The isolation of gamma-linolenic acid producing mucoralean fungi

[摘要] English: Members of Mucorales are known to produce the high value fatty acidgamma-linolenic acid [18:3(ω6)]. Although few studies have been conducted, it isknown that the type of carbon source included in the medium, influences theproduction of 18:3(ω6) by these fungi. The range of carbon sources on whichmucoralean fungi are able to grow and produce 18:3(ω6), is still mostly unknown.Another factor that influences the quantities of 18:3(ω6) that are being produced bythese fungi, is the specific fungal strain that is used in the process. Consequently,in this study it was decided first to investigate the ability of different mucoraleanfungi to grow and produce 18:3(ω6) on a wide range of carbon sources. Isolationmedia for obtaining new strains from nature, which utilize carbon sources obtainablefrom industrial effluents, would subsequently be developed.The influence of 38 different carbon sources on growth and consequent 18:3(ω6)content of the lipids produced by four mucoralean fungal strains were thereforeinvestigated. The strains represented the species Morlierella afpina, Mucorcircinelloides, Mucor ffavus and Thamnosfyfum piriforme. The representatives ofM. circinelloides and M. ffavus respectively utilized 25 and 23 of the 38 carbonsources in the series. The highest percentages 18:3(ω6) obtained with therepresentatives of M. circinelloides and M. ffavus were 27.17 % and 36.40 %respectively. In contrast, the highest percentages 18:3(ω6) obtained with therepresentatives of Mo. afpina and T. piriforme were only 5.61 % and 12.84 %respectively. These two strains could respectively utilize only seven and 17 of thecarbon sources. This study indicated that mucoralean fungi can grow and produce18:3(ω6) on a variety of carbon sources, including carbon sources present inindustrial effluents (e.g. starch, sucrose and acetic acid).Three selective media were subsequently developed in order to isolate mucoraleanfungi from soil, using the soil plate technique. The media, which were complex,respectively contained starch, sucrose and sodium acetate as carbon sources, aswell as 0.02 g/l of the anti-fungal agent, benlate. The selectivity of the media formembers of Mucorales was first determined by testing the media for the ability tosupport growth of 134 mucoralean fungal strains representing 66 species and sevengenera. The three isolation media supported growth of strains representing Absidia,Actinomucor, Backusella, Mucor, Rhizopus and Thamnostylum. The ability of theisolation media to select mucoralean fungi from a natural fungal population in soil,was then determined and representatives of the genera Absidia, Cunningham'ella,GongronelIa, Mucor and Rhizopus were obtained. The results further showed thatby using selective media in combination with a relatively non-selective medium,instead of the non-selective medium alone, more mucoralean taxa could be isolatedfrom a particular soil sample.Mucoralean fungal isolates that were obtained from the soil sample, weresubsequently evaluated for growth and 18:3(ω6) production in media containingstarch, sucrose or glucose as sole carbon sources. Isolates representing thefamilies Absidiaceae, Cunninghamellaceae and Mucoraceae were inoculated incomplex media containing the above mentioned carbon sources. It was found thatall the isolates were able to produce 18:3(ω6) on all three carbon sources.However, significant differences in volumetric 18:3(ω6) concentrations reached ondifferent carbon sources were noted for each isolate investigated. The highestvolumetric concentrations of 18:3(ω6) were obtained with an isolate representingR. stolonifer on starch (0.130 gii) and glucose (0.134 gii) as carbon sources.In order to prove that the isolates obtained using the above-mentioned isolationmedia, are able to grow in an industrial effluent, some of the isolates representingdifferent families, were grown in a medium prepared from an industrial effluentcontaining dextrins, galactans and starch as carbon sources. The lipids of theisolates which reduced the COD value of the effluent the most, were analysed. Itwas found that these isolates were able to produce 18:3(ω6).This study has therefore shown that it is possible to construct isolation media toisolate 18:3(ω6) producing mucoralean fungi from a natural fungal population. Itwas also found that such isolates can be used to produce biomass and 18:3(ω6)from carbon sources present in industrial effluents.

[发布机构] University of the Free State