Strain Selection for Biodiesel Production - Biotechnological Applications of Microalgae - page 34

routine, serial sub-culture and storage, preferably under suboptimal temperature and
light regimes that may be similar for most algae. In addition, the nature of the media
also plays a role in the frequency of transfer interval. But, for the achievement of
desirable physiological cultures or for mass cultures, optimal growth conditions are
vital, and this varies greatly with strains. In fact, algae poorly adapted to a specific
medium may alter morphological features, as in the case of Chlamydomonas , where
loss of functional flagella and some cyanobacteria may lose cell surface features.
Another concern needing emphasis, specifically in continuous culture systems, is
that culture conditions such as pH, nutrient content, oxygen level, etc. tend to change
over time, despite the fact that the external environment remains unchanged and the
limiting substrate concentration is at the required concentration. Some microalgae
having an absolute requirement for vitamin B12 at very low concentrations can be
grown without supplementing vitamin B12 in the culture medium for a number of
generations. Complementing medium with vitamins B1 or B12 helps in stimulating
the growth of certain algae. Another intrinsic phenomenon of some diatoms is that
the cell size eventually becomes too small during continuous vegetative propagation
to remain viable. A better alternative is to allow sexual reproduction of the culture
to regenerate large, new vigorous cells. To propagate indefinitely, some Dasycladales
are subjected to undergo periodic sexual reproduction.
One should appraise whether a particular alga strain would be best maintained
for long periods in liquid medium or in agar slants. This is influenced by many envi-
ronmental factors, including the habitat of the strain. A soil-water biphasic medium
favors the growth of filamentous green algae and euglenoids. In fact, the addition
of a soil phase directs the coccoid green algae to retain morphology, and a medium
without soil extract promotes the accumulation of starch granules or lipid droplets.
Hence, a choice of suitable culture medium specific to the strain is crucial. Second,
light intensity must also be considered. For long-term culturing and maintenance
of most microalgae, coupling subdued temperature with light intensities between
10 and 30 mmol photons m −2 s −1 is vital. Excessive light can cause photo-oxidative
stress in some algae. That is one of the reasons that some marine algae of tropical
open-ocean are killed by continuous light (Graham and Wilcox, 2000). Furthermore,
low light intensities are usually preferred by algae with phycobilisomes, while most
dinoflagellates often need higher light intensities (60 to 100 mmol photons m −2 s −1 ).
This directs most culture collections to vary the light:dark regimens between 12:12
and 16:8 hours light:dark. However, preserving algae from extreme environments
needs specific insight, as suggested by Elster et al. (2001). Third, the temperature of
storage is vital. Variations in temperature can more easily influence marine strains
than freshwater strains. In general, microalgal cultures are successfully conserved at
temperatures between 15°C and 20°C. Indeed, some larger service repositories such
as the Culture Collection of Algae at the University of Texas (UTEX) preserve algal
strains at 20°C. Prolonged maintenance at 20°C leads to cellular damage resulting
from photo-inhibition. And, alternatively, increased light intensities coupled with
incubation temperatures higher than 20°C can be employed. However, temperatures
above 20°C are mostly incorrect for conserving stocks at comprehensive transfer
cycles. One should note that the evaporation of the medium effectively regulates the
interval of their transfer cycles. Fourth, the frequency of transfer cycles is considered