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
2
3
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).