Microalgae are single-celled, ubiquitous, prokaryotic and eukaryotic primary photosynthetic microorganisms that are taxonomically and phylogenetically diverse. The advanced plant life of today is thought to have evolved from these simple microscopic plant-like entities. In general, the algae are a heterogeneous group of polyphyletic photosynthetic organisms with an estimated 350,000 known species (Brodie and Zuccarella, 2007). There are predominantly two prokaryotic divisions (Cyanophyta and Prochlorophyta) and nine eukaryotic divisions (Glaucophyta, Rhodophyta, Heterokontophyta, Haptophyta, Cryptophyta, Dinophyta, Euglenophyta, Chlorarachniophyta, and Chlorophyta). The biology of microalgae is interesting, and their enigma is due to their wide diversity as well as their plethora of habitats. The biology of microalgae is discussed extensively in Chapter 2 of this book.

Interest in microalgal cultivation is currently blossoming globally for a number of reasons. Microalgae are not extremely fastidious microorganisms but are found in diverse aquatic habitats. Microalgae can be found almost anywhere on Earth, in freshwater, marine, and hyper-saline environments (Williams and Laurens, 2010). The nutritional requirements of a wide array of microalgal strains are known, and the technology for microalgal cultivation is developing at a fast pace. The advent of genetic engineering protocols has brought new vistas to algal molecular systemat — ics. Recently, the general study of microalgae using genomics and molecular biol­ogy tools has attained phenomenal dimensions. The sheer number of microalgal strains from extreme environments that are yet to be discovered and identified is enormous (Brodie and Lewis, 2007). However, microalgal culture collection banks have been established as repository centers for these microorganisms (e. g., UTEX at The University of Texas at Austin).

The importance of microalgae in day-to-day life cannot be overemphasized. As the main primary producers, microalgal biomass is used for food and feed supplements (Lewis et al., 2000). Microalgae are important sources of commercial products such as polyunsaturated fatty acid (PUFA) oils (e. g., y-linolenic acid (GLA), arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA)) (Spolaore et al., 2006). In addition, microalgae such as Dunaliella and Haematococcus are important sources of carotenoids such as P-carotene and astaxanthin, respectively (Spolaore et al., 2006). Furthermore, the cyanobacterium Anthrospira and the rhodophyte Porphyridium are the main commercial producers of phycobiliproteins (i. e., phycoerythrin and phycocyanin), which are used as natu­ral dyes and for pharmaceutical applications (Spolaore et al., 2006). Potential bio­technological applications and value-added products generated from microalgae are discussed in Chapter 10 of this volume.

Chlorella, Arthrospira, and Nostoc are cultivated worldwide for human and animal nutrition, owing to their chemical composition (Spolaore et al., 2006). Microalgae have been hailed as the panacea for the dwindling petroleum-based fuels, and the preponderance of shorter-chain fatty acids has significance for their potential as diesel fuels (Chisti, 2007; Williams and Laurens, 2010). The efficacy of using microalgal biomass and lipids as alternative biofuels is currently a topical issue. Biofuels such as biodiesel, biomethane, biohydrogen, biobutanol, etc., can be generated from microalgae (Chisti, 2007). Current research is targeting other novel potential biotechnological applications in aquaculture, cosmetics, pharmaceuticals, and animal and human nutrition. It is envisaged that future research should focus on microalgal strain improvement through genetic engineering, in order to diversify and economically improve product competitiveness (Spolaore et al., 2006). Microalgal genetic manipulation is still in its infancy and is a pertinent area of investigation in order to improve the quality and quantity of products generated from microalgae. However, the development of nondestructive product recovery techniques from con­tinuous cultivation systems will greatly improve product yield.

Successful microalgal cultivation and generation of these products calls for metic­ulous and rigorous microalgal strain selection. Two important steps in obtaining a robust and suitable microalgal candidate are (1) bioprospecting of target microalgal strain samples from diverse habitats, and (2) strain selection, isolation, and purifica­tion using conventional and advanced microbiological methods (Grobbelaar, 2009; Mutanda et al., 2011). Suitable microalgal strains can be obtained commercially from registered authentic culture collection centers. The microalgal strain of choice is maintained under laboratory conditions, either as a freeze-dried sample or as a slant on solid media at 4°C with routine subculturing. The ever-growing field of phy — cology has introduced new, exciting, and efficient techniques for maintaining micro­algal cultures at ultra-low temperatures (i. e., cryopreservation). Microalgal strain selection for biodiesel production is discussed in detail in Chapter 3 of this volume.

