Advances in direct transesterification of algal oils from wet biomass

Cell rupture of Tetradesmus obliquus using high-pressure homogenization at the pilot scale and recovery of pigments and lipids

Microalgae are promising sources of intracellular metabolites such as proteins, polysaccharides, pigments, and lipids. Thus, this study applied high-pressure homogenization (HPH) techniques on a pilot scale to disrupt the cells of Tetradesmus obliquus. The effects of pressure (P; 150, 250, and 350 bar), suspension concentration (Cs; 1.0, 1.5, and 2.0 % w/v), and number of cycles (Nc; 5, 15, and 25) were evaluated in HPH via a Box-Behnken experimental design. Response surface methodology was applied to optimize the recovery rate (dTr) of pigments and lipids. The specific energy consumption (SEC) and color change gradient (ΔE) of the biomass during HPH were also assessed. The optimal HPH conditions for pigment extraction with 1.5 % Cs (w/v) were as follows: P = 312 bar and Nc = 22 for chlorophyll-a (0.83 g/100 g; dTr = 69 %; SEC = 47.50 kJ/g dry matter); P = 345 bar and Nc = 24 for chlorophyll-b (0.63 g/100 g; dTr = 80 %; SEC = 57.30 kJ/g dry matter); P = 345 bar and Nc = 24 for total carotenoids (0.53 g/100 g; dTr = 79 %; SEC = 54.12 kJ/g dry matter); and P = 350 bar and Nc = 25 for β-carotene (299 µg/g; dTr = 58 %; SEC = 62.08 kJ/g dry matter). The optimal HPH conditions for lipid extraction were P = 350 bar and Nc = 23, with a lipid recovery rate of ≥28 %. Cell disruption during HPH caused a change in the color of the biomass (ΔE) due to the release of intracellular biocompounds. Increasing P and Nc led to higher SECs, ΔE gradients, and pigment and lipid contents. Thus, the levels of recovered pigments and lipids can be indicators of cell disruption in T. obliquus.

Advancements in non-thermal technologies for enhanced extraction of functional triacylglycerols from microalgal biomass: A comprehensive review

Microalgae have emerged as a storehouse of biologically active components having numerous health benefits that can be used in the formulation of nutraceuticals, and functional foods, for human consumption. Among these biologically active components, functional triacylglycerols are increasingly attracting the attention of researchers owing to their beneficial characteristics. Microalgae are excellent sources of triacylglycerol containing omega-3 and omega-6 fatty acids and can be used by the vegan population as a replacement for fish oil. The functional triacylglycerols extracted using conventional processes have various drawbacks resulting in lower yield and inferior quality products. The non-thermal technologies are emerging as user-friendly and environment-friendly technologies that intensify the yield of final products and maintain the high purity of extracted products that can be used in food, cosmetic, pharmaceutical, and nutraceutical applications. The present review focuses on major non-thermal technologies that can probably be used for the extraction of high-quality functional triacylglycerols from microalgae.

Graphene-based catalysts for biodiesel production: Characteristics and performance

Biodiesel is a promising alternative to reduce the dependency on fossil fuels. However, biodiesel's cost is still higher than its petroleum counterpart, hence its production process must be modified to make it economically viable. Microalgae are an alternative feedstock to replace agricultural crops for biodiesel production, and offer several advantages such as fast growth, use of non-arable land, growth in saline and wastewater, and high lipid yield. Unfortunately, biodiesel production from microalgae is very energy-intensive and costly, mainly due to the high energy consumption required for dewatering and drying. Therefore, utilizing wet microalgal biomass instead of dry biomass can be a promising solution to reduce the biodiesel production cost Furthermore, the use of heterogeneous catalysts offers high efficiency, recoverability, and reusability, and is therefore very promising from the economic and environmental perspectives. The unique characteristics of graphene-based nano-catalysts, such as their high surface area, two-dimensional structure, and functional groups, make them suitable candidates for biodiesel production. In this review, the use of graphene-based catalysts for biodiesel production is analyzed in depth, and their efficiency compared to other heterogeneous catalysts is scrutinized. Moreover, their recoverability, reusability, and economic feasibility are critically discussed, and their potential to produce biodiesel from wet microalgae is explored as a sustainable and cost-effective approach.

