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Microalgae have been recognized as promising competitors for bioenergy production, because of their high lipid content and quick biomass development. In this work, freshwater microalgae Scenedesmus dimorphus was cultivated in a synthetic wastewater media with the object of investigating the bioenergy potential by physicochemical characterization. Batch experiments were performed photoautotrophically for biomass generation supplemented with different levels of CO2 concentration. Maximum biomass production was in the limit of 373 mg/L to 587 mg/L under different CO2 feed concentrations. Cultivation of microalgae along with CO2 supplementation found a substantial impact on buildup of lipid in microalgae biomass. Maximum 29% and 32% lipid content were achieved for 2% and 6% CO2 enrichment respectively, under nitrogen-depleted conditions. Fourier transform infrared spectroscopy, thermogravimetric analysis, and carbon, hydrogen, nitrogen, sulfur elemental analyzer instrumental tools were used to perform physicochemical characterization on biomass in order to determine its proximate analysis, ultimate analysis, higher heating value (HHV), and macromolecular composition, The higher heating value (HHV) of Scenedesmus dimorphus biomass achieved 33.26 MJ/kg with almost zero sulfur content. This HHV value is significantly higher than that of other microalgae sp. Moreover, this HHV value is also even higher than that of bituminous coal (24.3 MJ/kg) and other types of lignocellulosic biomass. Scenedesmus dimorphus biomass indicates that it can be a potential fuel feedstock for environmentally friendly bioenergy production. [ABSTRACT FROM AUTHOR]

Background: Metabolic engineering for hyperaccumulation of lipids in vegetative tissues is a novel strategy for enhancing energy density and biofuel production from biomass crops. Energycane is a prime feedstock for this approach due to its high biomass production and resilience under marginal conditions. DIACYLGLYCEROL ACYLTRANSFERASE (DGAT) catalyzes the last and only committed step in the biosynthesis of triacylglycerol (TAG) and can be a rate-limiting enzyme for the production of TAG. Results: In this study, we explored the effect of intron-mediated enhancement (IME) on the expression of DGAT1 and resulting accumulation of TAG and total fatty acid (TFA) in leaf and stem tissues of energycane. To maximize lipid accumulation these evaluations were carried out by co-expressing the lipogenic transcription factor WRINKLED1 (WRI1) and the TAG protect factor oleosin (OLE1). Including an intron in the codon-optimized TmDGAT1 elevated the accumulation of its transcript in leaves by seven times on average based on 5 transgenic lines for each construct. Plants with WRI1 (W), DGAT1 with intron (Di), and OLE1 (O) expression (WDiO) accumulated TAG up to a 3.85% of leaf dry weight (DW), a 192-fold increase compared to non-modified energycane (WT) and a 3.8-fold increase compared to the highest accumulation under the intron-less gene combination (WDO). This corresponded to TFA accumulation of up to 8.4% of leaf dry weight, a 2.8-fold or 6.1-fold increase compared to WDO or WT, respectively. Co-expression of WDiO resulted in stem accumulations of TAG up to 1.14% of DW or TFA up to 2.08% of DW that exceeded WT by 57-fold or 12-fold and WDO more than twofold, respectively. Constitutive expression of these lipogenic "push pull and protect" factors correlated with biomass reduction. Conclusions: Intron-mediated enhancement (IME) of the expression of DGAT resulted in a step change in lipid accumulation of energycane and confirmed that under our experimental conditions it is rate limiting for lipid accumulation. IME should be applied to other lipogenic factors and metabolic engineering strategies. The findings from this study may be valuable in developing a high biomass feedstock for commercial production of lipids and advanced biofuels. [ABSTRACT FROM AUTHOR]

