Lipid content (percentage of dry weight biomass = % dw) and fractions (neutral, glycolipid and phospholipid) in
The solar energy is an inexhaustible source, while other energy reserves, like fossil and nuclear fuels, are limited in quantity and are depleted as years go by. Renewable energy is necessary to replace petroleum-derived fuels. The first generation biofuels, which are produced from oil seeds and crops, are a possible alternative, but they are limited in their capacity to provide all the energy demanded in the world. Therefore, new sources for the sustainable production of renewable energy are being looked for. This concern has promoted the keen interest in developing second generation biofuels, which are produced from other feedstocks, such as microalgal oils (Schenk et al., 2008; Mata et al., 2010). Some microalgal species are capable of producing biomass yields containing high percentages of oils (Aaronson et. al., 1980). In addition, microalgal systems can use low value natural resources, such as arid lands and saline water, thus offering the potential for large biomass energy contributions without competing for prime agricultural or forest land. Most microalgae grow photoautotrophically by using solar energy and mainly carbon dioxide as carbon source. Alternatively, some species can grow heterotrophically or mixotrophically using organic compounds as energy and carbon sources (Kitano et al., 1997; Hu & Gao, 2003; Xu et al., 2006; Liang et al., 2009).
Some microalgae are called oleaginous because they synthesize and accumulate substantial amounts of neutral lipids, mainly as triacylglycerol (TAG), under diverse stress conditions (Bigogno et al., 2002; Hu et al., 2008; Gardner et al., 2010; Damiani et al., 2010). TAGs as storage lipids are the best substrate to produce biodiesel (Xu et al., 2006; Schenk et al., 2008). This biofuel is obtained by transesterification of oil or fat with a monohydric alcohol, yielding the corresponding mono-alkyl esters (Knothe, 2005). Since transesterification maintains the relative ratio of fatty acids present in the feedstock (Costa Neto et al., 2000), the profile of the fatty acid methyl esters is a reflection of the feedstock fatty-acid composition (Lang et al. 2001; Ferrari et al. 2005). Biodiesel production from microalgae is technically feasible (Xu et al., 2006; Patil et al., 2008; Francisco et al., 2010), but for an effective use of this renewable resource as biofuel, it is necessary to be able to modify microalgal growth conditions in order to obtain high biomass productivity and the desired lipid quantity and quality. Those interested in microalgal biomass production and lipid productivity are referred to recent reviews by Griffiths & Harrison (2009), Rodolfi et al. (2009) and Pruvost et al. (2011). In addition, it is important to have information about the various fatty-acid profiles of diverse microalga oils in order to evaluate their suitability as feedstocks for fuel-conversion processes (Gouveia & Oliveira, 2009; Damiani et al., 2010; Sobczuk & Chisti, 2010; Francisco et al., 2010; Popovich et al., 2011). Unlike land plants, oils of some microalga species have a significant amount of polyunsaturated fatty acids with four and more double bonds (Belarbi et al., 2000; Harwood & Guschina, 2009), which are valuable oils. This is a feature that limits the microalgal species that may be used for biodiesel production.
This chapter aims to provide an overview of the current status of research on microalgal feedstocks as regards biodiesel production. Since there are many species of microalgae with varied biological characteristics and lipid composition, a diversity of approaches for biodiesel production have been analysed. In this review the following relevant topics will be considered: 1) diagnostic characteristics of some microalgal main groups, such as Chlorophyceae, Eustigmatophyceae and Bacillariophyceae classes 2) triggering of lipid production, and 3) oil composition, i.e. lipid fractions, content of each lipid class and fatty acid composition of each fraction. In this context, how the latter might affect the biodiesel quality will be discussed. We hope this information provides a framework for future screening of oleaginous microalgae employed as feedstock for biofuel production.
2. Diagnostic characteristics of some main microalgal groups
In recent years plenty of research has been focused on promising microalgal species aiming at the development of sustainable, commercially feasible and economic processes for biodiesel production (Rodolfi et al., 2009; Mandal & Mallick, 2009; Mata et al., 2010). The first step for these studies includes the species selection, essential for a reliable analysis. This step requires knowledge of diagnostic characteristics of different microalgal groups to achieve correct species identification. Thus, contradictory or erroneous information about fatty acid profiles and other important features as reported by Zhukova & Aizdaicher (1995) and Goldberg & Boussiba (2011) can be avoided.
