Research review paperMicroalgal lipids biochemistry and biotechnological perspectives
Introduction
The microalgae are photosynthetic microorganisms that play a key role in the natural ecosystems supplying organic matter and specific molecules, such as polyunsaturated fatty acids (PUFAs), to many higher organisms. Microalgae applications range from human and animal nutrition to cosmetics and the production of high value molecules.
The systematic development of microalgal applications started in the 20th century. Several species are currently cultivated in large scale, in artificial or natural ponds and rarely in photobioreactors (PBRs), producing up to some tons of biomass and/or various metabolites per year. Algal biomass is rich in PUFAs, minerals (e.g. Na, K, Ca, Mg, Fe, Zn and trace minerals) and vitamins, such as riboflavin, thiamin, carotene and folic acid and so on (Becker, 2004, García-Garibay et al., 2003, Samarakoona and Jeona, 2012). The microalgal biomass, especially that produced by the species Dunaliella, Arthrospira (Spirulina, cyanobacterium) and Chlorella, is already marketed in various forms designed for human nutrition or is incorporated into foods and beverages (Liang et al., 2004, Yamaguchi, 1997), as it is considered a healthy nutritional supplement (Apt and Behrens, 1999, Borowitzka, 1999, Jensen et al., 2001, Priyadarshani and Rath, 2012, Soletto et al., 2005). Similarly, the consumption of even small amounts of microalgal biomass can positively affect the physiology of animals by improving immune response, diseases' resistance, antiviral and antibacterial protection, improved gut function, probiotic colonization stimulation, as well as enhanced feed conversion, reproductive performance and weight control (Harel and Clayton, 2004). Although the quality of algal proteins lags behind that of animal proteins, it is superior to that of common plants (Barrow and Shahidi, 2008, Becker, 2004, Kay and Barton, 1991, Samarakoona and Jeona, 2012, Sydney et al., 2010, Um and Kim, 2009). Particular algal peptides, such as taurine, are of great nutritional and pharmaceutical interest (Houstan, 2005), while glycoproteins (lectins), extracted by marine algae, are considered a type of interesting proteins for biochemical and clinical research, and can be isolated with their carbohydrate moiety (Silva et al., 2010).
Some species contain considerable amounts of pigments that are used in cosmetics and as natural coloring agents. Many industrial production plants are established in China, Australia and USA (Brown et al., 1997, García-González et al., 2005, León et al., 2003) dealing with beta-carotene production (e.g. from Dunaliella salina) that is used as a food coloring (Metting, 1996). Other pigments such as phycobiliproteins have been extracted from various marine algae including Porphyridium cruentum and Synechococcus spp. (Viskari and Colyer, 2003).
Although the majority of applications concern biomass production destined for animal or human consumption — in fact, 30% of the current world algal production is sold for animal feed applications (Becker, 2004) — there has been an increased interest in the use of microalgal lipids in numerous commercial applications, such as in food, chemical and pharmaceutical industries and cosmetology. Indicative of the high interest in microalgal lipids is the noteworthy research that has been performed in the last decade on all aspects concerning microalgal lipid production. These include fundamental research on the mechanisms used for light energy conversion and on lipid biosynthesis and catabolism, as well as biotechnological research dealing with the various technical bottlenecks of the lipid production process. In the current review article the up-to-date level of knowledge in lipid biosynthesis and turnover in microalgae and the various biotechnological applications and future perspectives of microalgal lipids are comprehensively presented and discussed. Τaking into consideration the recent techno-economic analyses concluding that the algal lipid content is the most critical factor affecting the viability of large-scale applications, especially those related to biodiesel, the current research efforts aimed to reinforce algal liposynthetic machinery using genetic engineering are also discussed.
Section snippets
Microalgal lipids in the forefront of lipid biotechnology
Several microalgal species are able to accumulate appreciable lipid quantities, and therefore are characterized as oleaginous. Lipid content in microalgae can reach up to 80% in dry biomass, but even in these cases the lipid productivity is actually low. In widespread species belonging to the genera of Porphyridium, Dunaliella, Isochrysis, Nannochloropsis, Tetraselmis, Phaeodactylum, Chlorella and Schizochytrium, lipid content varies between 20 and 50%. However, higher lipid accumulation can be
Lipid metabolism
Most microalgae accumulate lipids under specific environmental stress conditions, such as nitrogen or phosphate limitation (Amaro et al., 2011, Bellou and Aggelis, 2012, Courchesne et al., 2009, Hu et al., 2008, Hu et al., 2013, Msanne et al., 2012). Therefore the management of the environmental conditions is a common approach used for improving lipid accumulation in the microalgal cell. Strain selection is also likely to be of critical importance. Recently, there has been an intense interest
Biotechnological perspectives of microalgal lipids
The production of microalgal lipids (intended for either as source of PUFAs or as feedstock for biodiesel production) can be performed through specific processes (i.e. planned for this purpose) or in combination with the production of other microalgal metabolic products having pharmaceutical and/or nutritional interest, or even may arise by exploiting the algal biomass produced during wastewater treatment.
An evaluation of the various systems used for microalgal oil production is illustrated in
Conclusions
The most significant bottlenecks that limit the production of microalgal oil in large scale are primarily the restricted lipid synthesis in the microalgal cell and secondarily the low growth rate of these organisms (Davis et al., 2011). Both constraints, negatively affecting oil productivity, are of biological origin and therefore their solutions should be sought in laboratories of molecular biology and biochemistry. Genetic engineering of strains with such enhanced performances (having
Acknowledgments
Financial support was provided by the King Abdulaziz University (Jeddah, Saudi Arabia). Project title: Biotechnological production of PUFA rich SCOs from microalgae.
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