Page 260 - Algae Anatomy, Biochemistry, and Biotechnology
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Algal Culturing                                                             243

                 around the world (Australia, Israel, Hawaii, Mexico, China). These algae are a source for viable and
                 inexpensive carotenoids, pigments, proteins, and vitamins that can be used for the production of
                 nutraceuticals, pharmaceuticals, animal feed additives, and cosmetics. Mass algal cultures in
                 outdoor ponds are applied in Taiwanese shrimp hatcheries where Skeletonema costatum is pro-
                 duced successfully in rectangular outdoor concrete ponds of 10–40 tons of water volume and a
                 water depth of 1.5–2 m.

                 Photobioreactors

                 An alternative to open ponds for large-scale production of microalgal biomass are photobioreactors.
                 The term “photobioreactor” is used to indicate only closed systems that do not allow direct
                 exchange of gases or contaminants between the algal culture they contain and the atmosphere.
                 These devices provide a protected environment for cultivated species, relatively safe from contami-
                 nation by other microrganisms, in which culture parameters such as pH, oxygen and carbonic
                 dioxide concentration, and temperature can be better controlled, and provided in known amount.
                 Moreover, they prevent evaporation and reduce water use, lower CO 2 losses due to outgassing,
                 permit higher cell concentration, thus reducing operating costs, and attain higher productivity.
                 However, these systems are more expensive to build and operate than ponds, due to the need of
                 cooling, strict control of oxygen accumulation, and biofouling, and their use must be limited to
                 the production of very high-value compounds from algae that cannot be cultivated in open
                 ponds. Different categories of photobioreactors exist, such as axenic photobioreactors; tubular or
                 flat photobioreactors; horizontal, inclined, vertical, or spiral; manifold or serpentine photobioreactors;
                 air or pump mixed; single phase, filled with culture suspension, with gas exchange taking place in a
                 separate gas exchanger, or two-phase, with both the gas and the liquid phase contained in the
                 photostage.
                     The use of these devices dates back to the late 1940s, as a consequence of the investigation on
                 the fundamental of photosynthesis carried out with Chlorella. Open systems were considered inap-
                 propriate to guarantee the necessary degree of control and optimization of the continuous cultiva-
                 tion process. From the first vertical tubular reactors set up in the 1950s for the culture of Chlorella
                 under both artificial light and sunlight, several types of photobioreactors have been designed and
                 experimented with. Most of these are small-scale systems, for which experimentation has been con-
                 ducted mainly indoors, and only few have been scaled up to commercial level. Significantly higher
                 photosynthetic efficiencies and a higher degree of system reliability have been achieved in recent
                 years, due in particular to the progress in understanding the growth dynamic and requirements of
                 microalgae under mass cultivation conditions. Notwithstanding these advances, there are only few
                 examples of photobioreactor technology that has expanded from the laboratory to the market,
                 proving to be commercially successful. In fact, the principle obstacle remains the scaling-up
                 phase, due to the difficulties of transferring a process developed at the laboratory scale to industrial
                 scale in a reliable and efficient way. Two of the largest commercial systems in operation at present
                 are the Klo ¨tze plant in Germany for the production of Chlorella biomass and the Algatechnologies
                 plant in Israel for the production of Haematococcus biomass. Both plants utilize tubular,
                 pump-mixed, single phase photobioreactors; in particular, the Klo ¨tze plant consists of compact
                 and vertically arranged horizontal running glass tubes of a total length of 500,000 m and a total
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                 PBR volume of 700 m . In a glasshouse requiring an area of only 10,000 m an annual production
                 of 130–150 tons dry biomass was demonstrated to be economically feasible under Central Euro-
                 pean conditions.
                     Other industrial plants actually operating are the plant built in Maui, Hawaii (USA) by Micro-
                 Gaia Ltd. (now BioReal, Inc. a subsidiary of Fuji Chemical Industry Co., Ltd.), which is based on a
                 rather complex design, called BioDome TM , for the cultivation of Haematococcus; the rigid, plastic
                 tubes photobioreactor of AAPS (Addavita Ltd., UK) and the flexible, plastic tubes photobioreactor
                 of the Mera Growth Module (Mera Pharmaceuticals, Inc., USA).
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