Progress on Conventional and Advanced Techniques of In Situ Transesterification of Microalgae Lipids for Biodiesel Production

Global warming and the depletion of fossil fuels have spurred many efforts in the quest for finding renewable, alternative sources of fuels, such as biodiesel. Due to its auxiliary functions in areas such as carbon dioxide sequestration and wastewater treatment, the potential of microalgae as a feedstock for biodiesel production has attracted a lot of attention from researchers all over the world. Major improvements have been made from the upstream to the downstream aspects related to microalgae processing. One of the main concerns is the high cost associated with the production of biodiesel from microalgae, which includes drying of the biomass and the subsequent lipid extraction. These two processes can be circumvented by applying direct or in situ transesterification of the wet microalgae biomass, hence substantially reducing the cost. In situ transesterification is considered as a significant improvement to commercially produce biodiesel from microalgae. This review covers the methods used to extract lipids from microalgae and various in situ transesterification methods, focusing on recent developments related to the process. Nevertheless, more studies need to be conducted to further enhance the discussed in situ transesterification methods before implementing them on a commercial scale.

Ellman's method-based determination of acibenzolar-S-methyl in tobacco by transesterification with methanol and SABP2-catalyzed hydrolysis

Acibenzolar-S-methyl (ASM) is the most commercially successful biological antibacterial agent used for crop through systemic acquired resistance (SAR). In this study, a reproducible and accurate procedure, based on the spectrophotometric/microplate reader analysis, has been developed to detect ASM in tobacco leaves. This method involves oxidation of methyl mercaptan by the Ellman's reagent 5,5'-dithio-bis (2-nitrobenzoic acid) (DTNB) to form the yellow derivative 5'-thio-2-nitrobenzoic acid (TNB2-), measurable at 412 nm. Methyl mercaptan can be produced by either the ASM transesterification with methanol or the SA-binding protein 2 (SABP2)-catalyzed ASM hydrolysis. The proportions of methanol, reaction time, temperature, the concentrations of EDTA and DTNB were optimized in a 96-well plate. The calibration curve of ASM was linear over the range of 25.2-315 μg g-1. The results of the intra- and inter-day accuracy and precision data were within the FDA acceptance criteria. With ASM as substrate, the turnover number of SABP2 was determined, with the kcat value of 31.1 min-1 using the Michaelis-Menten equation. In tobacco plants treated with 100 μM ASM, it was decreased as time elapsed in treated tobacco, reaching negligible values 72 h after treatment. The optimized method was applied for the determination of ASM transesterification with methanol and the kinetic data determination of SABP2-catalyzed ASM hydrolysis.

Chlorella vulgaris cultivation in pilot-scale to treat real swine wastewater and mitigate carbon dioxide for sustainable biodiesel production by direct enzymatic transesterification

This study firstly addressed real swine wastewater (RSW) treatment by an indigenous Chlorella vulgaris MBFJNU-1 in 5-m³ outdoor open raceway ponds and then direct enzymatic transesterification of the resulting lipids from the wet biomass for sustainable biodiesel production. Compared to the control group, C. vulgaris MBFJNU-1 at 3% CO₂ achieved higher microalgal biomass (478.5 mg/L) and total fatty acids content (21.3%), higher CO₂ bio-fixation (63.2 mg/L/d) and lipid (9.1 mg/L/d) productivities, and greater nutrients removals (total nitrogen, 82.1%; total phosphorus, 28.4%; chemical oxygen demand, 37.1%). The highest biodiesel conversion (93.3%) was attained by enzymatic transesterification of wet disrupted Chlorella biomass with 5% lipase TL and 5% phospholipase PLA. Moreover, the enzymatic transesterification gave around 83% biodiesel conversion in a 15-L stirred tank bioreactor. Furthermore, the integrated process was a cost-effective approach to treat RSW and mitigate CO₂ for microalgal biodiesel production, based on the mass and energy balances analysis.

Valorisation of algal biomass to value-added metabolites: emerging trends and opportunities

Algal biomass is a promising feedstock for sustainable production of a range of value-added compounds and products including food, feed, fuel. To further augment the commercial value of algal metabolites, efficient valorization methods and biorefining channels are essential. Algal extracts are ideal sources of biotechnologically viable compounds loaded with anti-microbial, anti-oxidative, anti-inflammatory, anti-cancerous and several therapeutic and restorative properties. Emerging technologies in biomass valorisation tend to reduce the significant cost burden in large scale operations precisely associated with the pre-treatment, downstream processing and waste management processes. In order to enhance the economic feasibility of algal products in the global market, comprehensive extraction of multi-algal product biorefinery is envisaged as an assuring strategy. Algal biorefinery has inspired the technologists with novel prospectives especially in waste recovery, carbon concentration/sequestration and complete utilisation of the value-added products in a sustainable closed-loop methodology. This review critically examines the latest trends in the algal biomass valorisation and the expansive feedstock potentials in a biorefinery perspective. The recent scope dynamics of algal biomass utilisation such as bio-surfactants, oleochemicals, bio-stimulants and carbon mitigation have also been discussed. The existing challenges in algal biomass valorisation, current knowledge gaps and bottlenecks towards commercialisation of algal technologies are discussed. This review is a comprehensive presentation of the road map of algal biomass valorisation techniques towards biorefinery technology. The global market view of the algal products, future research directions and emerging opportunities are reviewed.

