Commercial-scale production of biofuels has recently been intensified due to its cost-effectiveness, sustainability, market stability, alternative fuel energy composition and greener output, etc. Lignocellulosic biomass attracted the attention of researchers as a renewable feedstock for fermentative conversion into biofuels because of its high productivity, low agricultural input requirements, being environmentally friendly and no competition with the food crops. The field of bioenergy is rapidly evolving with discoveries that are being reported daily. In this review, the economical production of biofuels using different feedstocks and bioprocessing strategies with the aim of integral utilization of agricultural or organic wastes are discussed. This review also highlights the potential of pyrolytic oil as fermentable substrates for different types of microbes (yeast, bacteria and algae) to generate biofuels. Although there is continuous technologic improvement for cost reduction in biomass conversion by microbial fermentation. Still, there are many techno-economic challenges such as recalcitrant nature of substrates, removal of inhibitors, metabolic engineering of microbial strains and detoxification strategies for the elimination of inhibitors. So, this review also summarizes some of these major challenges that should be addressed to make biofuel production cost effective.
Mucor circinelloides serves as a model organism to investigate the lipid metabolism in oleaginous microorganisms. It is considered as an important producer of γ-linolenic acid (GLA) that has vital medicinal benefits. In this study, we used WJ11, a high lipid-producing strain of M. circinelloides (36% w/w lipid, cell dry weight, CDW), to examine the role in lipid accumulation of two mitochondrial malic enzyme (ME) genes malC and malD. The homologous overexpression of both malC and malD genes enhanced the total lipid content of WJ11 by 41.16 and 32.34%, respectively. In parallel, the total content of GLA was enhanced by 16.73 and 46.76% in malC and malD overexpressing strains, respectively, because of the elevation of total lipid content. The fact that GLA content was enhanced more in the strain with lower lipid content increase and vice versa, indicated that engineering of mitochondrial MEs altered the fatty acid profile. Our results reveal that mitochondrial ME plays an important role in lipid metabolism and suggest that future approaches may involve simultaneous overexpression of distinct ME genes to boost lipid accumulation even further.
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