Thermophilic β-mannanases from bacteria: production, resources, structural features and bioengineering strategies

β-mannanases are pivotal enzymes that cleave the mannan backbone to release short chain mannooligosaccharides, which have tremendous biotechnological applications including food/feed, prebiotics and biofuel production. Due to the high temperature conditions in many industrial applications, thermophilic mannanases seem to have great potential to overcome the thermal impediments. Thus, structural analysis of thermostable β-mannanases is extremely important, as it could open up new avenues for genetic engineering, and protein engineering of these enzymes with enhanced properties and catalytic efficiencies. Under this scope, the present review provides a state-of-the-art discussion on the thermophilic β-mannanases from bacterial origin, their production, engineering and structural characterization. It covers broad insights into various molecular biology techniques such as gene mutagenesis, heterologous gene expression, and protein engineering, that are employed to improve the catalytic efficiency and thermostability of bacterial mannanases for potential industrial applications. Further, the bottlenecks associated with mannanase production and process optimization are also discussed. Finally, future research related to bioengineering of mannanases with novel protein expression systems for commercial applications are also elaborated.

Lipid-rich particles of processed food waste for microalgae harvest through lipid-enriched floating biomat formation

Food waste was collected from the campus canteen and lipid-rich particles (LRP) phase was evaluated to harvest Tetradesmus obliquus. Box-Behnken design showed the highest harvest efficiency (HE) of 84.69 % in run#1 (LRP = 30 %; initial OD680 = 1.75; and harvest time = 6 h). Numerical optimization ramps suggested 24.15 % (v/v) LRP ratio, initial OD680 3.00, and harvest time 3.82 h for maximum HE. Two flocs were observed, a precipitate at the bottom (B-Floc) and a floating biomat (F-Floc). Experimental results showed HE of 88.3 %, with 67 % and 33 % of the harvested biomass forming F-Floc and B-Floc, respectively. Pre-heating of LRP in a boiling water bath for 10 min (HFB-T10) promoted F-Floc proportion up to 91.6 %. In addition, HFB-T10 showed the highest FAMEs yield of 11.17 g/L of the total used volume, which was significantly higher than that of the centrifuged cells and heat-untreated biomat. Moreover, HFB-T10 showed better iodine value and cetane number of the produced biodiesel.

Integrated municipal wastewater treatment and lipid accumulation by a self-flocculating/floating microalga Limnothrix sp

The present study evaluated the growth, self-flocculation, lipid content, and pollutants removal by Limnothrix sp. BASMWW-9 isolated from municipal wastewater treatment system and cultivated in municipal wastewater. The biomass yield and lipid content after 6 days of cultivation were 1.07 g dw/L and 27.34 %dw, respectively. In addition, its self-flocculating ability reached up to 90 % after harvesting time of 180 min. Moreover, COD,NH3-N, TN, and TP removalefficiencies were 71.65 %, 81.89 %, 74.64 %, and 80.16 %, respectively. The self-flocculation performance of Limnothrix sp. was greatly associated to its morphology and production of extracellular polymeric substances (EPS), with significant positive impact of the high calcium and magnesium content in municipal wastewater. Interestingly, blue light irradiation during harvest enhanced the aggregation and floc formation as a floating biomat, which was attributed to enhanced polysaccharides production. This study provides innovative harvest method for Limnothrix sp. BASMWW-9 cultivated in wastewater using blue light for enhanced lipid recovery.

A comparative life cycle assessment on recycled concrete aggregates modified by accelerated carbonation treatment and traditional methods

Recycling of construction and demolition wastes contributes to achieve carbon summit and carbon neutrality early in the construction industry. Accelerated carbonation is a promising new technology for enhancing the properties of recycled concrete aggregates (RCAs) as well as mitigating global warming. This study performed a comparative life cycle assessment on RCAs modified by accelerated carbonation treatment and traditional methods. The effect of different treatment methods on environmental impacts of concrete was evaluated. The key contributors of environmental impacts for concrete incorporating carbonated RCAs were identified. Moreover, a sensitivity analysis on the transport distance of concrete incorporating carbonated RCAs was conducted. Results demonstrated that incorporating carbonated RCAs could significantly reduce the energy demand, environmental impacts and environmental cost compared with natural aggregate concrete. Accelerated carbonation treatment exhibited greater potential than the normal two-stage crushing and heating treatment in mitigating environmental burden, especially for the global warming potential. Cement production and transportation were the primary contributors to environmental impacts of concrete incorporating carbonated RCAs. Sensitivity analysis indicated incorporating carbonated RCAs as alternatives of natural aggregates contributes to lower the environmental impacts of concrete when the natural aggregates are far from urban areas while the recycling center is near the city.