The reuse of the culture media after biomass growth can be a cost-effective strategy for large-scale microalgae production, especially when the target product is biofuel. Therefore, this study aimed to investigate the effect of recycling the microalgae Tetradesmus obliquus culture medium, focusing on the improved production of biomass and lipids, biodiesel properties, and cost reduction. The experiments were carried out in duplicate and three sequential steps with successive recycling (50% of the clarified medium and 50% of the standard). It was found that cells from recycled cultures had a similar overall performance as those from the fresh medium (standard). From this verification, the main findings of this research pointed out that (i) the consumption of the nutrients was efficient, with values in the order of 99% nitrate, 20% phosphate, 64% total iron, and 9% potassium; (ii) the average contents of biomass and lipids in the recycled medium increased around 20% to 24%, respectively, concerning the standard; (iii) the polyunsaturated fatty acids pool present in the microalgal oil dropped from 20.71 to 9.78% in the recycled culture, which represents a high-quality feedstock for biofuel; and (iv) the biodiesel properties evaluated, including iodine and saponification values, were 48 g I2 100 g−1 and 254 mg KOH/goil−1, respectively. Finally, this recyclability bioprocess can reduce overall costs by about 34%, thus improving the economic viability of microalgae production. [ABSTRACT FROM AUTHOR]

This scientometric review and bibliometric analysis aimed to characterize trends in scientific research related to algae, photobioreactors and astaxanthin. Scientific articles published between 1995 and 2020 in the Web of Science and Scopus bibliographic databases were analyzed. The article presents the number of scientific articles in particular years and according to the publication type (e.g., articles, reviews and books). The most productive authors were selected in terms of the number of publications, the number of citations, the impact factor, affiliated research units and individual countries. Based on the number of keyword occurrences and a content analysis of 367 publications, seven leading areas of scientific interest (clusters) were identified: (1) techno-economic profitability of biofuels, bioenergy and pigment production in microalgae biorefineries, (2) the impact of the construction of photobioreactors and process parameters on the efficiency of microalgae cultivation, (3) strategies for increasing the amount of obtained lipids and obtaining biodiesel in Chlorella microalgae cultivation, (4) the production of astaxanthin on an industrial scale using Haematococcus microalgae, (5) the productivity of biomass and the use of alternative carbon sources in microalgae culture, (6) the effect of light and carbon dioxide conversion on biomass yield and (7) heterotrophy. Analysis revealed that topics closely related to bioenergy production and biofuels played a dominant role in scientific research. This publication indicates the directions and topics for future scientific research that should be carried out to successfully implement economically viable technology based on microalgae on an industrial scale. [ABSTRACT FROM AUTHOR]

Microalgae are photosynthetic organisms functioning as the green bio-factories for various pharmaceutical and biofuel products. To date, numerous attempts have been carried out to manipulate culture conditions to maximize the production of the desired metabolites. Because light is the energy source of microalgae for their growth and metabolites biosynthesis, it has been one of the most investigated variables emphasized on the deep understanding of how microalgae respond towards light changes as an external stimulus. This review discusses the effects of different light sources, light intensities, light wavelengths and length of photoperiod on various microalgae species, especially in terms of biomass and lipid productivity. Additionally, the relationship between photoregulation processes and lipid productivity of microalgae are also deliberated. The current available approaches of microalgae mass cultivation, including different types of open and closed systems are recapitulated with the intention to highlight the significant insights for the design of future photoreactors. [ABSTRACT FROM AUTHOR]

Freshwater deficiency and growing requests for renewable energy have led to increased concern about water and energy security. Therefore, the present study attempted to evaluate the feasibility of using wastewater (WW) as a culture medium for microalgal cultivation with the aim of nutrient removal and biodiesel production. The green microalga Asterarcys quadricellulare (Chlorophyta) was isolated from a wastewater treatment plant in Menoufia, Egypt, and grown for 24 days in Bold's Basal Medium as a control and at different concentrations of secondary treated WW in distilled water (25%, 50%, 75%, and 100%). The results of 75% WW treatment recorded 96.6%, 98.4%, and 89.9% removal efficiency for nitrate, ammonia, and total phosphorus, respectively. Also, it revealed high biomass yield and biomass productivity (1.44 g L−1 and 69.0 mg L−1 day−1, respectively). In addition, it showed higher lipid production (360.6 mg L−1) and lipid productivity (17.2 mg L−1 day−1). Asterarcys quadricellulare fatty acid profile estimation revealed elevated proportions of oleic (54.92%), palmitic (20.54%), and linoleic acids (13.9%). Most of the properties of biodiesel derived from the studied microalga are consistent with those recommended by the international standards. The current study concluded that A. quadricellulare could be used for wastewater bioremediation and biomass production with high potential as biodiesel feedstock. [ABSTRACT FROM AUTHOR]

