Strategies for economic utilization of rice straw residues into value-added by-products and prevention of environmental pollution

Rice straw management, along with the prevalent practice of residue burning, poses multifaceted challenges with substantial environmental and human health implications. After harvest, a considerable amount of straw is left behind, often disposed of through burning, releasing several pollutants into the environment. Carbon dioxide (CO2) dominates at 70%, accompanied by methane (CH4) at 0.66%, carbon monoxide (CO) at 7%, and nitrous oxide (N2O) at 2.09%. This process further compounds issues by depleting soil nutrients like nitrogen and organic matter. This review focuses on strategies for residue management and using straw as value-added by-products. We address research gaps and offer potential recommendations for rice straw management using economically feasible and practical routes. We elaborate that to improve rice straw digestibility, utilization in mushroom cultivation, and other value-added products, low silica (Si) rice varieties must be developed using modern technologies including marker-assisted selection breeding or genome editing. Developing low Si rice could also reduce arsenic uptake by rice, as rice plants use the same transporters for the uptake of both elements. Conversely, silica is also indispensable for quality rice production; hence, optimizing silicon content in rice is worth investigating. More research is required to understand the extent of silicon's effect on the utilization of straw for various purposes. This review also discusses the importance of educating farmers about the straw burning issue and its environmental consequences. We highlight the significance of tailoring rice straw management methods to local suitability, moving away from a universal approach. More extension work is needed to encourage farmers to opt for environmentally and economically sound options for rice straw management. Policy intervention to incentivize farmers and develop technologies for the widespread use of rice straw for various industries and product development could help in the management of rice straw and will also create a circular economy.

Kinetics of growth and co-production of amylase and protease in novel marine actinomycete, Streptomyces lopnurensis KaM5

Amylases and proteases are among the industrially most important enzymes for food processing, animal feed, brewing, starch processing, detergents, healthcare, leather processing, and biofuel production. In this study, we investigated the growth kinetics and statistically optimized the co-production of amylase and protease in a phylogenetically novel haloalkaliphilic actinomycete, Streptomyces lopnurensis KaM5 of seawater. The Plackett-Berman design using Minitab 14.0 software was employed to assess the impact of the nutritional factors, temperature, pH, and incubation time. Further, starch, yeast extract, NaCl concentrations, and incubation time were optimized by Box-Behnken design at their three levels. The Pareto charts, contour, surface plots, and individual factorial analysis expressed the variability and levels for the optimal enzyme production. ANOVA analysis admitted the statistical fitness and significance level among the variables. A two-fold increase in enzyme production was achieved by cost-effective co-production media. The study was further extended to growth kinetics associated with enzyme production. Specific growth rate (μ), maximal cell mass (X_max), volumetric product formation (P_max), rate of product formation (Q_p), and generation time (g) were computed and analyzed. These parameters significantly improved when compared with the pre-optimized conditions, and the production economics of the enzyme was industrially viable. The initial studies on the characteristics of the enzymes suggested its ability to function under the combination of alkaline pH and high salt concentrations. The co-production of enzymes from extremophiles can be a potentially viable option for large-scale production and applications.

Pathway dissection, regulation, engineering and application: lessons learned from biobutanol production by solventogenic clostridia

The global energy crisis and limited supply of petroleum fuels have rekindled the interest in utilizing a sustainable biomass to produce biofuel. Butanol, an advanced biofuel, is a superior renewable resource as it has a high energy content and is less hygroscopic than other candidates. At present, the biobutanol route, employing acetone-butanol-ethanol (ABE) fermentation in Clostridium species, is not economically competitive due to the high cost of feedstocks, low butanol titer, and product inhibition. Based on an analysis of the physiological characteristics of solventogenic clostridia, current advances that enhance ABE fermentation from strain improvement to product separation were systematically reviewed, focusing on: (1) elucidating the metabolic pathway and regulation mechanism of butanol synthesis; (2) enhancing cellular performance and robustness through metabolic engineering, and (3) optimizing the process of ABE fermentation. Finally, perspectives on engineering and exploiting clostridia as cell factories to efficiently produce various chemicals and materials are also discussed.

