Shuvashish Behera

Scope of algae as third generation biofuels

An initiative has been taken to develop different solid, liquid, and gaseous biofuels as the alternative energy resources. The current research and technology based on the third generation biofuels derived from algal biomass have been considered as the best alternative bioresource that avoids the disadvantages of first and second generation biofuels. Algal biomass has been investigated for the implementation of economic conversion processes producing different biofuels such as biodiesel, bioethanol, biogas, biohydrogen, and other valuable co-products. In the present review, the recent findings and advance developments in algal biomass for improved biofuel production have been explored. This review discusses about the importance of the algal cell contents, various strategies for product formation through various conversion technologies, and its future scope as an energy security.

A new search for thermotolerant yeasts, its characterization and optimization using response surface methodology for ethanol production

The progressive rise in energy crisis followed by green house gas (GHG) emissions is serving as the driving force for bioethanol production from renewable resources. Current bioethanol research focuses on lignocellulosic feedstocks as these are abundantly available, renewable, sustainable and exhibit no competition between the crops for food and fuel. However, the technologies in use have some drawbacks including incapability of pentose fermentation, reduced tolerance to products formed, costly processes, etc. Therefore, the present study was carried out with the objective of isolating hexose and pentose fermenting thermophilic/thermotolerant ethanologens with acceptable product yield. Two thermotolerant isolates, NIRE-K1 and NIRE-K3 were screened for fermenting both glucose and xylose and identified as Kluyveromyces marxianus NIRE-K1 and K. marxianus NIRE-K3. After optimization using Face-centered Central Composite Design (FCCD), the growth parameters like temperature and pH were found to be 45.17°C and 5.49, respectively for K. marxianus NIRE-K1 and 45.41°C and 5.24, respectively for K. marxianus NIRE-K3. Further, batch fermentations were carried out under optimized conditions, where K. marxianus NIRE-K3 was found to be superior over K. marxianus NIRE-K1. Ethanol yield (Yx∕s), sugar to ethanol conversion rate (%), microbial biomass concentration (X) and volumetric product productivity (Qp) obtained by K. marxianus NIRE-K3 were found to be 9.3, 9.55, 14.63, and 31.94% higher than that of K. marxianus NIRE-K1, respectively. This study revealed the promising potential of both the screened thermotolerant isolates for bioethanol production.

Enhancement in xylose utilization using Kluyveromyces marxianus NIRE-K1 through evolutionary adaptation approach

The evolutionary adaptation was carried out on the thermotolerant yeast Kluyveromyces marxianus NIRE-K1 at 45xa0°C up to 60 batches to enhance its xylose utilization capability. The adapted strain showed higher specific growth rate and 3-fold xylose uptake rate and short lag phase as compared to the native strain. During aerobic growth adapted yeast showed 2.81-fold higher xylose utilization than that of native. In anaerobic batch fermentation, adapted yeast utilized about 91xa0% of xylose in 72xa0h and produced 2.88 and 18.75xa0gxa0l−1 of ethanol and xylitol, respectively, which were 5.11 and 5.71-fold higher than that of native. Ethanol yield, xylitol yield and specific sugar consumption rate obtained by the adapted cells were found to be 1.57, 1.65 and 4.84-fold higher than that of native yeast, respectively. Aforesaid results suggested that the evolutionary adaptation will be a very effective strategy in the near future for economic lignocellulosic ethanol production.

Bioprospecting thermostable cellulosomes for efficient biofuel production from lignocellulosic biomass

The adverse climatic conditions due to continuous use of fossil-derived fuels are the driving factors for the development of renewable sources of energy. Current biofuel research focuses mainly on lignocellulosic biomass (LCB) such as agricultural, industrial and municipal solid wastes due to their abundance and renewability. Although many mesophilic cellulolytic microorganisms have been reported, efficient and economical bioconversion to simple sugars is still a challenge. Thermostable cellulolytic enzymes play an indispensible role in degradation of the complex polymeric structure of LCB into fermentable sugar stream due to their higher flexibility with respect to process configurations and better specific activity than the mesophilic enzymes. In some anaerobic thermophilic/thermotolerant microorganisms, few cellulases are organized as unique multifunctional enzyme complex, called the cellulosome. The use of cellulosomal multienzyme complexes for saccharification seems to be a promising and cost-effective alternative for complete breakdown of cellulosic biomass. This paper aims to explore and review the important findings in cellulosomics and forward the path for new cutting-edge opportunities in the success of biorefineries. Herein, we summarize the protein structure, regulatory mechanisms and their expression in the host cells. Furthermore, we discuss the recent advances in specific strategies used to design new multifunctional cellulosomal enzymes, which can function as lignocellulosic biocatalysts and evaluate the roadblocks in the yield and stability of such designer thermozymes with overall progress in lignocellulose-based biorefinery.

