Engineering challenges in biodiesel production from microalgae

Utilizing CO2 in industrial off-gas for microalgae cultivation: considerations and solutions

The utilization of microalgae to treat carbon dioxide (CO2)-rich industrial off-gas has been suggested as both beneficial for emissions reduction and economically favorable for the production of microalgal products. Common sources of off-gases include coal combustion (2-15% CO2), cement production (8-15% CO2), coke production (18-23% CO2), and ore smelting (6-7% CO2). However, industrial off-gas also commonly contains other acid gas components [typically nitrogen oxides (NOX) and sulfur dioxide (SO2)] and metals that could inhibit microalgae growth and productivity. To utilize industrial off-gas effectively in microalgae cultivation systems, a number of solutions have been proposed to overcome potential inhibitions. These include bioprospecting to identify suitable strains, genetic modification to improve specific cellular characteristics, chemical additions, and bioreactor designs and operating procedures.In this review, results from microalgae experiments related to utilizing off-gas are presented, and the outcomes of different conditions discussed along with potential solutions to resolve limitations associated with the application of off-gas.

Microalgae as Contributors to Produce Biopolymers

Biopolymers are very favorable materials produced by living organisms, with interesting properties such as biodegradability, renewability, and biocompatibility. Biopolymers have been recently considered to compete with fossil-based polymeric materials, which rase several environmental concerns. Biobased plastics are receiving growing interest for many applications including electronics, medical devices, food packaging, and energy. Biopolymers can be produced from biological sources such as plants, animals, agricultural wastes, and microbes. Studies suggest that microalgae and cyanobacteria are two of the promising sources of polyhydroxyalkanoates (PHAs), cellulose, carbohydrates (particularly starch), and proteins, as the major components of microalgae (and of certain cyanobacteria) for producing bioplastics. This review aims to summarize the potential of microalgal PHAs, polysaccharides, and proteins for bioplastic production. The findings of this review give insight into current knowledge and future direction in microalgal-based bioplastic production considering a circular economy approach. The current review is divided into three main topics, namely (i) the analysis of the main types and properties of bioplastic monomers, blends, and composites; (ii) the cultivation process to optimize the microalgae growth and accumulation of important biobased compounds to produce bioplastics; and (iii) a critical analysis of the future perspectives on the field.

Interplay between interfacial behaviour, cell structure and shear enables biphasic lipid extraction from whole diatom cells (Navicula sp.)

HYPOTHESIS

Bacillariophyceae (i.e., diatoms) are an important class of algae with potential use in the production of proteins and lipids including long-chain ω-3 polyunsaturated fatty acids. Biphasic extraction of microalgae lipids using water-immiscible solvents such as hexane, can avoid the excessive energy required to distil solvents from water, but generally requires energy-intensive rupture of the cells. The unique cell structure and surface chemistry of diatoms compared to other microalgae species might allow biphasic lipid extraction without prior cell rupture.

EXPERIMENTS

The kinetics of biphasic lipid extraction from intact Navicula sp. cells was investigated during low-shear and high-shear mixing, and with prior or simultaneous application of ultrasound (20 kHz at 0.57 W/mL). Dynamic interfacial tension measurements and electron microscopic analysis were used to investigate lipid extraction in relation to interfacial behaviour and cell structure.

RESULTS

High yields (>80%) of intracellular lipids were extracted from intact cells over the course of hours upon low-shear contacting with hexane. The cells associated with and stabilised the hexane-water interface, allowing hexane to infiltrate pores in the frustule component of the cell walls and access the intracellular lipids. It was shown that mucilaginous extracellular polymeric substances (EPS) bound to the cell walls acted as a barrier to solvent penetration into the cells. This EPS could be removed by prior ultrasonication. Biphasic extraction was greatly accelerated by shear applied by rotor-stator mixing or ultrasound. High-shear could remove mucilaginous EPS from the cell surfaces to facilitate direct contact of the cell surface with hexane and produced smaller emulsion droplets with increased surface area. The combination of high-shear in the presence of hexane resulted in the in-situ rupture of the cells, which greatly accelerated lipid extraction and allowed high yields of neutral lipid (>95%) to be recovered from freshly harvested cells within less than 5 min. The study demonstrated the ability of shear to enable simultaneous cell rupture and lipid extraction from a diatom alga based on its cell structure and interfacial behaviour.