Recent trends in biobutanol production

Finite availability of conventional fossil carbonaceous fuels coupled with increasing pollution due to their overexploitation has necessitated the quest for renewable fuels. Consequently, biomass-derived fuels are gaining importance due to their economic viability and environment-friendly nature. Among various liquid biofuels, biobutanol is being considered as a suitable and sustainable alternative to gasoline. This paper reviews the present state of the preprocessing of the feedstock, biobutanol production through fermentation and separation processes. Low butanol yield and its toxicity are the major bottlenecks. The use of metabolic engineering and integrated fermentation and product recovery techniques has the potential to overcome these challenges. The application of different nanocatalysts to overcome the existing challenges in the biobutanol field is gaining much interest. For the sustainable production of biobutanol, algae, a third-generation feedstock has also been evaluated.

Hydrolytic pre-treatment methods for enhanced biobutanol production from agro-industrial wastes

Brewery industry liquid waste (BLW), brewery spent grain (BSG), apple pomace solid wastes (APS), apple pomace ultrafiltration sludge (APUS) and starch industry wastewater (SIW) have been considered as substrates to produce biobutanol. Efficiency of hydrolysis techniques tested to produce fermentable sugars depended on nature of agro-industrial wastes and process conditions. Acid-catalysed hydrolysis of BLW and BSG gave a total reducing sugar yield of 0.433 g/g and 0.468 g/g respectively. Reducing sugar yield from microwave assisted hydrothermal method was 0.404 g/g from APS and 0.631 g/g from APUS, and, 0.359 g/g from microwave assisted acid-catalysed SIW dry mass. Parameter optimization (time, pH and substrate concentration) for acid-catalysed BLW hydrolysate utilization using central composite model technique produced 307.9 g/kg glucose with generation of inhibitors (5-hydroxymethyl furfural (20 g/kg), furfural (1.6 g/kg), levulinic acid (9.3 g/kg) and total phenolic compound (0.567 g/kg)). 10.62 g/L of acetone-butanol-ethanol was produced by subsequent clostridial fermentation of the substrate.

Enhanced butanol production by solvent tolerance Clostridium acetobutylicum SE25 from cassava flour in a fibrous bed bioreactor

To enhance the butanol productivity and reduce the material cost, acetone, butanol, and ethanol fermentation by Clostridium acetobutylicum SE25 was investigated using batch, repeated-batch and continuous cultures in a fibrous bed bioreactor, where cassava flour was used as the substrate. With periodical nutrient supplementation, stable butanol production was maintained for about 360h in a 6-cycle repeated-batch fermentation with an average butanol productivity of 0.28g/L/h and butanol yield of 0.32g/g-starch. In addition, the highest butanol productivity of 0.63g/L/h and butanol yield of 0.36g/g-starch were achieved when the dilution rate were investigated in continuous production of acetone, butanol, and ethanol using a fibrous bed bioreactor, which were 231.6% and 28.6% higher than those of the free-cell fermentation. On the other hand, this study also successfully comfirmed that the biofilm can provide an effective protection for the microbial cells which are growing in stressful environment.

Combined Detoxification and In-situ Product Removal by a Single Resin During Lignocellulosic Butanol Production

Phragmites australis (an invasive plant in North America) was used as feedstock for ABE (acetone-butanol-ethanol) fermentation by Clostridium saccharobutylicum. Sulphuric acid pretreated phragmites hydrolysate (SAEH) without detoxification inhibited butanol production (0.73 g/L butanol from 30 g/L sugars). The treatment of SAEH with resin L-493 prior the fermentation resulted in no inhibitory effects and an ABE titer of 14.44 g/L, including 5.49 g/L butanol was obtained, corresponding to an ABE yield and productivity of 0.49 g/g and 0.60 g/L/h, respectively. Dual functionality of the resin was realized by also using it as an in-situ product removal agent. Integrating in-situ product removal allowed for the use of high substrate concentrations without the typical product inhibition. Resin-detoxified SAEH was supplemented with neat glucose and an effective ABE titer of 33 g/L (including 13.7 g/L acetone, 16.4 g/L butanol and 1.9 g/L ethanol) was achieved with resin-based in-situ product removal, corresponding to an ABE yield and productivity of 0.41 g/g and 0.69 g/L/h, respectively. Both detoxification of the substrate and the products was achieved by the same resin, which was added prior the fermentation. Integrating hydrolysate detoxification and in-situ butanol removal in a batch process through single resin can potentially simplify cellulosic butanol production.

