovery of fatty acids from freeze-dried biomass of two lipid-producing microheterotrophs was examined. Two procedures were used: the extraction of lipids from biomass followed by transesterification of the fatty acids (extraction-transesterification); and the direct transesterification of biomass to produce fatty acid methyl esters (i.e. without the initial extraction step). Variable factors in the extraction-transesterification experiment were the sequence in which solvents were added to the samples, the relative amount of methanol in the solvent mix, and sonication of biomass while in the solvent mix. Variable factors in the direct transesterification experiment were sample size, and reaction duration. Statistical analysis of data (level of significance P<0.05) showed that: (1) extraction of total fatty acids prior to transesterification was significantly more efficient when solvents were added in the order of increasing polarity; (2) neither sonication nor increasing the proportion of methanol in the extraction solvent significantly affected extraction of fatty acids prior to transesterification; (3) efficiency of direct transesterification of fatty acids increased significantly with reaction time; (4) efficiency of direct transesterification of fatty acids was not significantly affected by sample size; (5) the most efficient method for extraction of fatty acids prior to transesterification yielded significantly less fatty acids than the most effective direct transesterification method. While the study examined only two strains, our results suggest that fatty acid analysis methodology for microheterotrophs under consideration for biotechnological exploitation requires optimisation and validation.

delta(6)-desaturase of Mucor rouxii with high similarity to plant delta(6)-desaturase and its heterologous expression in Saccharomyces cerevisiae

Gamma-linolenic acid (GLA, gamma-C18:3) is an essential fatty acid that plays a vital role in biological structures and cellular functions. Based on available sequence information and using polymerase chain reaction (PCR) technique, we cloned from the fungus Mucor rouxii the entire coding sequence of a delta(6)-desaturase enzyme, which is responsible for the transformation of linoleic acid into GLA. The deduced amino acid sequence of M. rouxii gene showed the highest homology with the plant delta(6)-desaturase. It comprises the characteristics of membrane-bound desaturases, including histidine-rich motifs and hydrophobic regions. A cytochrome b(5)-like domain was observed at the N-terminus. In addition to three conserved histidine-rich motifs, we found an additional histidine-rich motif, HKHHSH, downstream of the cytochrome b(5)-like domain, which is not present in previously cloned delta(6)-desaturase genes. Heterologous expression of the M. rouxii cDNA in Saccharomyces cerevisiae resulted in the synthesis and accumulation of GLA.

Isolation and use of a homologous histone H4 promoter and a ribosomal DNA region in a transformation vector for the oil-producing fungus Mortierella alpina

Mortierella alpina was transformed successfully to hygromycin B resistance by using a homologous histone H4 promoter to drive gene expression and a homologous ribosomal DNA region to promote chromosomal integration. This is the first description of transformation in this commercially important oleaginous organism. Two pairs of histone H3 and H4 genes were isolated from this fungus. Each pair consisted of one histone H3 gene and one histone H4 gene, transcribed divergently from an intergenic promoter region. The pairs of encoded histone H3 or H4 proteins were identical in amino acid sequence. At the DNA level, each histone H3 or H4 open reading frame showed 97 to 99% identity to its counterpart but the noncoding regions had little sequence identity. Unlike the histone genes from other filamentous fungi, all four M. alpina genes lacked introns. During normal vegetative growth, transcripts from the two histone H4 genes were produced at approximately the same level, indicating that either histone H4 promoter could be used in transformation vectors. The generation of stable, hygromycin B-resistant transformants required the incorporation of a homologous ribosomal DNA region into the transformation vector to promote chromosomal integration.

Two fatty acid delta9-desaturase genes, ole1 and ole2, from Mortierella alpina complement the yeast ole1 mutation

Genes encoding two distinct fatty acid delta9-desaturases were isolated from strains of the oleaginous fungus Mortierella alpina. Two genomic sequences, delta9-1 and delta9-2, each containing a single intron, were cloned from strain CBS 528.72 while one cDNA clone, LM9, was isolated from strain CBS 210.32. The delta9-1 gene encoded a protein of 445 aa which shared 99% identity with the LM9 gene product. These proteins also showed 40-60% identity to the delta9-desaturases (Ole1p) of other fungi and contained the three conserved histidine boxes, C-terminal cytochrome b5 fusion and transmembrane domains characteristic of endoplasmic reticulum membrane-bound delta9-desaturases. LM9 and delta9-1 are therefore considered to represent the same gene (ole1). The ole1 gene was transcriptionally active in all M. alpina strains tested and its function was confirmed by complementation of the Saccharomyces cerevisiae ole1 mutation. Fatty acid analysis of yeast transformants expressing the CBS 210.32 ole1 gene showed an elevated level of oleic acid (18:1) compared to palmitoleic acid (16:1), the major fatty acid component of wild-type S. cerevisiae. This indicated that the M. alpina delta9-desaturase had a substrate preference for stearic acid (18:0) rather than palmitic acid (16:0). Genomic clone delta9-2 (ole2) also encoded a protein of 445 aa which had 86% identity to the delta9-1 and LM9 proteins and whose ORF also complemented the yeast ole1 mutation. The transcript from this gene could only be detected in one of the six M. alpina strains tested, suggesting that its expression may be strain-specific or induced under certain physiological conditions.

