Bioprospecting thermophilic/thermotolerant microbes for production of lignocellulosic ethanol: A future perspective

The progressive depletion of non-renewable energy sources worldwide, together with the fact that their overexploitation has resulted in environmental deterioration and public health problems, has led to consider alternative sources of energy. Lignocellulose-based bioethanol is a leading option among alternatives to petroleum-derived transportation fuels due to its potential sustainability. The production of ethanol through microbial fermentations has generated considerable research interests. Several thermophilic/thermotolerant ethanologenic species i.e. Clostridium thermocellum, C. thermohydrosulfuricum, C. thermosaccharolyticum, Caldicellulosiruptor sp., Thermotoga sp., Thermoanaerobium brockii, Thermoanaerobacter ethanolicus, T. thermo-hydrosulfuricus, T. mathranii, etc., have been isolated and identified as the potential lignocellulosic ethanol producers. Use of lignocellulolytic organisms alone at high temperatures could potentially reduce the cellulase requirement. Moreover, such cultures facilitate the ethanol production at high temperature and offer the possibility of in-situ ethanol recovery. However, more research on the metabolic pathways, regulation of end-product formation and construction of genetically engineered thermophilic/thermotolerant microorganisms with high tolerance to ethanol is required for optimal utilization of such microbes in industrial fermentations. Therefore, the present review has been focused on thermophilic/thermotolerant microbes for the production of ethanol, especially on their catabolic pathways, end-product formation and their future perspectives for industrial applications.