Bioprospecting lipid-producing microorganisms: From metagenomic-assisted isolation techniques to industrial application and innovations

Traditionally, lipid-producing microorganisms have been obtained via conventional bioprospecting based on isolation and screening techniques, demanding time and effort. Thus, high-throughput sequencing combined with conventional microbiological approaches has emerged as an advanced and rapid strategy for recovering novel oleaginous microorganisms from target environments. This review highlights recent developments in lipid-producing microorganism bioprospecting, following (i) from traditional cultivation techniques to state-of-the-art metagenomics approaches; (ii) related topics on workflow, next-generation sequencing platforms, and knowledge bioinformatics; and (iii) biotechnological potential of the production of docosahexaenoic acid (DHA) by Aurantiochytrium limacinum, arachidonic acid (ARA) by Mortierella alpina and biodiesel by Rhodosporidium toruloides. These three species have been shown to be highly promising and studied in research articles, patents and commercialized products. Trends, innovations and future perspectives of these microorganisms are also addressed. Thus, these microbial lipids allow the development of food, feed and biofuels as alternative solutions to animal and vegetable oils.

A critical review on different harvesting techniques for algal based biodiesel production

The fuels retrieved from renewable sources which are usually employed as both carbon and energy sources are termed as neutral based biofuels. The most promising feedstock from renewable sources with great potentiality in contributing to the inclining energy demand is microalgae. These microalgae can be harnessed readily in terms of obtaining qualitative biodiesel with greater energy consumption under limited operational cost. The process of harvesting or dewatering microalgae could be carried under single or sequential combinations of operations. The major drawback of harvesting such as huge operational cost could be lowered by increasing the level of automation than cost of investments. The present review concentrates and explores on the techno-economic analysis of the microalgal harvesting and dewatering processes on a large scale. Along with these advanced techniques enclosing the utilization of nanoparticles for harvesting has also been explored. And it also adds with the impacts of concerning facts on energy consumption, processing cost and recovery of resources during harvesting.

Review on integrated biofuel production from microalgal biomass through the outset of transesterification route: a cascade approach for sustainable bioenergy

In recent years, microalgal feedstocks have gained immense potential for sustainable biofuel production. Thermochemical, biochemical conversions and transesterification processes are employed for biofuel production. Especially, the transesterification process of lipid molecules to fatty acid alkyl esters (FAAE) is being widely employed for biodiesel production. In the case of the extractive transesterification process, biodiesel is produced from the extracted microalgal oil. Whereas In-situ (reactive) transesterification allows the direct conversion of microalgae to biodiesel avoiding the sequential steps, which subsequently reduces the production cost. Though microalgae have the highest potential to be an alternate renewable feedstock, the minimization of biofuel production cost is still a challenge. The biorefinery approaches that rely on simple cascade processes involving cost-effective technologies are the need of an hour for sustainable bioenergy production using microalgae. At the same time, combining the biorefineries for both (i) high value-low volume (food and health supplements) and (ii) low value- high volume (waste remediation, bioenergy) from microalgae involves regulatory and technical problems. Waste-remediation and algal biorefinery were extensively reviewed in many previous reports. On the other hand, this review focuses on the cascade processes for efficient utilization of microalgae for integrated bioenergy production through the transesterification. Microalgal biomass remnants after the transesterification process, comprising carbohydrates as a major component (process flow A) or the carbohydrate fraction after bio-separation of pretreated microalgae (process flow B) can be utilized for bioethanol production. Therefore, this review concentrates on the cascade flow of integrated bioprocessing methods for biodiesel and bioethanol production through the transesterification and biochemical routes. The review also sheds light on the recent combinatorial approaches of transesterification of microalgae. The applicability of spent microalgal biomass residue for biogas and other applications to bring about zero-waste residue are discussed. Furthermore, techno-economic analysis (TEA), life cycle assessment (LCA) and challenges of microalgal biorefineries are discussed.