Algae are (with numerous exceptions) aquatic organisms that (with frequent exceptions) are photosynthetic and oxygenic autotrophs. They are typically smaller, except for the seaweeds, and less structurally complex than land plants (Graham & Wilcox, 2000). Microscopic algae are commonly named microalgae that live as solitary cells or as colonies. They vary a great deal with respect to their cell sizes, pigments, storage products, cell wall compositions and life cycles (van den Hoek, 1995). This highly specialised group of micro-organisms has the potential to adapt to diverse habitats as well as the ability to efficiently modify its lipid metabolism in response to changes in environmental conditions (Guschina & Harwood, 2006). Oleaginous microalgae can be found in diverse taxonomic groups and their total lipid content and fatty acid composition may vary noticeably among individual species or strains within and between taxonomic groups (Hu et al., 2008).
According to van den Hoek (1995) algae can be classified in ten major algal groups (Divisions). A number of characteristics has been traditionally used to distinguish these algal groups. The most prominent features are the types of photosynthetic pigments, storage reserves and the nature of the cell covering. One of the greatest groups is represented by Chlorophyta Division, commonly known as green algae because they look bright grass green. This colour is because the chlorophylls are usually unmasked by large amounts of accessory pigments. However, chlorophytes may not always have green colouring. Widely encountered examples include the flagellates
Ultrastructural and molecular evidences obtained within the past few decades have demonstrated the existence of several distinct green algal lineages (classes). Each lineage is characterized by specific differences in cellular features and primary habitat (Graham & Wilcox, 2000).
In Chlorophyta Division, Chlorophyceae Class represents the largest taxonomic group where oleaginous candidates have been identified (Hu et al., 2008). The Chlorophyceae Class includes some very familiar green algal genera. For example,
Heterokontophyta Division includes nine classes (van den Hoek, 1995). In this review some oleaginous species included in Eustigmatophyceae and Bacillariophyceae classes will be reviewed. The chloroplasts of all members of Heterokontophyta are enclosed by four membranes instead of two, as found in green algae and land plant chloroplasts (van den Hoek, 1995; Bozarth et al., 2009).
The Eustigmatophyceae Class includes small (2-32 µm) unicellular, coccoid microalgae. Cells have one or more yellow-green chloroplasts that only contain chlorophyll a. Violaxanthin is the major accessory pigment, which is also the main pigment involved in light harvesting (Whittle & Casselton, 1975). The storage product’s chemical structure is unknown. Polysaccharide walls were indicated by van den Hoek (1995). Non-hydrolysable macromolecular constituents, i.e. algaenans, were also isolated from two species (Gelin et al., 1996, 1997). Asexual reproduction occurs by autospores production or in some cases by zoospores. Because of the similarities in morphology, reproduction, cell colour and chloroplast structure, eustigmatophyceans are commonly mistaken for coccoid green microalgae at the light-microscopy level. Its identification requires cell examination by transmission electron microscopy and/or pigment analysis by chromatography (Graham & Wilcox, 2000). There are about seven genera, most occurring in freshwater or in soil, but there are also some marine forms (van den Hoek, 1995). The oleaginous microalga
The Bacillariophyceae Class includes diatoms. They occur only as single cells or chains of cells. These microalgae are ubiquitous, occurring in marine and freshwaters, where they may be principally planktonic (they live suspended or growing in a fluid environment) or benthic (they live in the lowest level of a water body, often attached to the substrate bottom). There are diatoms in an immense variety of shapes. Circular, triangular, and modified square shapes are common. These diatoms are known as centric. Other diatoms, especially benthic forms, display varying types of bilateral symmetry and are termed pennate diatoms. Cell sizes range from less than 15 µm to 1 mm in length. Some species have been indicated as oleaginous microalgae (McGinnis, et al., 1997; Hu et al., 2008; Matsumoto et al., 2010; Yu et al., 2009; Popovich et al., 2011). The chloroplasts are usually golden-brown, because the chlorophylls a and c are masked by the accessory pigment fucoxanthin. The reserve polysaccharide is chrysolaninaran, a ß-1,3 linked glucan that is formed outside the chloroplast. They also store carbon in the form of natural oils (Bozarth et al., 2009). The cell wall is siliceous and is termed frustule (van den Hoek, 1995). Diatoms normally reproduce asexually by cell division. In terms of contributions to global primary productivity, diatoms are among the most important aquatic photosynthesizers. They dominate the phytoplankton of the oceans and recently circulated in lake waters.