This research aims to evaluate the effect of molasses as a source of organic carbon on the growth and lipid profile of Dunaliella salina cultivated under different light regimes to determine its strategy and potential use to produce biodiesel. Two sources of nutrients (F/2 medium, F/2; molasses, M) and two light conditions (12 h light: 12 h darkness, 12L:12D) conformed four treatments (F/2 12L:12D, F/2 D, M 12L:12D, and MD) that were investigated with three replicates each. The concentration of microalgae produced by the groups under light conditions was more than double that of those grown in darkness. The highest biomass produced (46.91 ± 6.47 g L-1) was obtained in the F/2 medium with 12L:12D treatment, while the lowest (18.99 ± 0.78 g L-1) was in the M 12L:12D group. Regardless of the culture conditions, the lipids were mainly composed of the C16:0 and C18 fatty acids. The use of molasses as a culture medium produced the highest amount of C18:1, regardless of the light regime. Molasses promoted the highest production of fatty acids methyl esters. Depending on the culture medium, D. salina cells showed different percentages of saturated (SFA), monounsaturated (MFA), and polyunsaturated (PUFA) fatty acids. The highest amount of saturated fatty acids was obtained in F/2 12L:12D. The highest amount of polyunsaturated fatty acids was presented in the mixotrophic culture (molasses and light). The cultures grown with molasses were rich in monounsaturated fatty acids. An inexpensive industrial by-product, Cane molasses could be a low-cost feedstock for biodiesel in cultivating D. salina. [ABSTRACT FROM AUTHOR]

• Influence of phytohormones on algal biomass and lipid accumulation was investigated. • Auxin and cytokinin exihibited a synergystic effect on biomass and lipid yield. • Phytohormones influenced the activities of antioxidant enzymes SOD, CAT and APX. • Phytohormones modified the FAME contents that were desired for high-grade biofuels. Microalgae are considered as a promising resource for biodiesel. Nevertheless, their commercial exploitation necessitates considerable impetus towards the development of approaches for increased biomass and lipid production. The present work elucidates the impact of exogenously supplemented auxins, i.e., indole-3-acetic acid (IAA), indole-3-butyric acid (IBA) and indole-3-propionic acid (IPA) and cytokinins, i.e., benzylaminopurine (BAP) and thidiazuron (TDZ) on biomass, lipid content and fatty acid profile of Desmodesmus sp. JS07. Among auxins, IBA improved the biomass and lipid content up to 1.96 ± 0.11 g/L and 34.88 ± 3.87 %, respectively while BAP among cytokinins increased the biomass and lipid content up to 1.88 ± 0.061 g/L and 31.84 ± 1.33 % respectively. Further, the cumulative impact of IBA (10 mg/L) and BAP (5 mg/L) resulted in their synergistic effect by stimulating biomass and lipid content up to 2.34 ± 0.032 g/L and 42.43 ± 1.88 % respectively. Auxins stimulated the superoxide dismutase activity, and cytokinins increased the enzymes (catalase and ascorbate peroxidase), scavenging reactive oxidative species, thereby regulating ROS homeostasis in microalgae. A significant alteration in the fatty acid profile owing to the type and dosage of phytohormones was detected. Hence, the strategy employing phytohormones could prove to be a meaningful approach for biofuel production. [ABSTRACT FROM AUTHOR]