Bioenergy production from swine wastewater based on a combined process of anaerobic dynamic membrane reactor and microalgae cultivation: Feasibility and performance

Enhanced methane production and sustainable reduction of pollutants from anaerobic digestate are crucial for swine wastewater treatment. In this study, anaerobic dynamic membrane bioreactor (AnDMBR) was introduced to enhance methane production, then microalgae were cultivated on the digestate for nutrients recovery and lipid production. Results showed that pollutants can be effectively removed under various hydraulic retention time (HRT) conditions during long-term operation. Methanogenesis was enhanced with the reduction of HRT from 20 days to 10 days (0.23 L-CH4/g-CODremoved), but inhibited by shortening HRT to 5 days (0.09 L-CH4/g-CODremoved). Ammonia and phosphate in the digestate were effectively removed after microalgae cultivation. In addition, the highest microalgal biomass and lipid productivity (1.7 g/L and 17.5 mg/(L·d), respectively) were obtained using digestate ratio of 20 %, while microalgal growth was seriously restricted at high digestate content (>50 %). This work provides a prospective pathway for pollutants control and energy production from swine wastewater through integrating of AnDMBR technology with microalgae cultivation.

Response surface optimization of product yields and biofuel quality during fast hydrothermal liquefaction of a highly CO2-tolerant microalgae

The effects of biochemical components and processing variables (e.g., temperatures, solid-liquid ratio, ethanol concentration, and time) during fast hydrothermal liquefaction of a highly CO₂-tolerant microalgae (Micractinium sp.) on the product yields and biofuel quality were explored using response surface methodology coupled with central composite design. Results showed that the maximum bio-oil yield (51.4 %) was obtained at 321 °C for 49 min at ethanol concentration of 75 % and solid-liquid ratio of 15.3 %. Among different studied parameters, ethanol concentration showed the highest significant impact on the bio-oil yield due to the low P-value and high F-value in ANOVA analysis. Furthermore, the chemical compositions of bio-oils were determined, which showed that the increase of ethanol concentration in the solvent not only increased the bio-oil yield but also promoted the bio-oil quality by reduction of carboxylic acids and nitrogen-containing compounds with simultaneous enhancement of esters in the bio-oil. The present results show that fast hydrothermal liquefaction is a promising approach to convert the microalgae into high quality biofuels rich in esters.

Maximizing the energy recovery from rice straw through two-step conversion using eggshell-catalytic pyrolysis followed by enhanced anaerobic digestion using calcium-rich biochar

Anaerobic digestion of lignocelluloses for biogas production is greatly restricted by the poor biomass degradability. Herein, a novel approach is suggested to enhance the energy recovery from rice straw through a two-step conversion using eggshell-based catalytic pyrolysis followed by biochar-based anaerobic co-digestion. Pyrolysis with eggshell significantly enhanced the crude bio-oil yield by 4.6 %. Anaerobic digestion of rice straw using 4 g L^-1 of rice straw biochar (RB) showed the highest recorded biogas yield of 503.7 L kg^-1 VS, with 268.6 L kg^-1 VS biomethane yield. However, 4 g L^-1 of calcium-enriched eggshell rice straw biochar (ERB) enhanced the biomethane yield to 281.8 L kg^-1 VS, which represented 95.6 % higher than the control. It was attributed to enhancement of biomethanation, which resulted in 74.5 % maximum recorded biomethane content at the 7th day of anaerobic digestion. Microbial analysis confirmed that Methanosarciniales was the most dominant Archael group in the control (14.84 %), which increased sharply to 73.91 % and 91.66 % after addition of 4 g L^-1 RB and ERB, respectively. The suggested route enhanced the energy recovery in the form of bio-oil and biomethane by 41.6 %.