3. Triggering of lipid production and oil composition
As it is usual in photosynthetic cells, microalgae contain polar and neutral lipids. Polar lipids include glycolipids and phospholipids. The glycolipids (monogalactosyl-, digalactosyl- and sulphoquinovosyldiacylglycerol) and phosphatidylglycerol have been attributed to chloroplast membranes, while the phospholipids (phosphatidylcholine and phosphatidylethanolamine) are considered more characteristic of extrachloroplastic membranes. Neutral lipid fraction is very diverse and compounds as different as sterols, free-fatty acids and acyl lipids (mono-, di- and triacylglycerols) can be found (Harwoord & Jones, 1989; Berge, et al., 1995). TAG accumulation specifically occurs in oil droplets distributed in the cytoplasm. Nile red fluorescence (Figs 1- 2) is a technique that has been used in some microalgae as a rapid screening method to detect TAG presence (Damiani et al. 2010; Popovich et al., 2011), as well as to determine the relative neutral lipid content (McGinnis et al., 1997; Yu et al., 2009).
Even though it is known that the intrinsic ability to produce large quantities of lipids is species- and strain specific (Hu et al., 2008), the energy storage capacity can be maximized by controlling the organism’s metabolism. Tuning the microalgae’s metabolism can lead to enhanced production of energy-rich compounds, such as fatty acids and glycerol. A single microalgal species may show remarkable variation in its metabolism, according to the conditions to which it is exposed, such as carbon dioxide supply, light intensity, temperature, nutrient concentrations and salinity (Shifrin & Chisholm, 1981; Roessler, 1990). Synthesis and accumulation of large amounts of TAGs accompanied by considerable alterations in lipid and fatty acid composition occur in the cell when oleaginous microalgae are placed under stress conditions imposed by chemical or physical environmental stimuli (Hu et al., 2008). Nutrient starvation (as nitrogen, phosphorus, and silicate), salinity and growth-medium pH are the major chemical stimuli employed, whereas light intensity and temperature are frequently used as physical stimuli (Hu et al., 2008). In addition, it is well known that certain levels of carbon dioxide supplementation in microalgal cultures can increase lipid content, especially TAG fraction (Gordillo et al., 1998; Huntley & Redalje, 2007; Tang et al., 2010; Francisco et al., 2010). Besides, the lipid content and fatty acid composition also depend on the age of culture and different life-cycle stages (Siron et al., 1989; López Alonso et al., 2000; Spolaore et al., 2006; Hu et al., 2008).
From a chemical point of view, oils from different sources have different fatty acid compositions. The fatty acids vary in their carbon chain length and in their number of unsaturated bonds. The fatty acids in land plant oils are very well studied and stearic acid (18:0), palmitic acid (16:0), oleic acid (18:1n9c), linoleic acid (18:2n6c) and linolenic acid (18:3n3) are commonly found (Durrett et al., 2008; Singh & Singh, 2010). As regards microalgae, there is a greater diversity of fatty-acid profiles of oils among different classes. Moreover, their information is very limited at present and most of the analyses of fatty acid composition have used total lipid content rather than the examination of individual lipid fractions (Hu et al., 2008). As previously indicated, the fatty acid composition can widely vary both quantitatively and qualitatively with the microalgae’s physiological status and the environmental conditions (Hu et al., 2008; Rodolfi et al., 2009), making it difficult to compare microalgal species/strains across experimental conditions (Molina Grima et al., 1994).
Fatty-acid composition was used to predict the quality of fatty acid methyl esters of oils for use as biodiesel (Knothe, 2005). The most important characteristics include the ignition quality (i.e. cetane number), cold-flow properties and oxidative stability. For example, saturated oils produce a biodiesel with superior oxidative stability and a higher cetane number, but rather poor low-temperature properties. On the other hand, the biodiesel produced from feedstocks rich in polyunsaturated fatty acids (PUFAs) has good cold-flow properties. However, these fatty acids are particularly susceptible to oxidation (Knothe, 2005; Hu et al., 2008). Among PUFAs, some fatty acids should be taken into account. European standard EN 14214 limits linolenic acid’s methyl ester for vehicle use to 12% and methyl esters with four and more double bonds to a maximum of 1%, (CEN EN-14214, 2003).