To elucidate the potential of Chlorococcum oleofaciens KF584224.1, the effects of different ferric ion concentrations (0, 10−7, 10−6, 10−5, 10−4 mol L−1) on growth, lipid accumulation, and fatty acid profile has been studied. The quantity and quality of extracted lipid was also analyzed by gravimetric and gas chromatographic methods, respectively, to evaluate their suitability according to the international standards for biodiesel (ASTM D6751 and EN 14214). An increase in ferric ion concentration up to 10−6 mol L−1 was associated with an increased specific growth rate by 1.83 folds and expanded exponential growth rate by 10 days compared with that in 0 mol L−1 ferric ion concentration. The highest biomass productivity (310.17 ± 2.19 mg L−1 d−1) and lipid productivity (138.81 ± 3.57 mg L−1 d−1) were achieved in culture medium supplemented with 10−6 and 10−5 mol L−1 ferric ion. The proportion of PUFAs and SFAs tends to decrease and increase by enhancing the ferric ion concentration, respectively. Although the quality of biodiesel produced form C. oleofaciens was found to be in accordance with the specifications in the American (ASTM D6751) and European (EN 14214) biodiesel standards, ferric ion supplementation enhanced the lipid characteristics obtained from C. oleofaciens for biodiesel production. • C. oleofaciens grew up 2.19-fold faster at 10−6 mol L−1 ferric ion concentration. • Maximum lipid productivity was obtained under 10−5 mol L−1 ferric ion concentration. • Enhancing in ferric ion supply played a positive role in SFAs accumulation. • The produced FAMEs met the biodiesel specifications of international standards. • Ferric ion concentration up to 10−5 mol L−1 improved the biodiesel quality. [ABSTRACT FROM AUTHOR]

Discovery of high-yielding microalgae rich in valuable lipids, proteins, pigments, or carbohydrates is essential to the growth of nutraceutical and biofuel industries. Therefore, bioprospecting of microalgae from diverse environmental conditions has become highly significant to meet the demand of such bioresources globally. In this context, bioprospecting of three species of microalgae isolated from bloomed freshwaters of Kerala, India, was carried out; morphological and molecular characterizations revealed two of the algae as new species to science and named accordingly. The industrial potentials of these algae (Fasciculochloris boldii (MH992105.1), Sphaerocystis antoni-kadavilaii Ray & Prasanth sp. nov. (MK005299.1), and Chlorella zachariaii Ray & Prasanth sp. nov. (MH930447.1)) were assessed in terms of biomass yield and biochemical profile. The alga Sphaerocystis antoni-kadavilaii with a biomass yield of 176.8 ± 2.0% increase L−1 day−1, lipid productivity of 4.4 ± 0.3 mg L−1 day−1, the protein content of 46.1%, is found promising, in comparison with the known species such as Spirulina sp. and Chlorella sp. It is high in chlorophyll a, and total carotenoid content as well. The alga Fasciculochloris boldii showed high lipid content of 26.8 ± 1.1% and the alga Chlorella zachariaii showed 30.1% of omega-3 fractions in its total fatty acids. The Fourier transform infrared spectroscopy confirmed biodiesel feasibility of lipids from all these algae. However, the alga Fasciculochloris boldii with high proportions of saturated fatty acids in its lipid (71.2%) can be considered as a potential species for biodiesel as the biodiesel from the same also shows the required cetane number (63.3) and low iodine value (38.9). [ABSTRACT FROM AUTHOR]

Use of oleaginous microorganisms as 'micro-factories' for accumulation of single cell oils for biofuel production has increased significantly to mitigate growing energy demands, resulting in efforts to upgrade industrial waste, such as second-generation lignocellulosic residues, into potential feedstocks. Dilute-acid pretreatment (DAP) is commonly used to alter the physicochemical properties of lignocellulosic materials and is typically coupled with simultaneous saccharification and fermentation (SSF) for conversion of sugars into ethanol. The resulting DAP residues are usually processed as a waste stream, e.g. burned for power, but this provides minimal value. Alternatively, these wastes can be utilized as feedstock to generate lipids, which can be converted to biofuel. DAP-SSF residues were generated from pine, poplar, and switchgrass. High performance liquid chromatography revealed less than 0.13% monomeric sugars in the dry residue. Fourier transform infrared spectroscopy was indicative of the presence of lignin and polysaccharides. Gel permeation chromatography suggested the bacterial strains preferred molecules with molecular weight ~ 400-500 g/mol. DAP-SSF residues were used as the sole carbon source for lipid production by Rhodococcus opacus DSM 1069 and PD630 in batch fermentations. Depending on the strain of Rhodococcus employed, 9-11 lipids for PD630 and DSM 1069 were observed, at a final concentration of ~ 15 mg/L fatty acid methyl esters (FAME) detected. Though the DAP-SSF substrate resulted in low FAME titers, novel analysis of solid-state fermentations was investigated, which determined that DAP-SSF residues could be a viable feedstock for lipid generation. [ABSTRACT FROM AUTHOR]