Aquatic Plants, Landoltia punctata, and Azolla filiculoides as Bio-Converters of Wastewater to Biofuel

The aquatic plants, Azolla filiculoides, and Landoltia punctate, were used as complementing phytoremediators of wastewater containing high levels of phosphate, which simulates the effluents from textile, dyeing, and laundry detergent industries. Their complementarities are based on differences in capacities to uptake nitrogen and phosphate components from wastewater. Sequential treatment by L. punctata followed by A. filiculoides led to complete removal of NH4, NO3, and up to 93% reduction of PO4. In experiments where L. punctata treatment was followed by fresh L. punctata, PO4 concentration was reduced by 65%. The toxicity of wastewater assessed by shrimps, Paratya australiensis, showed a four-fold reduction of their mortality (LC50 value) after treatment. Collected dry biomass was used as an alternative carbon source for heterotrophic marine protists, thraustochytrids, which produced up to 35% dry weight of lipids rich in palmitic acid (50% of total fatty acids), the key fatty acid for biodiesel production. The fermentation of treated L. punctata biomass by Enterobacter cloacae yielded up to 2.14 mol H2/mole of reduced sugar, which is comparable with leading terrestrial feedstocks. A. filiculoides and L. punctata can be used as a new generation of feedstock, which can treat different types of wastewater and represent renewable and sustainable feedstock for bioenergy production.

Vortex Fluidic Ethenolysis, Integrating a Rapid Quench of Ruthenium Olefin Metathesis Catalysts

Ruthenium-catalysed ethenolysis occurs in a vortex fluidic device (VFD) – a scalable, thin-film microfluidic continuous flow process. This process takes advantage of the efficient mass transfer of gaseous reagents into the dynamic thin film of liquid. Also reported is the rapid quenching of the ruthenium-based olefin metathesis catalyst by the addition of a saturated solution of N-acetyl-l-cysteine in MeCN, as a convenient alternative to previously reported quenching methods.

Enhancing oil production and harvest by combining the marine alga Nannochloropsis oceanica and the oleaginous fungus Mortierella elongata

BACKGROUND

Although microalgal biofuels have potential advantages over conventional fossil fuels, high production costs limit their application in the market. We developed bio-flocculation and incubation methods for the marine alga, Nannochloropsis oceanica CCMP1779, and the oleaginous fungus, Mortierella elongata AG77, resulting in increased oil productivity.

RESULTS

By growing separately and then combining the cells, the M. elongata mycelium could efficiently capture N. oceanica due to an intricate cellular interaction between the two species leading to bio-flocculation. Use of a high-salt culture medium induced accumulation of triacylglycerol (TAG) and enhanced the contents of polyunsaturated fatty acids (PUFAs) including arachidonic acid and docosahexaenoic acid in M. elongata. To increase TAG productivity in the alga, we developed an effective, reduced nitrogen-supply regime based on ammonium in environmental photobioreactors. Under optimized conditions, N. oceanica produced high levels of TAG that could be indirectly monitored by following chlorophyll content. Combining N. oceanica and M. elongata to initiate bio-flocculation yielded high levels of TAG and total fatty acids, with ~ 15 and 22% of total dry weight (DW), respectively, as well as high levels of PUFAs. Genetic engineering of N. oceanica for higher TAG content in nutrient-replete medium was accomplished by overexpressing DGTT5, a gene encoding the type II acyl-CoA:diacylglycerol acyltransferase 5. Combined with bio-flocculation, this approach led to increased production of TAG under nutrient-replete conditions (~ 10% of DW) compared to the wild type (~ 6% of DW).

CONCLUSIONS

The combined use of M. elongata and N. oceanica with available genomes and genetic engineering tools for both species opens up new avenues to improve biofuel productivity and allows for the engineering of polyunsaturated fatty acids.

Lipid production in aquatic plant Azolla at vegetative and reproductive stages and in response to abiotic stress

The aquatic plant Azolla became increasingly popular as bioenergy feedstock because of its high growth rate, production of biomass with high levels of biofuel-producing molecules and ability to grow on marginal lands. In this study, we analysed the contribution of all organs of Azolla to the total yield of lipids at vegetative and reproductive stages and in response to stress. Triacylglycerol-containing lipid droplets were detected in all (vegetative and reproductive) organs with the highest level in the male microsporocarps and microspores. As a result, significantly higher total yields of lipids were detected in Azolla filiculoides and Azolla pinnata at the reproductive stage. Starving changed the yield and composition of the fatty acid as a result of re-direction of carbon flow from fatty acid to anthocyanin pathways. The composition of lipids, in regard the length and degree of unsaturation of fatty acids, in Azolla meets most of the important requirements for biodiesel standards. The ability of Azolla to grow on wastewaters, along with their high productivity rate, makes it an attractive feedstock for the production of biofuels.

