Biology Reference
In-Depth Information
of lipids, respectively. Other potentials are production of fuels (through
trans-esterifi cation of lipids) and liquid hydrocarbons (Campo et al., 2007;
Liebezeit, 2005; Jeffrey et al., 1997; Arad and Yaron, 1992). Production
of these compounds is essentially based on the relatively effi cient
photosynthetic machinery of these algae and selection of species for the
required applications. In culturing marine invertebrates, basically three
different approaches can be followed, viz., in situ cultivation, bioreactor
and cell cultures (Liebezeit, 2005). Microalgae can be cultivated in ponds
having large amounts of surface area to be economically viable. The
growth rate and maximum biomass yield of microalgae are infl uenced
by cultivation parameters (light, temperature and pH) and nutritional
status (CO 2 , nitrogen and phosphate concentrations). Stringent nitrogen
limitation conditions are required to stimulate the organisms to produce
required compounds such as lipids. The open ponds must be maintained
at low densities of the organism to allow light to penetrate into the system
for photosynthesis. However, this creates processing problems since large
volumes of water must be processed to recover the small volumes of
biomass. On the other hand, increasing the density of cultures decreases
photon availability to individual cells, since light penetration of microalgal
cultures is poor, especially at high cell densities, adversely affecting specifi c
growth rates. The open culture systems, however, offer advantages such
as low construction cost and ease of operation. Some of the open systems
include shallow ponds, which may be unstirred or paddle-wheeled,
slopping cascade, tubular reactors (helix, plane or double layered),
or laminar reactors. Microalgae such as Chlorella, Dunaliella, Spirulina,
Porphyridium, and Haematococcus spp. have been commercially cultured.
Dunaliella salina is cultured in large (up to 250 ha, approximately) shallow
open-air ponds, with no artifi cial mixing. Similarly, Chlorella are grown
outdoors in either paddle-wheel mixed ponds or circular ponds with a
rotating mixing arm of up to about 1 ha in area per pond.. The production
of microalgae for aquaculture feed is carried out indoors, in 20 to 40 litre
carboys or in large plastic bags of up to 1000 litre. In India, initiated by
Indo-German cooperation, large scale cultivation of Spirulina platensis
was conducted with the aim of utilizing the microalga as animal feed and
nutritional supplement. (Becker and Venkataraman, 1984) The alga was
cultured in large shallow open-air ponds with no artifi cial mixing or using
paddle wheel mixed ponds or circular ponds. Cultivation was carried out
in a high salt and bicarbonate/carbonate (alkaline pH of about 10) medium
under light irradiation and aeration to help optimal photosynthesis by the
alga. Batch tests performed at 25°C in open tanks suggested that inorganic
carbon is preferentially assimilated in the form of bicarbonate and that
its utilization effi ciency depended either on pH. For the rapid growth of
cells often CO 2 is injected into the water. The effi ciencies of photosynthesis
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