3.1. Chlorophyceae class
3.1.1. Triggering of lipid production
Nitrogen is the most commonly reported nutritional-limiting factor that triggers total lipid accumulation, mainly TAG in green microalgae (Hu et al., 2008; Pruvost et al., 2009, 2011). When nitrogen deprivation is imposed upon a culture exposed to suitable irradiances, photosynthesis continues, albeit at a reduced rate, and the flow of fixed carbon is diverted from protein to either lipid or carbohydrate synthesis (Shifrin & Chisholm, 1981). Some oleaginous microalgae seem to have the capacity for synthesizing
In recent years special attention has also been given to
Moreover, formation of chloroplastic and extraplastidial lipid bodies containing both TAGs and carotenoids under nitrogen starvation was reported in
There are few studies in green microalgae regarding phosphorus deficiency. According to Reitan et al. (1994), phosphorus deprivation results in decreased lipid content in
On the other hand, Liu et al. (2008) reported that high iron concentration stimulated lipid storage in
Regarding physical stimuli, it is extensively known that low light intensity induces the formation of polar lipids, whereas high light intensity decreases total polar lipid content with a concomitant increase in the amount of neutral lipids (Hu et al., 2008 and cites therein). In
The effects of temperature on the total lipid content have only been reported for a few species in green microalgae; though a general trend cannot be established. A decrease in the growth temperature from 30 to 25ºC led to an increase in the lipid content of
|15.61 1.46 (b)||9.20 0.67 (1)||3.70 0.38(3)||1.87 0.05(4)|
|34.85 0.78 (c)||19.80 0.14 (2)||7.85 1.77(3)||9.50 0.00 (3)|
3.1.2. Lipid composition
Although variations have been reported in the fatty acid composition of some representatives of Chlorophyceae Class, in general the most abundant fatty acids saturated and mono-unsaturated are palmitic acid (C16:0) and oleic acid (C18:1n9c), respectively. In turn, the major polyunsaturated fatty acids found in green algae are linoleic acid (C18:2n6c) and linolenic acid (C18:3n3). PUFAs above C18 are not usually present as majority fatty acids (Hu et al., 2008; Gouveia & Oliveira 2009; Gouveia et al., 2009; Damiani et al., 2010; Ho et al., 2010). Some examples of fatty acids profiles of green microalgae are shown in Table 2. As to saturated fatty acids (SFAs), a significant percentage (25-17%) of palmitic acid was indicated in
As to stress conditions, a marked increase in the level of SFAs and MUFAs with a concomitant decrease in PUFAs is usually associated with nitrogen deficiency (Piorreck et al., 1984). For example,
(% of total fatty acids)
|Source||Gouveia & Oliveira 2009||Gouveia & Oliveira 2009||Ho et al.2010||Gouveia & Oliveira 2009||Gouveia & Oliveira 2009||Leonardi et al. 2008|
acids (Gouveia et al., 2009). In contrast, linolenic acid proportion was below 12%, which was lower than the content (17.43%) reported for the same strain grown under sufficient nitrogen (Gouveia et al., 2009; Gouveia & Oliveira, 2009). In a similar way, Mendoza Guzmán et al. (2011) reported in
(% of total fatty acids)
Control exponential growth phase
Control exponential growth phase
High content of TAGs (more than 70% of the total lipids) containing mainly palmitic and oleic acids was reported in
|SFA %||27.81 0.42 (a)||30.36 1.19 (b)|
|MUFA %||20.07 0.06 (g)||19.91 0.12 (g)|
|PUFA %||45.80 0.18 (k)||43.15 0.68 (l)|
light intensity (300 μmol photons m-2 s-1) conditions during 14 days of growth. The fatty acid profile was similar under both culture conditions, and the major components were palmitic, stearic, oleic, linoleic, linolenic and linolelaidic acids (Table 4). The percentage of SFAs was significantly higher in cultures grown under high light intensity, when compared to the control. The palmitic acid content slightly declined under stress condition. In contrast, the stearic acid’s relative content increased. The MUFAs showed no significant differences between control and stress conditions. In addition, PUFAs presented a significant decrease in the stress condition compared to the control, with a concomitant decrease in linolenic acid.
Even though the concept of using microalgae as a source of biofuel is old (Sheehan et al., 1998), at present there are few studies related to lipid valorisation as biodiesel using chlorophycean and most of the microalgae tested are species currently cultivated for aquaculture or for human nutritional products. Some studies concerning to the potential use of green algae for biodiesel include the analysis of the total fatty acid quality- for example,
Recently, Chinnasamy et al. (2010) evaluated the feasibility of producing biodiesel from microalgal consortium of fifteen isolates (consisting of chlorophycean and cyanobacterial species) grown in treated wastewater.