Assessment of Chlorella vulgaris and indigenous microalgae biomass with treated wastewater as growth culture medium

The aim of the present work was to evaluate the feasibility of microalgae cultivation using secondary treated domestic wastewater. Two Chlorella vulgaris strains (CICESE and UTEX) and an indigenous consortium, were cultivated on treated wastewater enriched with and without the fertilizer Bayfolan®. Biomass production for C. vulgaris UTEX, CICESE and the indigenous consortium grown in treated wastewater was 1.167±0.057, 1.575±0.434 and 1.125±0.250g/L, with a total lipid content of 25.70±1.24, 23.35±3.01and 20.54±1.23% dw, respectively. The fatty acids profiles were mainly composed of C16 and C18. Regardless of the media used, in all three strains unsaturated fatty acids were the main FAME (fatty acids methyl esters) accumulated in a range of 45-62%. An enrichment of treated wastewater with Bayfolan® significantly increased the production of biomass along with an increase in pigments and proteins of ten and threefold, respectively.

A novel culture medium designed for the simultaneous enhancement of biomass and lipid production by Chlorella vulgaris UTEX 26

A novel culture medium to enhance the biomass and lipid production simultaneously by Chlorella vulgaris UTEX 26 was designed in three stages of optimization. Initially, a culture medium was inferred applying the response surface method to adjust six factors [NaNO3, NH4HCO3, MgSO4·7H2O, KH2PO4, K2HPO4 and (NH4)2HPO4], which were selected on the basement of BBM (Bold's Basal Medium) and HAMGM (Highly Assimilable Minimal Growth Medium) culture media. Afterwards, the nitrogen source compound was optimized to reduce both, ammonium and nitrate concentrations. As result of the optimization process, the proposed culture medium improved 40% the biomass (0.73gL(-1)) compared with the BBM medium and 85% the lipid concentration (281mgL(-1)), with respect to HAMGM medium. Some culture media components concentrations were reduced up to 50%. Gas chromatography analysis revealed that C16:0, C18:0, C18:1, C18:2 and C18:3 were the major fatty acids produced by C. vulgaris UTEX 26.

Escherichia coli as a fatty acid and biodiesel factory: current challenges and future directions

Biodiesel has received widespread attention as a sustainable, environment-friendly, and alternative source of energy. It can be derived from plant, animal, and microbial organisms in the form of vegetable oil, fats, and lipids, respectively. However, biodiesel production from such sources is not economically feasible due to extensive downstream processes, such as trans-esterification and purification. To obtain cost-effective biodiesel, these bottlenecks need to be overcome. Escherichia coli, a model microorganism, has the potential to produce biodiesel directly from ligno-cellulosic sugars, bypassing trans-esterification. In this process, E. coli is engineered to produce biodiesel using metabolic engineering technology. The entire process of biodiesel production is carried out in a single microbial cell, bypassing the expensive downstream processing steps. This review focuses mainly on production of fatty acid and biodiesel in E. coli using metabolic engineering approaches. In the first part, we describe fatty acid biosynthesis in E. coli. In the second half, we discuss bottlenecks and strategies to enhance the production yield. A complete understanding of current developments in E. coli-based biodiesel production and pathway optimization strategies would reduce production costs for biofuels and plant-derived chemicals.

Aquatic plant Azolla as the universal feedstock for biofuel production

BACKGROUND

The quest for sustainable production of renewable and cheap biofuels has triggered an intensive search for domestication of the next generation of bioenergy crops. Aquatic plants which can rapidly colonize wetlands are attracting attention because of their ability to grow in wastewaters and produce large amounts of biomass. Representatives of Azolla species are some of the fastest growing plants, producing substantial biomass when growing in contaminated water and natural ecosystems. Together with their evolutional symbiont, the cyanobacterium Anabaena azollae, Azolla biomass has a unique chemical composition accumulating in each leaf including three major types of bioenergy molecules: cellulose/hemicellulose, starch and lipids, resembling combinations of terrestrial bioenergy crops and microalgae.

RESULTS

The growth of Azolla filiculoides in synthetic wastewater led up to 25, 69, 24 and 40 % reduction of NH₄-N, NO₃-N, PO₄-P and selenium, respectively, after 5 days of treatment. This led to a 2.6-fold reduction in toxicity of the treated wastewater to shrimps, common inhabitants of wetlands. Two Azolla species, Azolla filiculoides and Azolla pinnata, were used as feedstock for the production of a range of functional hydrocarbons through hydrothermal liquefaction, bio-hydrogen and bio-ethanol. Given the high annual productivity of Azolla, hydrothermal liquefaction can lead to the theoretical production of 20.2 t/ha-year of bio-oil and 48 t/ha-year of bio-char. The ethanol production from Azolla filiculoides, 11.7 × 10³ L/ha-year, is close to that from corn stover (13.3 × 10³ L/ha-year), but higher than from miscanthus (2.3 × 10³ L/ha-year) and woody plants, such as willow (0.3 × 10³ L/ha-year) and poplar (1.3 × 10³ L/ha-year). With a high C/N ratio, fermentation of Azolla biomass generates 2.2 mol/mol glucose/xylose of hydrogen, making this species a competitive feedstock for hydrogen production compared with other bioenergy crops.