Increased Microbial Butanol Tolerance by Exogenous Membrane Insertion Molecules

Butanol is an ideal biofuel, although poor titers lead to high recovery costs by distillation. Fluidization of microbial membranes by butanol is one of the major factors limiting titers in butanol-producing bioprocesses. Starting with the hypothesis that certain membrane insertion molecules would stabilize the lipid bilayer in the presence of butanol, we applied a combination of in vivo and in vitro techniques within an in silico framework to describe a new approach to achieve solvent tolerance in bacteria. Single-molecule tracking of a model supported bilayer showed that COE1-5C, a five-ringed oligo-polyphenylenevinylene conjugated oligoelectrolyte (COE), reduced the diffusion rate of phospholipids in a microbially derived lipid bilayer to a greater extent than three-ringed and four-ringed COEs. Furthermore, COE1-5C treatment increased the specific growth rate of E. coli K12 relative to a control at inhibitory butanol concentrations. Consequently, to confer butanol tolerance to microbes by exogenous means is complementary to genetic modification of strains in industrial bioprocesses, extends the physiological range of microbes to match favorable bioprocess conditions, and is amenable with complex and undefined microbial consortia for biobutanol production. Molecular dynamics simulations indicated that the π-conjugated aromatic backbone of COE1-5C likely acts as a hydrophobic tether for glycerophospholipid acyl chains by enhancing bilayer integrity in the presence of high butanol concentrations, which thereby counters membrane fluidization. COE1-5C-mitigated E. coli K12 membrane depolarization by butanol is consistent with the hypothesis that improved growth rates in the presence of butanol are a consequence of improved bilayer stability.

Development of an electrotransformation protocol for genetic manipulation of Clostridium pasteurianum

BACKGROUND

Reducing the production cost of, and increasing revenues from, industrial biofuels will greatly facilitate their proliferation and co-integration with fossil fuels. The cost of feedstock is the largest cost in most fermentation bioprocesses and therefore represents an important target for cost reduction. Meanwhile, the biorefinery concept advocates revenue growth through complete utilization of by-products generated during biofuel production. Taken together, the production of biofuels from low-cost crude glycerol, available in oversupply as a by-product of bioethanol production, in the form of thin stillage, and biodiesel production, embodies a remarkable opportunity to advance affordable biofuel development. However, few bacterial species possess the natural capacity to convert glycerol as a sole source of carbon and energy into value-added bioproducts. Of particular interest is the anaerobe Clostridium pasteurianum, the only microorganism known to convert glycerol alone directly into butanol, which currently holds immense promise as a high-energy biofuel and bulk chemical. Unfortunately, genetic and metabolic engineering of C. pasteurianum has been fundamentally impeded due to lack of an efficient method for deoxyribonucleic acid (DNA) transfer.

RESULTS

This work reports the development of an electrotransformation protocol permitting high-level DNA transfer to C. pasteurianum ATCC 6013 together with accompanying selection markers and vector components. The CpaAI restriction-modification system was found to be a major barrier to DNA delivery into C. pasteurianum which we overcame by in vivo methylation of the recognition site (5'-CGCG-3') using the M.FnuDII methyltransferase. With proper selection of the replication origin and antibiotic-resistance marker, we initially electroporated methylated DNA into C. pasteurianum at a low efficiency of 2.4 × 101 transformants μg-1 DNA by utilizing conditions common to other clostridial electroporations. Systematic investigation of various parameters involved in the cell growth, washing and pulse delivery, and outgrowth phases of the electrotransformation procedure significantly elevated the electrotransformation efficiency, up to 7.5 × 104 transformants μg-1 DNA, an increase of approximately three order of magnitude. Key factors affecting the electrotransformation efficiency include cell-wall-weakening using glycine, ethanol-mediated membrane solubilization, field strength of the electric pulse, and sucrose osmoprotection.

CONCLUSIONS

C. pasteurianum ATCC 6013 can be electrotransformed at a high efficiency using appropriately methylated plasmid DNA. The electrotransformation method and tools reported here should promote extensive genetic manipulation and metabolic engineering of this biotechnologically important bacterium.

