Lignocellulosic hydrolysates contain a mixture of hexose (C6)/pentose (C5) sugars and pretreatment-generated inhibitors (furans, weak acids and phenolics). Therefore, robust yeast isolates with characteristics of C6/C5 fermentation and tolerance to pretreatment-derived inhibitors are pre-requisite for efficient lignocellulosic material based biorefineries. Moreover, use of thermotolerant yeast isolates will further reduce cooling cost, contamination during fermentation, and required for developing simultaneous saccharification and fermentation (SSF), simultaneous saccharification and co-fermentation (SScF), and consolidated bio-processing (CBP) strategies. In this study, we evaluated thirty-five yeast isolates (belonging to six genera including Saccharomyces, Kluyveromyces, Candida, Scheffersomyces, Ogatea and Wickerhamomyces) for pretreatment-generated inhibitors {furfural, 5-hydroxymethyl furfural (5-HMF) and acetic acid} and thermotolerant phenotypes along with the fermentation performances at 40 °C. Among them, a sugarcane distillery waste isolate, Saccharomyces cerevisiae NGY10 produced maximum 49.77 ± 0.34 g/l and 46.81 ± 21.98 g/l ethanol with the efficiency of 97.39% and 93.54% at 30 °C and 40 °C, respectively, in 24 h using glucose as a carbon source. Furthermore, isolate NGY10 produced 12.25 ± 0.09 g/l and 7.18 ± 0.14 g/l of ethanol with 92.81% and 91.58% efficiency via SHF, and 30.22 g/l and 25.77 g/l ethanol with 86.43% and 73.29% efficiency via SSF using acid- and alkali-pretreated rice straw as carbon sources, respectively, at 40 °C. In addition, isolate NGY10 also produced 92.31 ± 3.39 g/l (11.7% v/v) and 33.66 ± 1.04 g/l (4.26% v/v) ethanol at 40 °C with the yields of 81.49% and 73.87% in the presence of 30% w/v glucose or 4× concentrated acid-pretreated rice straw hydrolysate, respectively. Moreover, isolate NGY10 displayed furfural- (1.5 g/l), 5-HMF (3.0 g/l), acetic acid- (0.2% v/v) and ethanol-(10.0% v/v) tolerant phenotypes. A sugarcane distillery waste isolate NGY10 demonstrated high potential for ethanol production, C5 metabolic engineering and developing strategies for SSF, SScF and CBP.

The urgent need to address environmental issues associated with the use of conventional fossil fuels has driven the rapid evolution of the global energy landscape. This review explores the background and significance of 3-G biofuel production, emphasizing the shift towards sustainable alternatives amidst escalating greenhouse gas emissions. While various renewable energy sources have gained prominence, biofuels have emerged as a promising solution for the transportation and industrial sectors, particularly from microalgal biomass. The rationale for focusing on microalgal biomass is based on its technical and environmental advantages. Unlike traditional feedstocks, microalgae boast a high lipid content, enhancing biofuel production efficiency. Their rapid growth rates and efficient carbon dioxide sequestration make microalgae frontrunners in scalable and sustainable biofuel production. This review aims to comprehensively analyze recent breakthroughs in 3-G biofuel production from microalgal biomass, filling gaps in the existing literature. The topics covered included species diversity, cultivation techniques, harvesting, pretreatment, lipid extraction methods, and biofuel production pathways. Genetic engineering, downstream processing, energy-efficient practices, and emerging trends, such as artificial intelligence and cross-disciplinary collaboration, will be explored. This study aims to consolidate recent research findings, identify challenges and opportunities, and guide future directions in microalgal biomass-based biofuel production. By synthesizing unpublished research, this review seeks to advance our knowledge and provide insights for researchers to foster sustainable and efficient 3-G biofuel production.