CONCLUSIONS

The high productivity, the ability to grow on wastewaters and unique chemical composition make Azolla species the most attractive, sustainable and universal feedstock for low cost, low energy demanding, near zero maintenance system for the production of a wide spectrum of renewable biofuels.

Fungal-assisted algal flocculation: application in wastewater treatment and biofuel production

BACKGROUND

The microalgal-based industries are facing a number of important challenges that in turn affect their economic viability. Arguably the most important of these are associated with the high costs of harvesting and dewatering of the microalgal cells, the costs and sustainability of nutrient supplies and costly methods for large scale oil extraction. Existing harvesting technologies, which can account for up to 50% of the total cost, are not economically feasible because of either requiring too much energy or the addition of chemicals. Fungal-assisted flocculation is currently receiving increased attention because of its high harvesting efficiency. Moreover, some of fungal and microalgal strains are well known for their ability to treat wastewater, generating biomass which represents a renewable and sustainable feedstock for bioenergy production.

RESULTS

We screened 33 fungal strains, isolated from compost, straws and soil for their lipid content and flocculation efficiencies against representatives of microalgae commercially used for biodiesel production, namely the heterotrophic freshwater microalgae Chlorella protothecoides and the marine microalgae Tetraselmis suecica. Lipid levels and composition were analyzed in fungal-algal pellets grown on media containing alternative carbon, nitrogen and phosphorus sources from wheat straw and swine wastewater, respectively. The biomass of fungal-algal pellets grown on swine wastewater was used as feedstock for the production of value-added chemicals, biogas, bio-solids and liquid petrochemicals through pyrolysis. Co-cultivation of microalgae and filamentous fungus increased total biomass production, lipid yield and wastewater bioremediation efficiency.

CONCLUSION

Fungal-assisted microalgal flocculation shows significant potential for solving the major challenges facing the commercialization of microalgal biotechnology, namely (i) the efficient and cost-effective harvesting of freshwater and seawater algal strains; (ii) enhancement of total oil production and optimization of its composition; (iii) nutrient supply through recovering of the primary nutrients, nitrogen and phosphates and microelements from wastewater. The biomass generated was thermochemically converted into biogas, bio-solids and a range of liquid petrochemicals including straight-chain C12 to C21 alkanes which can be directly used as a glycerine-free component of biodiesel. Pyrolysis represents an efficient alternative strategy for biofuel production from species with tough cell walls such as fungi and fungal-algal pellets.

Lipid production in association of filamentous fungi with genetically modified cyanobacterial cells

BACKGROUND

Numerous strategies have evolved recently for the generation of genetically modified or synthetic microalgae and cyanobacteria designed for production of ethanol, biodiesel and other fuels. In spite of their obvious attractiveness there are still a number of challenges that can affect their economic viability: the high costs associated with (1) harvesting, which can account for up to 50 % of the total biofuel's cost, (2) nutrients supply and (3) oil extraction. Fungal-assisted bio-flocculation of microalgae is gaining increasing attention due to its high efficiency, no need for added chemicals and low energy inputs. The implementation of renewable alternative carbon, nitrogen and phosphorus sources from agricultural wastes and wastewaters for growing algae and fungi makes this strategy economically attractive.

RESULTS

This work demonstrates that the filamentous fungi, Aspergillus fumigatus can efficiently flocculate the unicellular cyanobacteria Synechocystis PCC 6803 and its genetically modified derivatives that have been altered to enable secretion of free fatty acids into growth media. Secreted free fatty acids are potentially used by fungal cells as a carbon source for growth and ex-novo production of lipids. For most of genetically modified strains the total lipid yields extracted from the fungal-cyanobacterial pellets were found to be higher than additive yields of lipids and total free fatty acids produced by fungal and Synechocystis components when grown in mono-cultures. The synergistic effect observed in fungal-Synechocystis associations was also found in bioremediation rates when animal husbandry wastewater was used an alternative source of nitrogen and phosphorus.