Effect of Evolutionary Adaption on Xylosidase Activity in Thermotolerant Yeast Isolates Kluyveromyces marxianus NIRE-K1 and NIRE-K3

Efficient use of xylose along with glucose is necessary for the economic production of lignocellulosic based biofuels. Xylose transporters play an important role in the microorganisms for efficient utilization of xylose. In the present study, a novel method has been developed for a rapid assay of xylose transport activity in the xylose-utilizing isolates and other known yeasts. An assay was conducted to compare the activity of β-xylosidase using p-nitrophenyl-β-d-xylopyranoside (pNPX) in the intact, intracellular, and extracellular yeasts cells showing xylose transporter. Saccharomyces cerevisiae (MTCC 170) showed no xylosidase activity, while little growth was observed in the xylose-containing medium. Although other yeasts, i.e., Kluyveromyces marxianus NIRE-K1 (MTCC 5933), K. marxianus NIRE-K3 (MTCC 5934), and Candida tropicalis (MTCC 230), showed xylosidase activity in intact, intracellular, and extracellular culture. The xylosidase activity in intact cell was higher than that of extracellular and intracellular activity in all the yeast cells. The enzyme activity was higher in case of K. marxianus NIRE-K1 and K. marxianus NIRE-K3 rather than the C. tropicalis. Further, better xylosidase activity was observed in adapted K. marxianus cells which were 2.79–28.46xa0% higher than that of native (non-adapted) strains, which indicates the significant improvement in xylose transportation.

Potential Role of Xylose Transporters in Industrial Yeast for Bioethanol Production: A Perspective Review

Sustainable development in lignocellulosic bioethanol production has major challenge due to high cost of production. There are several issues such as efficient utilization of pentose sugars present in lignocelluloses, economical production of lignocellulolytic enzymes with high specificity and economical product recovery, etc. In line, genetically modified yeast strains have been approached to utilize pentose and hexose sugars for bioethanol production. However, these strains showed limited xylose consumption. For efficient utilization of xylose, it is necessary to provide efficient molecular transportation of xylose to the yeast cells. The yeast strains which have been found prominent are Saccharomyces cerevisiae, Candida intermedia, C. tropicalis, Kluyveromyces marxianus and Scheffersomyces stipitis which have been engineered and examined for xylose transporter genes for xylose utilization. Several transporter genes of interest have been targeted; however, there are major bottleneck in this approach such as xylose transporters which are significantly inhibited by glucose and other hexose sugars. Hence, there are several molecular approaches that have been applied to engineer the yeasts which could improve the xylose transportation. This review has been focused to discuss the molecular advancements in xylose transporter genes and its complexity.

Prospects of Solvent Tolerance in Butanol Fermenting Bacteria

Butanol tolerance is a critical factor affecting the ability of microorganisms to produce economically viable quantities of butanol through acetone-butanol-ethanol (ABE) fermentation using renewable feedstocks. However, ABE process has certain challenges like maintaining strict anaerobic conditions, slow growth rate of microorganisms, the rapid shift of pH, sensitivity to acetic acid, low butanol titer, solvent tolerance, and product inhibition. Separation of fermentation products through distillation, gas stripping, pervaporation, and adsorption also makes the process costly. Despite their importance at a biofuel platform, a limited number of butanol-tolerant bacteria have been identified so far. This problem can be eradicated through the isolation of solvent tolerating bacteria, development of bacteria through evolutionary engineering, mutation, and genetic engineering with promising product recovery techniques. In the present chapter, an overview of the butanol tolerating microbes, their solvent survival strategies, and the techniques to overcome the problem for a high concentration of butanol have been discussed.

Xylose transport in yeast for lignocellulosic ethanol production: Current status

Lignocellulosic ethanol has been considered as an alternative transportation fuel. Utilization of hemicellulosic fraction in lignocelluloses is crucial in economical production of lignocellulosic ethanol. However, this fraction has not efficiently been utilized by traditional yeast Saccharomyces cerevisiae. Genetically modified S.xa0cerevisiae, which can utilize xylose, has several limitations including low ethanol yield, redox imbalance, and undesired metabolite formation similar to native xylose utilizing yeasts. Besides, xylose uptake is a major issue, where sugar transport system plays an important role. These genetically modified and wild-type yeast strains have further been engineered for improved xylose uptake. Various techniques have been employed to facilitate the xylose transportation in these strains. The present review is focused on the sugar transport machineries, mechanisms of xylose transport, limitations and how to deal with xylose transport for xylose assimilation in yeast cells. The recent advances in different techniques to facilitate the xylose transportation have also been discussed.

Evolutionary Adaptation of Kluyveromyces marxianus NIRE-K3 for Enhanced Xylose Utilization

The evolutionary adaptation was approached on the thermotolerant yeast Kluyveromyces marxianus NIRE-K3 at 45oC on xylose as a sole source of carbon for enhancement of xylose uptake. After 60 cycles, evolved strain K. marxianus NIRE-K3.1 showed comparatively 3.75-fold and 3.0-fold higher specific growth and xylose uptake rates, respectively than that of native strain. Moreover, the short lag phase was also observed on adapted strain. During batch fermentation with xylose concentration of 30 g l-1, K. marxianus NIRE-K3.1 could utilize about 96 % of xylose in 72 h and produced 4.67 and 15.7 g l-1 of ethanol and xylitol, respectively, which were 9.72-fold and 4.63-fold higher than that of native strain. Similarly, specific sugar consumption rate, xylitol and ethanol yields were 5.07-fold, 1.15-fold and 2.44-fold higher as compared to the native strain, respectively. The results obtained after evolutionary adaptation of K. marxianus NIRE-K3 show the significant improvement in the xylose utilization, ethanol and xylitol yields, and productivities. By understanding the results obtained, the significance of evolutionary adaptation has been rationalized, since the adapted culture could be more stable and could enhance the productivity.