Enhanced fatty acid methyl esters recovery through a simple and rapid direct transesterification of freshly harvested biomass of Chlorella vulgaris and Messastrum gracile

Conventional microalgae oil extraction applies physicochemical destruction of dry cell biomass prior to transesterification process to produce fatty acid methyl esters (FAMEs). This report presents a simple and rapid direct transesterification (DT) method for FAMEs production and fatty acid profiling of microalgae using freshly harvested biomass. Results revealed that the FAMEs recovered from Chlorella vulgaris were 50.1 and 68.3 mg with conventional oil-extraction-transesterification (OET) and DT method, respectively. While for Messastrum gracile, the FAMEs recovered, were 49.9 and 76.3 mg, respectively with OET and DT methods. This demonstrated that the DT method increased FAMEs recovery by 36.4% and 53.0% from C. vulgaris and M. gracile, respectively, as compared to OET method. Additionally, the DT method recovered a significantly higher amount of palmitic (C16:0) and stearic (C18:0) acids from both species, which indicated the important role of these fatty acids in the membranes of cells and organelles. The DT method performed very well using a small volume (5 mL) of fresh biomass coupled with a shorter reaction time (~ 15 min), thus making real-time monitoring of FAMEs and fatty acid accumulation in microalgae culture feasible.

Algae: Biomass to Biofuel

Worldwide demand for ethanol alternative fuel has been emerging day by day owing to the rapid population growth and industrialization. Culturing microalgae as an alternative feedstock is anticipated to be a potentially significant approach for sustainable bioethanol biofuel production. Microalgae are abundant in nature, which grow at faster rates with a capability of storing high lipid and starch/cellulose contents inside their cells. This process offers several environmental advantages, including the effective utilization of land, good CO2 sequestration without entering into "food against fuel" dispute. This chapter focuses on the methods and processes used for the production of bioethanol biofuels from algae. Thus, it also covers significant achievements in the research and developments on algae bioethanol production, mainly including pretreatment, hydrolysis, and fermentation of algae biomass. The processes of producing biodiesel, biogas, and hydrogen have also been discussed.

Switchable solvent N, N, N', N'-tetraethyl-1, 3-propanediamine was dissociated into cationic surfactant to promote cell disruption and lipid extraction from wet microalgae for biodiesel production

Switchable solvent N, N, N', N'-tetraethyl-1,3-propanediamine (TEPDA) was proposed to extract lipids from wet Nannochloropsis oceanica with a 5% higher extraction efficiency than chloroform-methanol. It was found that TEPDA acted mainly as an organic solvent to soften and dissolve lipids, while a small amount of TEPDA was dissociated into tertiary amine ion, i.e.,(C₂H₅)₂N-(CH₂)₃-NH⁺(C₂H₅)₂. This cation acted as a surfactant to promote cell disruption and lipid separation. With moisture increasing from 0 to 84 wt%, more TEPDA was dissociated into cationic surfactant to induce local rearrangement of phospholipid bilayers in cell membranes through electrostatic interaction, resulting in the fractal dimension of disrupted cells increased from 1.49 to 1.66. Accordingly, the yield of fatty acid methyl ester (FAME) through transesterification of lipids extracted with TEPDA increased by 9%, while FAME yield from lipids extracted with chloroform and n-hexane decreased by 41% and 65%, respectively.

Recovery of Astaxanthin-Containing Oil from Haematococcus pluvialis by Nano-dispersion and Oil Partitioning

The feasibilities of cell disruption by homogenization-assisted high-pressure nano-dispersion and recovery of astaxanthin-containing oil by oil partitioning in oil-acetone-water solution were examined. The total fatty acid content of Haematococcus pluvialis was 414.6 mg/g cell, and the astaxanthin content was 4.4% of oil. Extra oil was added to the solution in order to recover oil through instability of dispersion status instead of solvent evaporation. A total amount of energy of 0.34 kWh/L was consumed for acetone evaporation at 50 °C, whereas fully 1.86 kWh/L of energy for water evaporation was consumed. When soybean oil was added to the solution after partial acetone evaporation, the oil-recovery yield was 97.8%, while the yield after full evaporation was 97.6% in 10-g/L solution. However, the energy consumed for partial evaporation (0.29 kWh/L) was much lower than that for full evaporation (0.40 kWh/L). When H. pluvialis oil was added to the solution after partial evaporation, the oil-recovery yield decreased to 90.6% due to the impurity of crude H. pluvialis oil in 10-g/L solution. Methods such as refining of H. pluvialis oil, increase of microalgae dosage for cell disruption, and increase of the injection amount of extra oil can help to enhance oil recovery.