CONCLUSION

Fungal assisted flocculation can complement and assist in large scale biofuel production from wild-type and genetically modified Synechocystis PCC 6803 strains by (1) efficient harvesting of cyanobacterial cells and (2) producing of high yields of lipids accumulated in fungal-cyanobacterial pellets.

Co-cultivation of fungal and microalgal cells as an efficient system for harvesting microalgal cells, lipid production and wastewater treatment

The challenges which the large scale microalgal industry is facing are associated with the high cost of key operations such as harvesting, nutrient supply and oil extraction. The high-energy input for harvesting makes current commercial microalgal biodiesel production economically unfeasible and can account for up to 50% of the total cost of biofuel production. Co-cultivation of fungal and microalgal cells is getting increasing attention because of high efficiency of bio-flocculation of microalgal cells with no requirement for added chemicals and low energy inputs. Moreover, some fungal and microalgal strains are well known for their exceptional ability to purify wastewater, generating biomass that represents a renewable and sustainable feedstock for biofuel production. We have screened the flocculation efficiency of the filamentous fungus A. fumigatus against 11 microalgae representing freshwater, marine, small (5 µm), large (over 300 µm), heterotrophic, photoautotrophic, motile and non-motile strains. Some of the strains are commercially used for biofuel production. Lipid production and composition were analysed in fungal-algal pellets grown on media containing alternative carbon, nitrogen and phosphorus sources contained in wheat straw and swine wastewater, respectively. Co-cultivation of algae and A. fumigatus cells showed additive and synergistic effects on biomass production, lipid yield and wastewater bioremediation efficiency. Analysis of fungal-algal pellet's fatty acids composition suggested that it can be tailored and optimised through co-cultivating different algae and fungi without the need for genetic modification.

Algal biomass analysis by laser-based analytical techniques--a review

Algal biomass that is represented mainly by commercially grown algal strains has recently found many potential applications in various fields of interest. Its utilization has been found advantageous in the fields of bioremediation, biofuel production and the food industry. This paper reviews recent developments in the analysis of algal biomass with the main focus on the Laser-Induced Breakdown Spectroscopy, Raman spectroscopy, and partly Laser-Ablation Inductively Coupled Plasma techniques. The advantages of the selected laser-based analytical techniques are revealed and their fields of use are discussed in detail.

Methylcrotonyl-CoA Carboxylase Regulates Triacylglycerol Accumulation in the Model Diatom Phaeodactylum tricornutum

The model marine diatom Phaeodactylum tricornutum can accumulate high levels of triacylglycerols (TAGs) under nitrogen depletion and has attracted increasing attention as a potential system for biofuel production. However, the molecular mechanisms involved in TAG accumulation in diatoms are largely unknown. Here, we employed a label-free quantitative proteomics approach to estimate differences in protein abundance before and after TAG accumulation. We identified a total of 1193 proteins, 258 of which were significantly altered during TAG accumulation. Data analysis revealed major changes in proteins involved in branched-chain amino acid (BCAA) catabolic processes, glycolysis, and lipid metabolic processes. Subsequent quantitative RT-PCR and protein gel blot analysis confirmed that four genes associated with BCAA degradation were significantly upregulated at both the mRNA and protein levels during TAG accumulation. The most significantly upregulated gene, encoding the β-subunit of methylcrotonyl-CoA carboxylase (MCC2), was selected for further functional studies. Inhibition of MCC2 expression by RNA interference disturbed the flux of carbon (mainly in the form of leucine) toward BCAA degradation, resulting in decreased TAG accumulation. MCC2 inhibition also gave rise to incomplete utilization of nitrogen, thus lowering biomass during the stationary growth phase. These findings help elucidate the molecular and metabolic mechanisms leading to increased lipid production in diatoms.

Mixotrophic cultivation of microalgae for biodiesel production: status and prospects

Biodiesel from microalgae provides a promising alternative for biofuel production. Microalgae can be produced under three major cultivation modes, namely photoautotrophic cultivation, heterotrophic cultivation, and mixotrophic cultivation. Potentials and practices of biodiesel production from microalgae have been demonstrated mostly focusing on photoautotrophic cultivation; mixotrophic cultivation of microalgae for biodiesel production has rarely been reviewed. This paper summarizes the mechanisms and virtues of mixotrophic microalgae cultivation through comparison with other major cultivation modes. Influencing factors of microalgal biodiesel production under mixotrophic cultivation are presented, development of combining microalgal biodiesel production with wastewater treatment is especially reviewed, and bottlenecks and strategies for future commercial production are also identified.

Perspectives and advances of microalgal biodiesel production with supercritical fluid technology

Biodiesel, a sustainable and clean energy source, has been greatly attracting interest to compete against serious challenges like energy crisis and environmental pollution.