Valorization of floral foam waste via pyrolysis optimization for enhanced phenols recovery

Utilization of phenol formaldehyde foams is becoming increasingly widespread, especially in floral bouquets, generating toxic microplastics in the environment. The present study evaluated phenols recovery from floral foam waste (FFW) of floral bouquets through optimization of pyrolysis conditions. Compared to the biomass portion in the floral bouquet, FFW showed 55.1% higher carbon content, 56.9% lower nitrogen content, and 44.6% lower oxygen content, with the highest recorded calorific value of 27.43 MJ kg^-1. Thermogravimetric analysis showed the relative thermal stability of FFW with gradual weight loss and numerous small peaks at 70 °C (representing short chain volatiles such as formaldehyde and phenol), 450 and 570 °C (due to phenolic and aromatic products release), indicating the richness of FFW with phenolic compounds. Optimization of pyrolysis conditions showed the highest significant biocrude yield of 36.0% at 700 °C for 20 min using FFW load of 2.5 g. However, optimization of phenolic production suggested 520 °C, 30 min, and 3.49 g FFW load as optimum conditions for high biocrude yield with enhanced phenolic proportion. Experimental results using the aforementioned conditions showed phenolics potential of 0.22 g phenolics/g FFW, with 78.8% phenolic compounds composed mainly of phenol and its methyl derivatives.

Recent Advances in Micro-/Nanoplastic (MNPs) Removal by Microalgae and Possible Integrated Routes of Energy Recovery

Reliance on plastic has resulted in the widespread occurrence of micro-/nanoplastics (MNPs) in aquatic ecosystems, threatening the food web and whole ecosystem functions. There is a tight interaction between MNPs and microalgae, as dominant living organisms and fundamental constituents at the base of the aquatic food web. Therefore, it is crucial to better understand the mechanisms underlying the interactions between plastic particles and microalgae, as well as the role of microalgae in removing MNPs from aquatic ecosystems. In addition, finding a suitable route for further utilization of MNP-contaminated algal biomass is of great importance. The present review article provides an interdisciplinary approach to elucidate microalgae-MNP interactions and subsequent impacts on microalgal physiology. The degradation of plastic in the environment and differences between micro- and nanoplastics are discussed. The possible toxic effects of MNPs on microalgal growth, photosynthetic activity, and morphology, due to physical or chemical interactions, are evaluated. In addition, the potential role of MNPs in microalgae cultivation and/or harvesting, together with further safe routes for biomass utilization in biofuel production, are suggested. Overall, the current article represents a state-of-the-art overview of MNP generation and the consequences of their accumulation in the environment, providing new insights into microalgae integrated routes of plastic removal and bioenergy production.

Enhanced waste hot-pot oil (WHPO) anaerobic digestion for biomethane production: Mechanism and dynamics of fatty acids conversion

Resource depletion and climate changes due to human activities and excessive burning of fossil fuels are the driving forces to explore alternatives clean energy resources. Anaerobic digestion of bio-waste provides a unique opportunity to fulfil this objective through biogas production. The present study aimed to evaluate waste hot-pot oil (WHPO) at different feeding ratios as a novel lipidic waste for anaerobic mono-digestion. The highest recorded maximum biomethane potential (M_max) was 274.1 L kg^-1 VS at 1.2% WHPO, which showed significant differences with those of 0.8% and 1.6% (227.09 and 237.62 L kg^-1 VS, respectively). The changes in volatile fatty acids (VFAs), medium chain fatty acids (MCFAs), and long-chain fatty acids (LCFAs) as intermediates of WHPO decomposition were investigated before and after anaerobic digestion. Results showed efficient production and utilization of VFAs at all studied WHPO ratios, whereas the maximum utilization of VFAs (90-95%) was recorded in the reactors with up to 1.2 %WHPO. Although lipid conversion efficiency decreased by increasing the WHPO ratio, 81.2% lipid conversion efficiency was recorded at the highest applied WHPO treatment, which confirms the potential of WHPO as a promising feedstock for anaerobic digestion. The present results will have major implications towards efficient energy recovery and biochemical management of lipidic-waste through efficient anaerobic digestion.