Evaluation of CO2-induced azole-based switchable ionic liquid with hydrophobic/hydrophilic reversible transition as single solvent system for coupling lipid extraction and separation from wet microalgae

The utilization of microalgae as bioenergy source was limited by the excessive cost and energy consumption during the process of lipid extraction and separation. CO₂-induced switchable ionic liquids (S-ILs) with reversible hydrophobic-hydrophilic conversion were synthesized and applied for lipid extraction and separation. The reversible transition mechanism of switchable IL is due to the formation of carbamate. The novel approach based on S-ILs was developed for lipid extraction from wet microalgae, which coupled microalgae cell disruption, lipid extraction, separation, and solvent recovery process without additional solvents. The highest lipid extraction efficiencies from wet microalgae were obtained by C₆DIPA-Im, and the lipids were recovered from the extraction phase by simply bubbling CO₂. Furthermore, C₆DIPA-Im maintained more than 83.6 ± 3.6% of its initial lipid extraction efficiency after recycling five times. The S-IL based extraction and separation method provides a new strategy for sustainable bioenergy production.

Algae biorefinery: Review on a broad spectrum of downstream processes and products

Algae biomass comprises variety of biochemicals components such as carbohydrates, lipids and protein, which make them a feasible feedstock for biofuel production. However, high production cost mainly due to algae cultivation remains the main challenge in commercializing algae biofuels. Hence, extraction of other high value-added bioproducts from algae biomass is necessary to enhance the economic feasibility of algae biofuel production. This paper is aims to deliberate the recent developments of conventional technologies for algae biofuels production, such as biochemical and chemical conversion pathways, and extraction of a variety of bioproducts from algae biomass for various potential applications. Besides, life cycle evaluation studies on microalgae biorefinery are presented, focusing on case studies for various cultivation techniques, culture medium, harvesting, and dewatering techniques along with biofuel and bioenergy production pathways. Overall, the algae biorefinery provides new opportunities for valorisation of algae biomass for multiple products synthesis.

Biodiesel Production by Lipids From Indonesian strain of Microalgae Chlorella vulgaris

The fatty acid methyl ester (FAME) production from Chlorella vulgaris has been studied by sequential investigation such as microalgae culturing, lipid extraction, and lipid conversion to FAME. The C. vulgaris could grow well in the BG-11 medium and had a doubling time 3.7 days for its growth using inocula 16% (v/v). The optimum of dry cell biomass as 11.6 g/L was obtained after the microalgae culture harvested for 6 days. Lipid extraction from the biomass was carried out in various solvents and ultrasonication power, resulted lipid as 31% (w/w) when extracted with a mixed solvent of n-hexane-ethanol in ratio 1:1 and ultrasonication treatment at power 25 kHz/270W for 30 min. The lipid then converted to FAME through transesterification reaction with methanol using H2SO4 catalyst at 45ºC for 2 h, and resulted FAME with area 32.26% in GC-MS analysis. The area was corresponded to FAME output as 13.68% (w/w). Fatty acid profiles of FAME obtained from GC-MS analysis showed the major peaks of fatty acids found in Chlorella vulgaris were palmitic acid (C16:0), stearic acid (C18:0) and margaric acid (C17:0), and nonadecanoic acid (C19:0). Optimization of the transesterification reaction will be developed in future to improve the FAME product.

Efficient Biodiesel Conversion from Microalgae Oil of Schizochytrium sp

Abstract: Microalgae oil has been regarded as a promising feedstock for biodiesel production. However, microalgae oil usually contains some non-lipid components, such as pigments. Microalgae oil could be converted to biodiesel effectively with a two-step process to decrease the negative effect caused by by-product glycerol generated in traditional biodiesel production process. Firstly, microalgae oil was hydrolysed to free fatty acids (FFAs) and then FFAs were converted to methyl ester. In this study, the hydrolysis of microalgae oil from Schizochytrium sp. was systematically investigated and microalgae oil could be hydrolysed effectively to FFAs at both non-catalytic and acid-catalytic conditions. The hydrolysis degree of 97.5% was obtained under non-catalytic conditions of 220 °C and a water to oil ratio of 10:1 (w:w). The hydrolysis degree of 97.1% was obtained with the optimized sulphuric acid catalytic conditions of 95 °C, and a ratio of water to oil 3:1. The lipase Novozym435-mediated esterification with the hydrolysed FFAs was explored and a FAME (Fatty Acids Methyl Ester) yield of 95.1% was achieved. The conversion of different FFAs also was compared and the results indicated that lipase Novozym435-mediated methanolysis was effective for the preparation of biodiesel as well as poly unsaturated fatty acids (PUFAs).