3.2. Eustigmatophyceae class
3.2.1. Oil composition and triggering of lipid production
The Eustigmatophyceae Class is characterized by a typical fatty acid composition that includes four abundant fatty acids: palmitic acid, palmitoleic acid (16:1n7), arachidonic acid (ARA, 20:4n6) and eicosapentaenoic acid (EPA, 20:5n3) (Volkman et al., 1993; Goldberg & Boussiba, 2011). In contrast with green algae, fatty acids with C18 chain length are present as relatively minor components. Regarding lipid fractions, there are only few studies related to TAG composition in
The effect of irradiance on lipid content has been reported for many strains of
(% of total fatty acids)
µmol quanta m-2 s-1
|SFA (%)||32.57||48.78||38.90 *||28.80 *||34.10 *||55.45 *|
|MUFA (%)||32.30||40.06||30.05 *||34.45 *||30.40 *||28.95 *|
|PUFA (%)||35.13||11.16||27.15 *||33.85 *||26.65 *||12.30 *|
|Total lipids (%dw)||29.09||20.20||17.65||32.10|
|Source||Fábregas et al. 2004||James et al. 1989|
The effect of temperature on lipid content and fatty acid composition of
3.3. Bacillariophyceae class
3.3.1. Oil composition and triggering of lipid production
The diatoms’ fatty acids have been studied more extensively than other microalgal classes. This interest is related to the wide use of diatoms in aquaculture (Zhukova & Aizdaicher, 1995; Lebeau & Robert, 2003a, 2003b; Bozarth et al., 2009). Diatoms are characterized by an unusual distribution of fatty acids compared to green algae and land plants (Darley, 1977). The C14, C16 and C20 acids comprise the bulk of the diatom fatty acids, while unsaturated C18 acids, particularly linolenic acid, are either absent or present at very low levels. In general, SFAs and MUFAs in diatoms are myristic acid, palmitic acid and palmitoleic acid (Reitan et al., 1994; Hu et al., 2008 and cites therein). Although this composition is relatively constant, some differences have been observed in the number of fatty acids detected among species (Dunstan et al., 1994; Rousch et al., 2003), and among individual microalgal strains within a species (Johansen et al., 1990). For example, the fatty-acid numbers detected in
The lipid content of oleaginous diatoms of freshwater and marine origin growing under normal and stress culture conditions was summarized by Hu et al. (2008). Statistical analysis indicated that the average lipid content of oleaginous diatoms was 22.7% dw when maintained under normal growth conditions, whereas a total lipid content of 44.6% dw was achievable under stress conditions. However, lipid content and lipid fraction composition in diatoms are subjected to variability during the growth cycle and nutrient-deficiency (Table 6). Specifically, TAG synthesis and accumulation take place naturally in the stationary growth-phase (Siron et al., 1989; Sicko-Goad & Andresen, 1991; Lombardi & Wangersky, 1995; López Alonso et al. 1998, 2000). This fact occurs when photosynthetic assimilation is carried out while cell division is blocked due to a nutritional deficiency (Siron et al., 1989; Dunahay et al. 1996). In addition, a marked increase in the level of SFAs and MUFAs (e.g. 16:0, 16:1n7 and 18:1n9), with a concomitant decrease in the levels of PUFAs (e.g. 16:3n4 and 20:5n3) with increasing culture age has been observed in
Regarding nutrient availability, high TAG amounts in diatoms have been related to silicon (Hu et al., 2008) and phosphorus deficiency (Siron et al., 1989; Reitan et al., 1994). Silicate is the essential compound of a diatom’s cell wall. This nutrient has a positive effect on growth (Turpin et al., 1999) and affects cellular lipid metabolism (Hu et al., 2008). The response to
et al., 1989
et al., 1989
et al., 1995
et al., 2011
et al., 1989
|López Alonso et al., 2000|
stress induced by silicate starvation indicates a rapid increase in neutral lipids. For example, after 6h of silicon deprivation, the total lipid fraction in
Light intensity is another factor that affects lipid composition in diatoms (Orcutt & Patterson, 1974; Roessler, 1990). For example, increased TAG levels were observed in
The number of genera and species of diatoms is in the order of 250 and 100,000, respectively (Norton et al., 1996; van den Hoek et al., 1995). In spite of their abundance and diversity in nature, cultures of diatoms of biotechnological interest are still at the early stage of development, except for aquaculture. This is most likely due to difficulties in their cultivation. In addition to EPA, which is usually extracted on a semi-industrial scale, and biomass for feeding in aquaculture, silicon production from diatoms’ frustules is the most promising application, particularly in the field of nanotechnology (Lebeau & Robert, 2003a, 2003b; Bozarth et al., 2009). Only a few authors have reported lipid valorisation as biodiesel using diatoms, for example in
4. Influence of oil composition on biodiesel quality
As was previously indicated, the most important properties of biofuel- i.e. cetane number (ignition quality), cold-flow properties, oxidative stability, and iodine value- are determined by the structure of fatty esters, which form essential part of the biodiesel (Knothe, 2005; Chisti, 2007). In turn, the properties of fatty esters are determined by the characteristics of fatty acid’s oil- i.e. carbon chain length, its unsaturation degree, and the alcohol moieties that comprise a fatty ester (Knothe, 2005). Thus, the fatty-acid composition of different oils has a significant effect on the characteristics of the produced biodiesel. For instance, palmitic, stearic, oleic, linoleic and linolenic acids were recognized as the most common fatty acids contained in biodiesel from land plants (Miao & Wu, 2006; Knothe, 2008). These fatty acids are well represented in some green microalgae recently examined.