The present study was conducted with microalga Nannochloropsis sp. to evaluate its tolerance responses and biofuel perspectives under different arsenic regime (0–1000 μM As(III)). Results showed that optimal biomass (61.00 ± 3.72 mg/L/d) and lipid productivity (20.27 mg/L/d) were obtained in culture treated with 100 μM As(III) in comparison to other treatments. In addition, fatty acid profile of alga was in accordance with European biodiesel standards (EN 14214), which reflects good oxidative stability of oil. High antioxidants viz., ascorbic acid, GSH and cysteine tolerance responses as well as lipid yield at 100 μM As(III), opens a new insight in the field of algal biology. Thus, microalgae Nannochloropsis sp. may be employ in remediation as well as biodiesel production. [ABSTRACT FROM AUTHOR]

Highlights: [•] Haematococcus pluvialis was cultivated in domestic secondary effluent. [•] The maximal biomass productivity of 27.8mgL−1 d−1 was achieved. [•] Nitrogen and phosphorus in the secondary effluent both were removed efficiently. [•] The maximal lipid accumulation rate of 12.0mgL−1 d−1 was obtained. [•] Haematococcus pluvialis outperformed most microalgal species in literatures. [ABSTRACT FROM AUTHOR]

There is a growing interest for the mixotrophic cultivation of microalgae using sustainable natural carbon sources. Chitin from shrimp waste was hydrolyzed using chitinase from Trichoderma asperellum and was exploited for the mixotrophic cultivation of microalgae. Chitin saccharification was optimized using Box-Behnken design to produce 50.41% (w/w) of reducing sugars using chitin, 0.83% (w/v), chitinase, 2.92 (v/v), temperature, 40 °C and time, 4.48 h. The biomass productivity of Tetradesmus obliquus and Aphanocapsa sp. was promoted to 39.1 and 40.57 mg L−1 day−1 in the presence of 0.5 g L−1 of N -acetylglucosamine oligomers (GlcNAc), which was estimated to be ∼1.9 and ∼2-folds higher than the autotrophic conditions, respectively. Modified Logistic kinetic model confirmed higher growth rates and shortened lag-time of the mixotrophic cultures. The lipid productivity of both microalgae species was >1.5-times higher than the autotrophic conditions. Nitrate removal from the culture medium was found to be an effective strategy to increase both lipid content and lipid productivity for T. obliquus , but this strategy was not effective for Aphanocapsa sp. Fatty acid analysis revealed an increase of saturated fatty acids under mixotrophic cultivation for both microalgae species. Additionally, several biodiesel evaluation parameters indicated superior characteristics for the mixotrophic culture compared to the autotrophic conditions. Accordingly, mixotrophic growth of microalgae using GlcNAc as a new, sustainable carbon and nitrogen source opens a new avenue for biofuel production with an effective waste conversion into valuable commodities. • Enzymatic production of reducing sugars from shrimp chitin was optimized to yield 50.41% (w/w). • Algal lipid productivity was enhanced >1.5 fold using chitin hydrolysate. • Nitrate removal enhanced lipid production of Tetradesmus obliquus. • Biodiesel from algal lipids obtained by mixotrophy had improved characteristics. [ABSTRACT FROM AUTHOR]

Abstract: Although there has been a large research effort associated with individual parts of various algal biofuel production pathways, few studies have tracked changes in product composition throughout an integrated biofuel production process. This study uses microscopy and chromatography to analyze the progression of lipid profile and cell structure of algal cells throughout a research-scale production pathway for biocrude. For the specific processing methods used in this pathway, it is shown that TAG content decreased, while DAG and FFA content increased during processing. The changes in the lipid content corresponded to cell degradation that was observed by SEM and TEM throughout processing. These results demonstrate the dynamic nature of lipid composition in an algal culture used for biofuel production and emphasize the need to monitor changes in lipid profile, as those changes can directly impact biofuel productivity. [Copyright &y& Elsevier]