Algal biorefinery: A sustainable approach to valorize algal-based biomass towards multiple product recovery

In recent years, ever-increasing socio-economic awareness, and negative impact of excessive petro consumption have redirected the research interests towards bio-resources such as algal-based biomass. In order to meet current bio-economy challenges to produce high-value multiple products at a time, new integrated processes in research and development are necessary. Though various strategies have been posited for conversion of algal-based biomass to fuel and fine chemicals, none of them has been proved as economically viable and energetically feasible. Therefore, a range of other bio-products needs to be pursued. In this context, the algal bio-refinery concept has appeared with notable solution to recover multiple products from a single operation process. Herein, an algal-based bio-refinery platform for fuel, food, and pharmaceuticals considering Bio-refinery Complexity Index (BCI) has been evaluated, as an indicator of techno-economic risks. This review presents recent developments on algal-biomass utilization for various value-added products as part of an integrated bio-refinery.

Biodiesel Production (FAEEs) by Heterogeneous Combi-Lipase Biocatalysts Using Wet Extracted Lipids from Microalgae

The production of fatty acids ethyl esters (FAEEs) to be used as biodiesel from oleaginous microalgae shows great opportunities as an attractive source for the production of renewable fuels without competing with human food. To ensure the economic viability and environmental sustainability of the microbial biomass as a raw material, the integration of its production and transformation into the biorefinery concept is required. In the present work, lipids from wet Isochrysis galbana microalga were extracted with ethyl acetate with and without drying the microalgal biomass (dry and wet extraction method, respectively). Then, FAEEs were produced by lipase-catalyzed transesterification and esterification of the extracted lipids with ethanol using lipase B from Candida antarctica (CALB) and Pseudomonas cepacia (PC) lipase supported on SBA-15 mesoporous silica functionalized with amino groups. The conversion to FAEEs with CALB (97 and 85.5 mol% for dry and wet extraction, respectively) and PC (91 and 87 mol%) biocatalysts reached higher values than those obtained with commercial Novozym 435 (75 and 69.5 mol%). Due to the heterogeneous nature of the composition of microalgae lipids, mixtures with different CALB:PC biocatalyst ratio were used to improve conversion of wet-extracted lipids. The results showed that a 25:75 combi-lipase produced a significantly higher conversion to FAEEs (97.2 mol%) than those produced by each biocatalyst independently from wet-extracted lipids and similar ones than those obtained by each lipase from the dry extraction method. Therefore, that optimized combi-lipase biocatalyst, along with achieving the highest conversion to FAEEs, would allow improving viability of a biorefinery since biodiesel production could be performed without the energy-intensive step of biomass drying.

Mexican Microalgae Biodiversity and State-Of-The-Art Extraction Strategies to Meet Sustainable Circular Economy Challenges: High-Value Compounds and Their Applied Perspectives

In recent years, the demand for naturally derived products has hiked with enormous pressure to propose or develop state-of-the-art strategies to meet sustainable circular economy challenges. Microalgae possess the flexibility to produce a variety of high-value products of industrial interests. From pigments such as phycobilins or lutein to phycotoxins and several polyunsaturated fatty acids (PUFAs), microalgae have the potential to become the primary producers for the pharmaceutical, food, and agronomical industries. Also, microalgae require minimal resources to grow due to their autotrophic nature or by consuming waste matter, while allowing for the extraction of several valuable side products such as hydrogen gas and biodiesel in a single process, following a biorefinery agenda. From a Mexican microalgae biodiversity perspective, more than 70 different local species have been characterized and isolated, whereas, only a minimal amount has been explored to produce commercially valuable products, thus ignoring their potential as a locally available resource. In this paper, we discuss the microalgae diversity present in Mexico with their current applications and potential, while expanding on their future applications in bioengineering along with other industrial sectors. In conclusion, the use of available microalgae to produce biochemically revenuable products currently represents an untapped potential that could lead to the solution of several problems through green technologies. As such, if the social, industrial and research communities collaborate to strive towards a greener economy by preserving the existing biodiversity and optimizing the use of the currently available resources, the enrichment of our society and the solution to several environmental problems could be attained.

Emerging techniques for cell disruption and extraction of valuable bio-molecules of microalgae Nannochloropsis sp

Microalgae of Nannochloropsis sp. present valuable source of bio-molecules (pigments, lipids, proteins) that have nutritional potential for the prevention and treatment of human diseases. Moreover, some species of Nannochloropsis are the promising sources of biofuels and excellent candidates for the replacement of classical biofuel crops. This review describes and compares the efficiency of different conventional and novel techniques that can be used for cell disruption and recovery of bio-molecules from Nannochloropsis sp. Classification of different extraction techniques includes chemical, enzymatic, mechanical and other physical methods. The detailed analysis of extraction efficiency assisted by pressure and temperature (subcritical and supercritical fluids, hydrothermal liquefaction), ultrasound, microwaves, and pulsed electric energy (pulsed electric fields and high voltage electrical discharges) is presented. The general discussion includes comparison between techniques, their effectiveness for cell disruption and selectivity of bio-molecules extraction from Nannochloropsis sp. The cost-effectiveness, benefits and limitations of different techniques are also analyzed.