Enhancement of gamma-linolenic acid production by the fungus Mucor sp LB-54 by growth temperature

As a relatively prolific producer of GLA, the strain of Mucor sp LB-54 was selected for a study at different growth temperatures in shaker flask culture. The strain used in our experiment was capable to accumulate a relatively high amount of intracellular lipid, 20.73 % of dry cell weight, and GLA content of 15 % of total fatty acids after 5 days of incubation at 28°C. As the growth temperature was decreased from 28 to 12°C the percentage of GLA increased from 15 to 24 % of total fatty acids. In order to optimize the culture conditions for rapid biomass production and lipid production with a high proportion of GLA, the fungus was grown at two temperature combinations associated supplies of carbon source (glucose) in the culture medium. Maximal production of GLA (74 mg/l) was obtained from the Mucor sp LB-54 strain after 5 days of incubation at 28°C in basal medium following glucose addition (7 % w/v) and incubation for an additional 3 days at 12°C. The identity of GLA found in the strain of Mucor sp LB-54 was confirmed by the coupled gas chromatography-mass spectrometry

Biosynthesis of triacylglycerol in the filamentous fungus Mucor circinelloides

Lipid metabolism was studied in 2-d-old liquid cultures of Mucor circinelloides grown at 25 degrees C. Under these conditions, oil accumulated to 0.5 g l-1 with a gamma-linolenic acid content (gamma 18:3) of 60 mg l-1. The major labelled lipids in cultures incubated with [14C]acetate were triacylglycerol (TAG), phosphatidylcholine (PC) and phosphatidylethanolamine (PE). The proportion of label declined in the phospholipids and increased in TAG with time. [14C]18:1 and [14C]18:2 rapidly appeared in PC and PE and later accumulated in [14C]gamma 18:3. TAG-synthesizing capacity was greatest in the microsomal membrane fraction, which accumulated high levels of phosphatidic acid in the presence of glycerol 3-phosphate and acyl-CoA substrates at pH 7.0. Further metabolism of phosphatidic acid to diacylglycerol and TAG was achieved by increasing the pH to 8.0. Lysophosphatidic acid: acyl-CoA acyltransferase (LPAAT) activity was particularly high and may have accounted for the rapid accumulation of phosphatidic acid in the membranes. The glycerol-3-phosphate: acyl-CoA acyltransferase (GPAAT) and LPAAT were non-specific for a range of saturated and unsaturated species of acyl-CoA although the GPAAT showed a marked selectivity for palmitoyl-CoA and the LPAAT for oleoyl- and linoleoyl-CoA. gamma-Linolenic acid was detected at all three positions of sn-TAG and was particularly enriched at the sn-3 position. The preparation of active in vitro systems (microsomal membranes) capable of the complete biosynthetic pathway for TAG assembly may be valuable in understanding the assembly of oils in future transgenic applications.

On the safety of Mortierella alpina for the production of food ingredients, such as arachidonic acid

Mortierella alpina is the most efficient production organism for arachidonic acid (AA) presently known. Since AA is being developed as a food ingredient, and since M. alpina has no history of use for such applications, we have undertaken this safety evaluation. M. alpina is a common soil fungus, to which humans are frequently exposed. The production strains are non-pathogenic and do not form potentially allergenic spores under production conditions. Moreover, there are no reliable reports in the literature connecting the species with disease or allergenic responses. No production of mycotoxins was observed, in line with the absence of literature reports describing such products, and with the results of toxicological tests. On solid growth media the strains showed antibiotic activity against Gram-positive bacteria. In submerged culture, which is used for AA production, no significant antibiotic activity was found. We conclude that M. alpina in general, and the AA production strains CBS 168.95 and CBS 169.95 in particular, should be considered safe for the submerged production of food ingredients.