The enumeration of microalgae poses a real challenge due to the requirement of sophisticated equipment such as flow cytometers. The use of optical microscopes for cell counting is relatively cheaper, although not very accurate as compared to faster automated cell counting techniques (Guillard and Sieracki, 2005; Marie et al., 2005). Microalgal cells are counted in order to estimate the size of the cultured pop­ulation and to estimate the rate of culture growth (i. e., determination of the rate of population increase) (Guillard and Sieracki, 2005). Microalgal enumeration methods are described in detail in Chapter 4 of this volume.

The important factors affecting microalgal growth are light intensity, tempera­ture, nutrients, CO2 availability, pH, and salinity (Bhola et al., 2011; Rosenberg et al., 2011). Other factors such as conductivity, oxidation/reduction potential (ORP), total dissolved solids (TDS), and biological factors such as protozoa are also important. These factors must be closely monitored to prevent failure of the cultivation system, especially when growing microalgae on a large commercial scale.

There are essentially two commonly used methods for microalgal cultivation, namely open raceway ponds and photobioreactors. The design, and the pros and cons, of these cultivation systems are discussed in detail in Chapter 5. The open raceway system is amenable to large-scale microalgal cultivation because it is simple and cost effective to operate. Despite these attractive features, microalgal biomass harvesting still remains a huge challenge. Harvesting microalgal biomass is technically difficult because the biomass exists as a dilute aqueous suspension. Furthermore, microalgal cells are very difficult to remove due to their miniscule size (<20 pm), similar in density to water (Lavoie and De la Noue, 1986), and strong negative surface charge, particularly during exponential growth (Moraine et al., 1979; Park et al., 2011). It is a relatively daunting task to surmount these drawbacks.

Several methods are available for dewatering and recovering microalgal biomass, such as centrifugation, flocculation, gravity settling, microfiltration, and dissolved air floatation (DAF) inter alia (Lavoie and de la Noue, 1986; Molina Grima et al., 2003). The technology for microalgal biomass harvesting is still in its infancy, and trials on suitable combinations of these methods are currently underway (Williams and Laurens, 2010). The use of the centrifugation technique on a large scale is not cost effective due the colossal amounts of power consumption (Mutanda et al., 2011). The techniques available for microalgal harvesting and dewatering are discussed at length in Chapter 6.

There are several techniques that are used for extracting lipids from microal­gal biomass (Lewis et al., 2000). Most of these methods are destructive; however, it is desirable to develop nondestructive methods for continuous extraction of l ipids from live microalgal cells. The solvent extraction system using a mixture of solvents such as hexane and methanol are commonly used. Other methods are sonication and microwave-assisted extraction. The Bligh and Dyer method (1959) has been commonly used in many applications, whereby lipids are extracted from biological material using a combination of chloroform and methanol (Lewis et al., 2000). Extracting lipids from microalgal biomass is a real challenge because it is intracellular and therefore requires a cell disruption step. Currently, research is ongoing to develop cost-effective and efficient lipid extraction strategies (Molina — Grima et al., 2003; Williams and Laurens, 2010). Subsequent to lipid extraction, it is desirable to accurately identify the lipid and characterize the lipids using highly analytical techniques. This is done to establish whether the lipids extracted are suit­able for application to biodiesel production. Techniques that are widely used for the analysis of lipids are gas chromatography with mass spectrometry (GC-MS), liquid chromatography (LC), matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF), thin-layer chromatography (TLC), etc. Chapter 7 explores in detail the lipid extraction and identification techniques that are commonly used.

Microalgal lipids are converted into biodiesel through transesterification steps. Transesterification of microalgal lipids into biodiesel is accomplished either chemically or biologically using lipolytic enzymes. These methods are outlined in Chapter 8. To establish the feasibility of biodiesel production from microalgae, it is prudent to perform a life cycle analysis (LCA). The procedures involved in LCA are discussed in Chapter 9. Apart from generating biofuels and other value-added prod­ucts, microalgae cultivation is also profoundly involved in climate change abatement through CO2 sequestration. This important application of microalgae is discussed in Chapter 11. Microalgae can use wastewater rich in nitrates and phosphates as sub­strates for growth while simultaneously removing these macronutrients and thereby arresting eutrophication. Therefore, microalgae are involved in the phycoremedia — tion of domestic and industrial wastewaters, and this is achieved in high-rate algal ponds (Chapter 12). Finally, Chapter 13 discusses general microalgal biotechnology in terms of its potential as today’s “green gold rush.” The chapter gives an overview of advanced techniques such as genetic engineering of microalgae so as to increase lipid yield.