Enhancement of black and odorous water treatment coupled with accelerated lipid production by microalgae exposed to 12C6+ heavy-ion beam irradiation

In this study, Auxenochlorella protothecoides (AP-CK) was selected due to its reported high growth potential in sterilized black and odorous water (SBOW). In order to improve the resource utilization level of microalgae for wastewater treatment, AP-CK was mutated using ¹²C⁶⁺ heavy-ion beam irradiation, and a high lipid-containing mutant (AP-34#) was isolated and further evaluated to treat original black and odorous water (OBOW). Compared with the wild type, the maximum removal rates of COD, NH₄⁺-N and TP of the mutant increased by 8.12 ± 0.33%, 10.43 ± 0.54% and 11.97 ± 0.16%, respectively, while maximum dissolved oxygen content increased from 0 to 4.36 ± 0.25 mg/L. Besides, the mutant lipid yield increased by 115.87 ± 3.22% over the wild type in OBOW. The fatty acid profile of AP-34# grown in SBOW and OBOW showed higher proportion of saturated fatty acids (C16:0 and C18:0) and valuable polyunsaturated fatty acids (mainly C20:5n3 and C22:6n3) which are more suitable for biodiesel production and value-added products, respectively. This work provides a new perspective on improving the characteristics of microalgae and an innovative approach for resource-based microalgae wastewater treatment through bioremediation of black and odorous water.

Enhanced energy production and biological treatment of swine wastewater using anaerobic membrane bioreactor: Fouling mechanism and microbial community

This study aimed to reveal the membrane fouling mechanisms during anaerobic membrane bioreactor (AnMBR) operation for swine wastewater treatment under different organic loading rates (OLR). Results showed that AnMBR could achieve high pollutant removal (71.9-83.6 %) and energy recovery (0.18-0.23 L-CH₄/g-COD) at an OLR range of 0.25-0.5 g-COD/g-VSS.d, realizing energy production. However, higher OLR would aggravate the membrane fouling due to accumulation of fine sludge particles, organic foulants, and extracellular polymeric substances (EPS) on cake layer. Based on the high-throughput sequencing, microbial communities significantly changed and fouling-causing bacteria (e.g. Pseudomonas, Methanosarcina and Methanothrix) enriched in the cake layer at higher OLR conditions, leading to lower membrane permeability. Backwash can effectively remove the cake layer from the membrane surface and recover membrane permeability. The present study provides important information about membrane fouling and microbial information that could have significant impact on large-scale AnMBR application.

One-step lignocellulose fractionation using acid/pentanol pretreatment for enhanced fermentable sugar and reactive lignin production with efficient pentanol retrievability

To valorize whole lignocellulosic biomass, this study proposed a biphasic solvent system using dilute acid (DA)/pentanol pretreatment. Effects of the key factors, i.e., temperature and pentanol concentration, on aspen were evaluated. Under identified optimal pretreatment conditions (160 °C, 60% pentanol), 85% and 91% of lignin and hemicellulose were solubilized in separate organic and liquid phases, respectively, while 91.1% of cellulose was retained in solid fraction. Enzymatic digestibility efficiency of pretreated cellulose was ∼ 6.4-times higher than that of untreated biomass. Notably, excellent pentanol recovery rates were obtained after four-times recycling (84%) with great cellulose digestibility (81%) and delignification (71%) performance. The recovered lignin contained low levels of contaminated sugars (<1%), while it could stabilize and protect high amounts of β-O-4 bonds. Besides, high phenolic OH content was found in lignin, which could be utilized for lignin-based biomaterials. Therefore, DA/pentanol pretreatment is an innovative promising technology for lignocellulosic valorization towards biorefinery.

Pharmaceutical applications and consequent environmental impacts of

Recently, microalgae cultivation for different applications, including the production of nutritional and pharmaceutical active compounds has received increasing attention. Among the different genera, Spirulina (Arthrospira sp.) is one of the most promising blue-green microalgae (Cyanophyta) because it is rich in antioxidants, essential amino acids (EAAs), minerals, proteins, polyunsaturated fatty acids and vitamins. It has a high protein content (60-70% of the dry weight), which is a complete protein, i.e. containing all EAAs. Therefore, Spirulina is currently a commercial product with high nutritional value and also a significant source of complementary and alternative medicine. The objective of the present work was to review the pharmaceutical and therapeutic applications of Spirulina, especially its antioxidant, anti-inflammatory, anti-cancer, anti-microbial, anti-diabetic, anti-obesity and anti-toxicity properties. The results were obtained from experiments in the literature performed in vitro and in vivo using experimental animals. The main reported active ingredients in Spirulina include phycocyanin, tocopherol, β-carotene, caffeic acids and chlorogenic acid, which showed individual or synergetic effects. In addition, the present review discusses the future perspectives of genetically modified Spirulina as a source for industrial products while producing valuable biomass photoautotrophically. Furthermore, the consequent environmental impacts of large-scale cultivation of Spirulina are discussed.