Page 256 - Algae Anatomy, Biochemistry, and Biotechnology

Algal Culturing 239

In practice, culture crashes can be caused by a variety of reasons, including the depletion of a
nutrient, oxygen deficiency, overheating, pH disturbance, or contamination. The key to the success
of algal production is maintaining all cultures in the exponential phase of growth. Also, the nutri-
tional value of the produced algae is inferior once the culture is beyond Phase 4 due to reduced
digestibility, deficient composition, and possible production of toxic metabolites.
In batch cultures, cell properties such as size, internal nutrient composition, and metabolic func-
tion vary considerably during the above growth phases. This can often make interpretation of the
results difficult. During the exponential growth phase, cell properties tend to be constant. However,
this phase usually only lasts for a short period of time, and if one wishes to estimate growth rates of
the exponential phase of batch cultures, daily sampling appeared to be insufficient to allow a
reasonably accurate estimate. Moreover, the accuracy of growth rate determination is highest in
artificial, defined media as compared to cells grown in natural surface water media.
A significant advantage of batch culture systems is their operational simplicity. The culture
vessels most often consist of an Erlenmeyer flask with a sample to flask volume ratio of about
0.2 in order to prevent carbon dioxide limitation. This volume ratio is only critical if the flasks
are shaken by hand once a day during the culturing run. If the flasks are cultured on a rotating
shaker table a sample to flask volume ratio of 0.5 is permitted.
Batch culture systems are widely applied because of their simplicity and flexibility, allowing to
change species and to remedy defects in the system rapidly. Although often considered as the most
reliable method, batch culture is not necessarily the most efficient method. Batch cultures are har-
vested just prior to the initiation of the stationary phase and must thus always be maintained for a
substantial period of time past the maximum specific growth rate. Also, the quality of the harvested
cells may be less predictable than that in continuous systems and, for example, vary with the timing
of the harvest (time of the day, exact growth phase).
Another disadvantage is the need to prevent contamination during the initial inoculation and
early growth period. Because the density of the desired phytoplankton is low and the concentration
of nutrients is high, any contaminant with a faster growth rate is capable of outgrowing the culture.
Batch cultures also require a lot of labor to harvest, clean, sterilize, refill, and inoculate the
containers.

CONTINUOUS CULTURES
In continuous cultures, resources are potentially infinite: cultures are maintained at a chosen point
on the growth curve by the regulated addition of fresh culture medium. In practice, a volume of
fresh culture medium is added automatically at a rate proportional to the growth rate of the alga,
while an equal volume of culture is removed. This method of culturing algae permits the mainten-
ance of cultures very close to the maximum growth rate, because the culture never runs out of
nutrients.
Fresh growth medium is stored in the large vessel. Air is pumped into the airspace in this
medium vessel. This air pressure will push the medium through a tube which is connected to the
culture vessel. By opening and closing the clamp on this medium line one can add medium to
the culture vessel. Air is also pumped into the culture vessel. This air passes down a long glass
tube to the bottom of the culture and bubbles up. This serves to keep the culture well suspended
as well as high in oxygen and CO 2 . The air flowing into the culture vessel flows out through an
outflow tube. As fresh medium is added to the culture vessel the level of the liquid in the
culture vessel rises; when that level reaches the bottom of the outflow tube, old medium and
cells flow out of the culture vessel into a waste flask. There is another glass tube in the culture
vessel, the sample port. When a sample of cells from the culture vessel is needed the clamp on
the sample port can be opened up and medium and cells flow out.