Cost-effective biodiesel production from wet microalgal biomass by a novel two-step enzymatic process

In this study, a novel two-step enzymatic process was firstly established to produce microalgae biodiesel using wet Chlorella biomass. In the first hydrolysis step, to reduce energy consumption and effectively disrupt microalgal cell wall, among cellulase, hemicellulase, papain, lysozyme and pectinase, the highest hydrolysis efficiency (67.52%) was obtained by cellulase at pH 5.0 with enzyme dosage of 200 U/g dry biomass at 40 °C for 12 h. In the second transesterification step, compared with liquid CAL-A/B from Candida antarctica and PLA from Aspergillus oryzae, liquid lipase TL from Thermomyces lanuginosus achieved the highest biodiesel conversion at 81.15:1 (v/w) ethanol/g TFAs ratio in 78-83% water content with 100 PLU/g TFAs lipase loading at 25 °C for 48 h. Moreover, similar results were obtained with three Chlorella species by this process. Overall, this two-step enzymatic process was a green, low-energy and efficient method for cost-effective biodiesel production using wet microalgal biomass.

Ultrasound-assisted in-situ transesterification of wet Aurantiochytrium sp. KRS 101 using potassium carbonate

A new in-situ transesterification method was developed for wet biomass: K₂CO₃ was used as an alkaline catalyst and, Aurantiochytrium sp. KRS 101 as oleaginous DHA-producing microalgae. It was found that the presence of water greatly impaired the overall efficiency even with the powerful catalyst that had worked surpassingly well with dry biomass, and thus a mechanical aid like ultrasonication was needed to make advantage of full potential of the alkaline catalyst. The total fatty acid ethyl ester (FAEE) recovery yield of 94.6% was achieved with sonication at 100 g/L of biomass (40% moisture), 3% of K₂CO₃, 70 °C and 30 min. All these suggest that the ultrasound assisted in-situ transesterification can offer a feasible means for FAEE recovery and it was so by way of overcoming the physical limitation of mass transfer caused the presence of water and providing effective contacts between reactants.

Conversion and recovery of saponifiable lipids from microalgae using a nonpolar solvent via lipase-assisted extraction

A single-step method for transesterifying and recovering lipids in concentrated slurries (ca 20% w/w solids) of ruptured microalgae is presented. A soluble Rhizomucor miehei lipase (RML) was used to directly transesterify the lipids in the marine microalgae Nannochloropsis salina. This allowed both triglycerides (TAG) and polar saponifiable lipids to be recovered as fatty acid methyl esters (FAME) using a nonpolar solvent (hexane). Up to 90 wt% of the total saponifiable lipids (SL) were converted to FAME within 24 h, approximately 75% of which was recovered in the hexane by centrifugation. Two pathways for the conversion and recovery of polar lipids were identified. The water in the slurry buffered against potential lipase inhibition by methanol, but necessitated a high methanol dose for maximal FAME conversion. Nonetheless the method enables the recovery of polar lipids as FAME while avoiding the need for both drying of the biomass and a downstream transesterification step.

Wet in situ transesterification of spent coffee grounds with supercritical methanol for the production of biodiesel

This work introduces biodiesel production from wet spent coffee grounds (SCGs) with supercritical methanol without any pre-drying process. Supercritical methanol and subcritical water effectively produced biodiesel via in situ transesterification by inducing more porous SCG and enhancing the efficiency of lipid extraction and conversion. It was also found that space loading was one of the critical factors for biodiesel production. An optimal biodiesel yield of 10.17 wt% of dry SCG mass (86.33 w/w% of esterifiable lipids in SCG) was obtained at reaction conditions of 270 °C, 90 bars, methanol to wet SCG ratio 5:1, space loading 58.4 ml/g and reaction time 20 min. Direct use of wet SCG waste as feedstock for supercritical biodiesel production eliminates the conventional dying process and the need of catalyst and also reduces environmental problems caused by landfill accumulation.

