Page 232 - Algae Anatomy, Biochemistry, and Biotechnology

Algal Culturing 215

tissue culture flasks can be purchased as single-use sterile units (Iwaki, Nunc, Corning) and used for
transport purposes.
The vessels are capped by non-adsorbent cotton-wool plugs, which will allow gas transfer but
prevent entry of microbial contaminants. A more efficient and costly way of capping is to use foam
plugs or silicone bubble stoppers (bungs), which also allow efficient gas transfer: they are re-usable
and autoclavable. Glass, polypropylene, or metal covers and caps are also used for flasks and tubes.
Materials which should generally be avoided during microalgal culturing include all types of
rubber and PVC.

MEDIA CHOICE AND PREPARATION

The correct maintenance of algal strains is dependent on the choice of growth media and culture
parameters. Two approaches are possible for selection of media composition:

. In theory it is best to work on the principle that if the alga does not need the addition of any
particular chemical substance to the culture media (i.e., if it has no observable positive
effect on growth rate), do not add it.
. In practice it is often easier to follow well-known (and presumably, therefore, well tried)
media recipes, and safer to add substances “just in case” (providing they have no observable
detrimental effect on algal growth).

When choosing a culture medium, the natural habitat of the species in question should be con-
sidered in order to determine its environmental requirements. It is important to know whether the
environment is eutrophic, hence nutrient rich, or oligotrophic, hence nutrient poor, and whether the
algae belong to a r-selected or a k-selected species. r-selected species are characterized by a rapid
growth rate, autotrophic metabolism, and a wide environmental plasticity, whereas k-selected
species shows a slow growth rate, mixotrophic or photoheterotrophic metabolism, and a low
environmental tolerance.
The media recipes currently available are not always adequate for many species, and the exact
choice for a particular species therefore is dependent on trial and error. It must be remembered that
in culturing in general there are (within limits) no right and wrong methods; culture media have
only developed trying out various additions, usually based on theoretical considerations. Refine-
ment of media composition for laboratory-maintained algal cultures have been the object of
research for several decades, resulting in many different media recipes being reported in the litera-
ture and being used in different laboratories.
Media can be classified as being defined or undefined. Defined media, which are often essential
for nutritional studies, have constituents that are all known and can be assigned a chemical formula.
Undefined media, on the other hand, contain one or more natural or complex ingredients, for
example, agar or liver extract and seawater, the composition of which is unknown and may
vary. Defined and undefined media may be further subdivided into freshwater or marine media.
In choosing or formulating a medium, it may be important to decide whether it is likely to
promote heavy bacterial growth. Richly organic media should be avoided unless the algae being
cultivated are axenic. For contaminated cultures, mineral media should be used. These may
contain small amount of organic constituents, such as vitamins or humic acids and, in either
case, provide insufficient carbon for contaminating organisms to outgrow the algae.
Culture collections have attempted to rationalize the number of media recipes, and to standar-
dize recipes for algal strain maintenance. In particular, the use of undefined biphasic media (soil/
water mixture) is declining, due to lack of reproducibility in media batches and occasional contami-
nation of the media from soil samples.