Lipid metabolism in Mucor genevensis and related species

English: In this study the lipid metabolism of Mucor genevensis and related mucoralean fungi have been explored. Consequently, endogenous lipid turnover during growth and development of the different asexual and sexual reproductive stages, was studied in Mucor genevensis MUFS 038. Lipid analyses were periodically performed on cultures growing on solid synthetic media. It was found that during all developmental stages, the lipid composition remained similar to that of a typical mucoralean fungus. However, zygospores were found to have fewer membranes (i.e. less phospholipids) with a lower degree of fluidity (i.e. less polyunsaturated fatty acids - PUFAs) than sporangiospores and mycelium. Decreases in neutral, glyco- and phospholipids were observed during sporogenesis. The aim of the next chapter was to investigate the uptake and incorporation of arachidonic acid [20:4((,)6)] into the various lipid fractions of Mucor genevensis MUFS 038 and Rhizopus oryzae PPRI 5172. The exogenous 20:4((·)6) was rapidly taken up by the fungi. The overall concentration of 20:4((,)6) in both fungi decreased due to catabolism of 20:4((1)6). The PUFA was incorporated into the neutral, glyco- and phospholipids of both fungi. 1t was found that Rhizopus oryzae has a greater ability to reduce the amount of cellular free 20:4((!)6) than M. genevensis. During incubation with 20:4((!)6), different changes in the concentrations of 16 and 18 carbon fatty acids were observed for the two fungi, indicating differences in lipid metabolism of these two fungi. In the next chapter, mucoralean fungi werescreened for oxidation products of exogenous PUFAs. Lipid extracts from biomass and aqueous phases of cultures of the following strains were analysed using gas chromatography-mass spectrometry: Mucor circinelloides f. circinelloides CBS 107.16, Mucor flavus CBS 234.35, Mucor genevensis MUFS 038, Mucor mucedo CBS 109.16, Mucor plumbeus CBS 111.07, Rhizopus oryzae PPRI 5172 and Thamnostylum piriforme MUFS 025. The following PUFAs were fed to the fungal cultures: linoleic acid, y-linolenic acid, dihomo-y-linolenic acid [20:3((,)6)], 20:4((1)6) and eicosapentaenoic acid. It was demonstrated that M. genevensis and M. circinelloides are able to produce 3-hydroxy-5,8-tetradecadienoic acid (3-HTDE) from 20:4(0)6) and 20'3((,)6), respectively No other 3-hydr0X~1 fatty acids (3-0H-FAs) were detected in any other strains. It was suggested that the 3-0H-FAs were produced at such low concentrations that detection was impossible in most of the strains. Another reason .' for the apparent absence of 3-0H-FAs may be that these compounds are rapidly utilised or further metabolised by the strains. Detection of 3-HTDE in the aqueous phase only, suggested that it was not a structural component of membranes, or that 3-HTDE was formed at a pnvsiotoqica: stage which did not contribute to the final biomass. Subsequently, immunofluorescence microscopy was used in the localisation of endogenous 3-0H-FAs in M. genevensis MUFS 038. It was found that 3-0H-FAs are associated with spore bearing structures. It was speculated that some of the lipids, which diminish during sporogenesis of M. genevensis, were transformed to hydroxylated fatty acids during sporogenesis.