Abstract: A 1.2kW, 2450MHz resonant continuous microwave processing system was designed and optimized for oil extraction from green algae (Scenedesmus obliquus). Algae-water suspension (1:1 w/w) was heated to 80 and 95°C, and subjected to extraction for up to 30min. Maximum oil yield was achieved at 95°C and 30min. The microwave system extracted 76–77% of total recoverable oil at 20–30min and 95°C, compared to only 43–47% for water bath control. Extraction time and temperature had significant influence (p <0.0001) on extraction yield. Oil analysis indicated that microwaves extracted oil containing higher percentages of unsaturated and essential fatty acids (indicating higher quality). This study validates for the first time the efficiency of a continuous microwave system for extraction of lipids from algae. Higher oil yields, faster extraction rates and superior oil quality demonstrate this system’s feasibility for oil extraction from a variety of feedstock. [ABSTRACT FROM AUTHOR]

The application of microalgae biomass of Chlorella sorokiniana as environmentally friendly biosorbents for removing potentially toxic elements (PTE) from water and as a source of biofuel has been thoroughly studied. In this paper, we investigate its physicochemical properties infrared spectroscopy (IR spectra), microstructure, adsorption properties); we have managed to isolate the lipid complex, which amounted to 20% of dry biomass. Studies of the lipid complex showed that 80.02% of lipids are unsaturated fatty acids (C18:1, C18:2, C18:3). Additionally, we have investigated the efficiency of using the residual biomass obtained after lipid extraction for water purification from rare-earth metals (REM) and PTE. To increase the sorption properties of residual biomass, its thermal modification was carried out and sorption materials based on heat-treated residual biomass and chitosan were created. The physicochemical and mechanical properties of the obtained sorption materials were studied. The total sorption capacity was 31.9 mg/g for REM and 349.7 mg/g for PTE. Moreover, we propose a new method for the disposal of spent sorbents as additional fuel. Spent sorbents can be considered to be biofuel in terms of energy content (20.7 MJ*kg−1). The results of this study provide the basis for increased use of microalgae. [ABSTRACT FROM AUTHOR]

The depletion of fossil fuels and the search for novel sustainable energy sources are challenges faced by countries throughout the world over the past few decades. Microalgae have received considerable interest as new oil sources for biofuel production. However, the enhancement of the lipid content in microalgae strains without decreasing the growth rate is a prerequisite for improving the economic viability of microalgae-derived biofuel production. The implementation of an appropriate cultivation strategy can increase both lipid accumulation and biomass production. One of the most-often suggested strategies is the cultivation of microalgae through two-stage cultivation strategy (TSCS). This paper aims to recapitulate the findings of recent studies and achievements in improving the lipid productivity and economic feasibility of TSCS. This article starts by highlighting different types of TSCS based on various stimuli. Afterwards, the most noteworthy culture parameters, including growth stage and initial cell density, are addressed. The existing experimental results show that lipid productivity can be enhanced by optimizing stress factor(s) and engineering processes. Moreover, it is reported that the modification or elimination of the pre-harvesting phase (which can be done by upgrading technical aspects and/or via stress induction strategies) can promote the economic feasibility of TSCSs. Image 1 • Focuses on a two-stage strategy for achieving maximal biomass/lipid productivity. • Discusses different types of a two-stage strategy based on biotic and abiotic stimuli. • Discusses nutrient starvation as the most reliable stress type in the lipid induction phase. • Focuses on growth stages in determining the time node and dose of stress factors. [ABSTRACT FROM AUTHOR]