The selection of appropriate microalgal strains is an important factor for the overall success of biofuel production from microalgae. Rigorous selection is challenging owing to the large number of microalgal species available, the limited characterization of these organisms and their varying sets of characteristics. At present no ideal species have been found for this purpose; however, some examples of promising microalgal species are the following. Regarding chlorophycean, some research has been performed with the aim to analyze fatty acid composition so as to infer biodiesel quality. Total fatty acid profiles of
As was reported, lipids produced from green microalgal species usually contain fatty acid profiles of mainly C16 and C18. In
Even though it is important to have information about total fatty acid profiles to screen microalgae for biodiesel production, transesterification includes TAG conversion to diglycerides, monoglycerides and then esters and glycerol (Mata et al., 2010). In this context, the utility of microalgal oils as biodiesel will depend on fatty acids’ quantity and composition in TAG fraction. The analysis of TAG profiles for Argentinian
In a similar way, the TAG profiles were analyzed in the diatoms
As to FAME analysis, there are few studies focused on biodiesel production from different microalgal species. It is well known that certain methyl esters allow us to improve biodiesel quality. For instance, methyl oleate has been suggested as a possible candidate since it exhibits a combination of improved fuel properties (Knothe, 2005). Mandal & Mallick (2009) reported a high amount of methyl palmitate (38.8%) and methyl oleate (35.4%) fatty acids in biodiesel from
Francisco et al. (2010) analyzed feedstocks of six microalgal strains for biodiesel production, taking into account important properties like ester content, cetane number, iodine value and unsaturation degree. In this study they found
As regards Eustigmatophyceae, Koberg et al. (2011) reported that the major composition of biodiesel produced from
The production of biodiesel from microalgae is an emergent area greatly promising for the gradual replacement of diesel fuel. This review covers only a small part of various aspects that should be taken into account when choosing a microalgal species to produce biodiesel. The results clearly indicate the need for more research on microalgal lipids, especially TAG and fatty-acid profiles. Besides, rigorous comparisons across experiments under different conditions are impossible to carry out. As has been shown, the lipid content and fatty acid composition can vary with: a) strain/species, b) environmental conditions, c) type of stress condition, d) duration of stress condition, e) nutrient’s concentration or irradiance’s intensity during cultivation, f) life cycle (exponential or stationary phase), g) culture age, h) culture strategies (one- or two-stage cultivation, indoor or outdoor), among others. Even though no generalization can be strictly made, stress conditions seem to be effective in promoting lipid accumulation and improving TAG amount in many microalgae. These features are often associated with low productivity of biomass and lipids. Thus, the major challenge in process development for microalgal biodiesel production consists in choosing the best strains and defining cultivation strategies in order to simultaneously achieve three objectives: to obtain high biomass yields, high lipid contents and lipids with adequate fatty acid profiles.
Thanks to Secretaría de Ciencia, Tecnología e Innovación Productiva (SeCyT) de la República Argentina, Estudio Exploratorio Nº E658/07PET29, Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET) PIP 112-200801-00234, Secretaría de Ciencia y Tecnología de la Universidad Nacional del Sur, PGI TIR and Agencia Nacional de Promoción Científica y Tecnológica, PICT-2010-0959. P.I.L. is Research Member of CONICET.