Although algae in general and transgenic algae in particular
describe a promising source for expression products and
other compounds, the commercial application of wild-type
algae is still limited (Borowitzka 1999) and that of transgenic
algae is in its infancy.
One limiting factor for growth is light. On the one hand
it is advantageous that (micro)algae grow up to high densities
in photo-bioreactors or even open pond systems, on the
other hand light becomes limiting in dense cultures beyond
the first few centimeters and this restricts cell growth. One
strategy to solve such problems is the development of special
bioreactors (Morita et al. 2002), the use of stirred tanks,
or shallow ponds. An alternative approach is the use of heterotrophic
algae and addition of the required organic substrate.
Another strategy is to transform photoautotrophic algae
into heterotrophic algae by introducing a gene for a
sugar transporter into their genome by genetic engineering.
This was already accomplished in Volvox carteri (Hallmann
and Sumper 1996) and Phaeodactylum tricornutum (Zaslavskaia
et al. 2001). The diatom Phaeodactylum tricornutum
grew heterotrophically even in the dark with glucose
as the only carbon source (Zaslavskaia et al. 2001).
Algal cultures in bioreactors are normally axenic, but
the culture volume is limited and sterilization is quite expensive.
When large scale production is done in open pond
systems, large starter cultures grown in closed photo-bioreactors
are necessary in order to avoid overrun of the species
of interest by other species in the unsterile open pond
system (Walker et al. 2005b). Alternatively, large closed
culture systems have to be developed that will, most likely,
be very expensive.
Notably, open pond systems also require the development
of strategies that reduce the probability and impact of
gene flow between transgenic algae and their wild relatives.
The likelihood that a transformed alga will escape and that
the transgene will spread in the environment depends on its
potential fitness impact, which is controllable to some extent
by the genetic engineer.
Last but not least, harvesting of algae from an open
pond is still inefficient and expensive, let alone extracting
and purifying bioproducts. So, clearly more work is needed
to optimize algal biotechnology for extensive commercia