A two-stage cultivation method involving the initial growth in optimized conditions for biomass production followed by those for lipid production in oleaginous brackish diatom Navicula phyllepta MACC8 resulted in a proportional increase of lipid concentration along with biomass production. The diatom was further subjected to stress conditions by altering the nutrient components such as nitrate, phosphate, silicate, and temperature. Silicon deprivation resulted in the highest lipid percentage of 28.78% of weight at the end of the 18th day of the second stage. A significant increase in lipid content was observed on the complete removal of the nutrients silicon and urea one at a time, while the biomass showed a considerable reduction. The application of multiple nutrient stress conditions had a profound influence on the increased rate of lipid production. A combination of phosphate deprivation, silicate limitation and temperature reduction resulted in a significant increase in lipid percentage of 32.13% at the cost of reduced biomass (1.1 g L−1), whereas phosphate deprivation, urea limitation, and temperature reduction resulted in lipid percentage of 27.58% with a biomass of 1.44 g L−1 at the end of the second stage. Further, the results were supported by Nile red staining, FTIR, fatty acid profile and oxidative stress marker analyses. The changes in biochemical composition and oxidative stress parameters within the various stress conditions demonstrated the profound influence of the selected stress factors on the biodiesel productivity of the diatom, besides its stress tolerance. A two-phase culturing system, with multifactor stress application, especially nitrogen limitation along with phosphate starvation and temperature stress, would be the suitable method for gaining maximum biomass productivity and lipid content in diatom Navicula phyllepta MACC8 towards biofuel production. [ABSTRACT FROM AUTHOR]

The use of fossil fuels has been strongly related to critical problems currently affecting society, such as: global warming, global greenhouse effects and pollution. These problems have affected the homeostasis of living organisms worldwide at an alarming rate. Due to this, it is imperative to look for alternatives to the use of fossil fuels and one of the relevant substitutes are biofuels. There are different types of biofuels (categories and generations) that have been previously explored, but recently, the use of microalgae has been strongly considered for the production of biofuels since they present a series of advantages over other biofuel production sources: (a) they don't need arable land to grow and therefore do not compete with food crops (like biofuels produced from corn, sugar cane and other plants) and; (b) they exhibit rapid biomass production containing high oil contents, at least 15 to 20 times higher than land based oleaginous crops. Hence, these unicellular photosynthetic microorganisms have received great attention from researches to use them in the large-scale production of biofuels. However, one disadvantage of using microalgae is the high economic cost due to the low-yields of lipid content in the microalgae biomass. Thus, development of different methods to enhance microalgae biomass, as well as lipid content in the microalgae cells, would lead to the development of a sustainable low-cost process to produce biofuels. Within the last 10 years, many studies have reported different methods and strategies to induce lipid production to obtain higher lipid accumulation in the biomass of microalgae cells; however, there is not a comprehensive review in the literature that highlights, compares and discusses these strategies. Here, we review these strategies which include modulating light intensity in cultures, controlling and varying CO2 levels and temperature, inducing nutrient starvation in the culture, the implementation of stress by incorporating heavy metal or inducing a high salinity condition, and the use of metabolic and genetic engineering techniques coupled with nanotechnology. [ABSTRACT FROM AUTHOR]

Background: Microalgae, being a phototroph, grow in the presence of light, and utilizing photons in narrow and specific range of wavelengths. There have been numerous attempts to take advantage of this trait of wavelength-dependent growth for the purpose of increasing biomass productivity. One potential option involves wavelength conversion of sunlight. In the present study, three fluorescent dyes with blue, red, and green emission spectra were employed with the aim of improving sunlight utilization efficiency and thus enhancing biomass and lipid productivity of Nannochloropsis gaditana. Results: When DPA and R101 were used to enrich blue and red spectra, biomass productivity of Nannochloropsis gaditana was increased by 35.1 and 40.3%, respectively. The maximum quantum yield values were higher than 0.6 at the early stage of growth for the cultures grown under DPA- and R101-modified solar radiation. Chlorophyll a content was also 57.0 and 32.3% higher than the control at the early growth stage under DPA- and R101-modified solar radiation, respectively. This stimulation of photosynthetic activity at the early growth stage correlated well with rapid growth under DPA- and R101-modified light during the first 4 days of cultivation. Lipid productivity consequently increased by 26.9 (DPA) and 39.4% (R101) after 10 days of cultivation. An immediate effect on lipid induction was observed in cultures under modified light, which exhibited 19.1% improvement in lipid content at the cost of some degree of impaired growth. Conclusion: Fluorescent dyes with the capability of enriching wavelengths of light favored by the algal photosystem could indeed be an effective means of promoting growth of Nannochloropsis gaditana. This strategy would be particularly powerful for mass cultivation where sunlight is the only economically viable option for illumination. [ABSTRACT FROM AUTHOR]