Recent developments of downstream processing for microbial lipids and conversion to biodiesel

With increasing global population and depleting resources, there is an apparent demand for radical unprecedented innovation to satisfy the basal needs of lives. Hence, non-conventional renewable energy resources like biodiesel have been worked out in past few decades. Biofuel (e.g. Biodiesel) serves to be the most sustainable answer to solve "food vs. fuel crisis". In biorefinery process, lipid extraction from oleaginous microbial lipids is an integral part as it facilitates the release of fatty acids. Direct lipid extraction from wet cell-biomass is favorable in comparison to dry-cell biomass because it eliminates the application of expensive dehydration. However, this process is not commercialized yet, instead, it requires intensive research and development in order to establish robust approaches for lipid extraction that can be practically applied on an industrial scale. This review aims for the critical presentation on cell disruption, lipid recovery and purification to support extraction from wet cell-biomass for an efficient transesterification.

Economic assessment of biodiesel production from wastewater sludge

Currently, there are mainly two pathways of the biodiesel production from wastewater sludge including 1) directly extracting the lipid in sludge and then converting the lipid to biodiesel through trans-esterification, and 2) employing sludge as medium to cultivate oleaginous microorganism to accumulate lipid and then transferring the lipid to biodiesel. So far, the study was still in research stage and its cost feasibility was not yet investigated. In this study, biodiesel production from wastewater sludge was designed and the cost was estimated with SuperPro Designer. With consideration of converting the lipid in raw sludge to biodiesel, the unit production cost was 0.67 US $/kg biodiesel (0.59 US $/L biodiesel). When the sludge was used as medium to grow oleaginous microorganism to accumulate lipid for producing biodiesel, the unit production cost was 1.08 US $/kg biodiesel (0.94 US $/L biodiesel). The study showed that sludge has great potential in biodiesel production.

Industrial potential of carotenoid pigments from microalgae: Current trends and future prospects

Microalgae are rich source of various bioactive molecules such as carotenoids, lipids, fatty acids, hydrocarbons, proteins, carbohydrates, amino acids, etc. and in recent Years carotenoids from algae gained commercial recognition in the global market for food and cosmeceutical applications. However, the production of carotenoids from algae is not yet fully cost effective to compete with synthetic ones. In this context the present review examines the technologies/methods in relation to mass production of algae, cell harvesting for extraction of carotenoids, optimizing extraction methods etc. Research studies from different microalgal species such as Spirulina platensis, Haematococcus pluvialis, Dunaliella salina, Chlorella sps., Nannochloropsis sps., Scenedesmus sps., Chlorococcum sps., Botryococcus braunii and Diatoms in relation to carotenoid content, chemical structure, extraction and processing of carotenoids are discussed. Further these carotenoid pigments, are useful in various health applications and their use in food, feed, nutraceutical, pharmaceutical and cosmeceutical industries was briefly touched upon. The commercial value of algal carotenoids has also been discussed in this review. Possible recommendations for future research studies are proposed.

Microalgae as feedstock for biodiesel production under ultrasound treatment - A review

The application of ultrasound in biodiesel production has recently emerged as a novel technology. Ultrasound treatment enhances the mass transfer characteristics leading to the increased reaction rate with short reaction time and potentially reduces the production cost. In this review, application of ultrasound-assisted biodiesel production using acid, base and enzyme catalysts is presented. A critical assessment of the current status of ultrasound in biodiesel production was discussed with the emphasis on using ultrasound for efficient microalgae biodiesel production. The ultrasound in the biodiesel production enhances the emulsification of immiscible liquid reactant by microturbulence generated by cavitation bubbles. The major benefit of the ultrasound-assisted biodiesel production is a reduction in reaction time. Several different methods have been discussed to improve the biodiesel production. Overall, this review focuses on the current understanding of the application of ultrasound in biodiesel production from microalgae and to provide insights into future developments.

Catalyst-free production of alkyl esters from microalgae via combined wet in situ transesterification and hydrothermal liquefaction (iTHL)

This study introduces a process combining wet in situ transesterification and hydrothermal liquefaction (iTHL) for fatty acid ethyl ester (FAEE) production from intact microalgae, Nannochloropsis gaditana without catalyst at temperatures higher than 160°C. It is found that the chlorinated hydrocarbon solvents, SolvCl (dichloromethane, chloroform, and dichloroethane (DCE)), enhances the FAEE production by providing hydrogen chloride in an ionized form that can act as an acid catalyst. The SolvCl effect on iTHL is compared to acid catalyst assisted wet in situ transesterification. The most effective solvent is DCE with the FAEE selectivity in biocrude equal to 91.85% (maximum transesterifiable lipid basis). The optimum point for maximizing the FAEE yield is 185.08°C with 4.69mL ethanol and 1.98mL DCE/g of dry algal cells based on the response surface methodology. iTHL with both Nannochloropsis and Chlorella species provides a possibility of the process applicable to the other algal species.