Continuous butanol fermentation from inexpensive sugar-based feedstocks by Clostridium saccharobutylicum DSM 13864

Corn stover hydrolysate (CSH) and cane molasses were studied for butanol fermentation by Clostridium saccharobutylicum DSM 13864 in continuous fermentation. Using cane molasses as substrate, solvent of 13.75 g/L (butanol 8.65 g/L) and productivity of 0.439 g/L/h were achieved in a four-stage continuous fermentation at a gradient dilution mode of 0.15-0.15-0.125-0.1 h(-1). In continuous fermentation using CSH as substrate, total solvent titer of 11.43 g/L (butanol 7.81 g/L) and productivity of 0.429 g/L/h were reached at a dilution rate of 0.15 h(-1), and the steady process was continuously operated for 220 h without compromise in solvent titer.

Biochemical and genetic engineering strategies to enhance hydrogen production in photosynthetic algae and cyanobacteria

As an energy carrier, hydrogen gas is a promising substitute to carbonaceous fuels owing to its superb conversion efficiency, non-polluting nature, and high energy content. At present, hydrogen is predominately synthesized via chemical reformation of fossil fuels. While various biological methods have been extensively explored, none of them is justified as economically feasible. A sustainable platform for biological production of hydrogen will certainly impact the biofuel market. Among a selection of biological systems, algae and cyanobacteria have garnered major interests as potential cell factories for hydrogen production. In conjunction with photosynthesis, these organisms utilize inexpensive inorganic substrates and solar energy for simultaneous biosynthesis and hydrogen evolution. However, the hydrogen yield associated with these organisms remains far too low to compete with the existing chemical systems. This article reviews recent advances of biochemical, bioprocess, and genetic engineering strategies in circumventing technological limitations to hopefully improve the applicative potential of these photosynthetic hydrogen production systems.

Utilization of ram horn peptone in the production of glucose oxidase by a local isolate Aspergillus niger OC-3

Glucose oxidase (GO) is an enzyme that is used in many fields. In this study, ram horn peptone (RHP) was utilized as the nitrogen source and compared with other nitrogen sources in the production of GO by Aspergillus niger. To obtain higher GO activity, 14 A. niger strains were isolated from soil samples around Erzurum, Turkey. Among these strains, the isolate that was named A. niger OC-3 achieved the highest GO production. The production of GO was carried out in 100 mL scaled batch culture. The fermentation conditions such as initial pH, temperature, agitation speed, and time were investigated in order to improve GO production. The results showed that the cultivation conditions would significantly affect the formation of GO, and the utilization of the RHP achieved the highest enzyme production (48.6 U/mL) if compared to other nitrogen sources. On the other hand, the maximum biomass was obtained by using the fish peptone (7.2 g/L), while RHP yielded 6.4 g/L. These results suggest that RHP from waste ram horns could effectively be used in the production of GO by A. niger OC-3.

Butanol production by Clostridium beijerinckii ATCC 55025 from wheat bran

Wheat bran, a by-product of the wheat milling industry, consists mainly of hemicellulose, starch and protein. In this study, the hydrolysate of wheat bran pretreated with dilute sulfuric acid was used as a substrate to produce ABE (acetone, butanol and ethanol) using Clostridium beijerinckii ATCC 55025. The wheat bran hydrolysate contained 53.1 g/l total reducing sugars, including 21.3 g/l of glucose, 17.4 g/l of xylose and 10.6 g/l of arabinose. C. beijerinckii ATCC 55025 can utilize hexose and pentose simultaneously in the hydrolysate to produce ABE. After 72 h of fermentation, the total ABE in the system was 11.8 g/l, of which acetone, butanol and ethanol were 2.2, 8.8 and 0.8 g/l, respectively. The fermentation resulted in an ABE yield of 0.32 and productivity of 0.16 g l(-1) h(-1). This study suggests that wheat bran can be a potential renewable resource for ABE fermentation.

Bioproduction of butanol from biomass: from genes to bioreactors

Butanol is produced chemically using either the oxo process starting from propylene (with H2 and CO over a rhodium catalyst) or the aldol process starting from acetaldehyde. The key problems associated with the bioproduction of butanol are the cost of substrate and butanol toxicity/inhibition of the fermenting microorganisms, resulting in a low butanol titer in the fermentation broth. Recent interest in the production of biobutanol from biomass has led to the re-examination of acetone-butanol-ethanol (ABE) fermentation, including strategies for reducing or eliminating butanol toxicity to the culture and for manipulating the culture to achieve better product specificity and yield. Advances in integrated fermentation and in situ product removal processes have resulted in a dramatic reduction of process streams, reduced butanol toxicity to the fermenting microorganisms, improved substrate utilization, and overall improved bioreactor performance.