Existing commercial microalgae culture systems range in volume from about 102
l to more than
l. However, aside from the specialized small-scale culture systems (<1000 l) other types of
culture systems predominate: large open ponds, circular ponds with a rotating arm to mix the
cultures, raceway ponds, or large bags.
There are several considerations as to which culture system to use. Factors to be considered
include: the biology of the alga, the cost of land, labor, energy, water, nutrients, climate (if the
culture is outdoors), and the type of final product. The various large-scale culture systems also
need to be compared on their basic properties such as their light utilization efficiency, ability to
control temperature, the hydrodynamic stress placed on the algae, the ability to maintain the
culture unialgal or axenic and how easy they are to scale up from laboratory scale to large-scale.
The final choice of system is almost always a compromise between all of these considerations to
achieve an economically acceptable outcome. Successful further development of the industry
requires significant improvements in the design and construction of the photobioreactors as well
as a better understanding of the physiology and physical properties of the microalgae to be grown.
A common feature of most of the algal species currently produced commercially (i.e. Chlorella
, and Dunaliella
) is that they grow in highly selective environments, which means
that they can be grown in open air cultures and still remain relatively free of contamination by
other algae and protozoa. Thus, Chlorella
grows well in nutrient-rich media, Spirulina
high pH and bicarbonate concentration, and Dunaliella salina
grows at very high salinity. Those
species of algae which do not have this selective advantage must be grown in closed systems.
This includes most of the marine algae grown as aquaculture feeds (e.g., Skeletonema
, and Isochrysis
) and the dinoflagellate Crypthecodinium cohnii
a source of long-chain polyunsaturated fatty acids, as well as almost all other species being considered
for commercial mass culture. In the particular case of C. cohnii
, a large scale fermentation
plant is operated by Martek Biosciences in Winchester (USA).