ACKNOWLEDGMENTS

The author hereby acknowledges the National Research Foundation (South Africa) for financial assistance.

REFERENCES

Bhola, V., Ramesh, D., Kumari-Santosh, S., Karthikeyan, S., Elumalai, S., and Bux, F. (2011). Effects of parameters affecting biomass yield and thermal behavior of Chlorella vulgaris. Journal of Bioscience and Bioengineering, 111: 377-382.

Bligh, E. G., and Dyer, W. J. (1959). A rapid method for total lipid extraction and purification.

Canadian Journal of Biochemistry and Physiology, 37: 911-917.

Brodie, J., and Lewis, J. (2007). Unravelling the Algae, the Past, Present, and Future of Algal Systematics. CRC Press, London.

Brodie, J., and Zuccarello, G. C. (2007). Systematics of the species-rich algae: Red algal classification, phylogeny and speciation. In The Taxonomy and Systematics of Large and Species-Rich Taxa: Building and Using the Tree of Life (Eds. T. R. Hodkinson and J. Parnell), Systematics Association Series, CRC Press, London, pp. 317-330.

Chisti, Y. (2007). Biodiesel from microalgae. Biotechnology Advances, 25: 294-306. Grobbelaar, J. U. (2009). From laboratory to commercial production: A case study of a Spirulina (Arthrospira) facility in Musina, South Africa. Journal of Applied Phycology, 21: 523-527.

Guillard, R. R.L., and Sieracki, M. S. (2005). Counting cells in cultures with the light micro­scope. In R. A. Andersen (Ed.). Algal Culturing Techniques (pp. 239-252). Elsevier Academic, Burlington, MA.

Lavoie, A., and De la Noue, J., (1987). Harvesting of Scenedesmus obliquus in wastewaters: Auto — or bioflocculation. Biotechnology and Bioengineering, 30: 852-859.

Lewis, A., Nichols, P. D., and McMeekin, T. A. (2000). Evaluation of extraction methods for recovery of fatty acids from lipid-producing microheterotrophs. Journal of Microbiological Methods, 43: 107-116.

Marie, D, Simon, N., and Vaulot, D. (2005). Phytoplankton cell counting by flow cytometry. In R. A. Andersen (Ed.). Algal Culturing Techniques (pp. 253-268). Elsevier Academic, Burlington, MA.

Molina Grima, E., Belarbi, E. H., Acien Fernandez, F. G., Robles Medina, A., and Chisti, Y. (2003). Recovery of microalgal biomass and metabolites: Process options and econom­ics. Biotechnology Advances, 20: 491-515.

Moraine, R., Shelef, G., Meydan, A., and Levi, A., (1979). Algal single cell protein from wastewater treatment and renovation process. Biotechnology and Bioengineering, 21: 1191-1207.

Mutanda, T., Ramesh, D., Karthikeyan, S., Kumari, S., Anandraj, A., and Bux, F. (2011). Bioprospecting for hyper-lipid producing microalgal strains for sustainable biofuel pro­duction. Bioresource Technology, 102: 57-70.

Park, J. B.K., Craggs, R. J., and Shilton, A. N. (2011). Wastewater treatment high rate algal ponds for biofuel production. Bioresource Technology, 102: 35-42.

Rosenberg, J. N., Mathias, A., Korth, K., Betenbaugh, M. J., and Oyler, G. A. (2011). Microalgal biomass production and carbon dioxide sequestration from an integrated ethanol biore­finery in Iowa: A technical appraisal and economic feasibility evaluation. Biomass and Bioenergy, 35: 3865-3876.

Spolaore, P., Joannis-Cassan, C., Duran, E., and Isambert, A. (2006). Commercial applications of microalgae. Journal of Bioscience and Bioengineering, 101, 87-96.

Williams, P. J.B., and Laurens, L. M.L. (2010). Microalgae as biodiesel and biomass feed­stocks: Review and analysis of the biochemistry, energetics and economics. Energy and Environmental Science, 3: 554-590.