Page 245 - Algae Anatomy, Biochemistry, and Biotechnology
P. 245
228 Algae: Anatomy, Biochemistry, and Biotechnology
metal chelators in undefined quantities, each batch being different, and hence having unpredictable
effects on microalgae. With increasing understanding of the importance of various constituents of
culture media, soil extract is less frequently used.
Buffers
The control of pH in culture media is important because certain algae grow only within narrowly
defined pH ranges in order to prevent the formation of precipitates. Except under unusual
conditions, the pH of natural seawater is around 8. Because of the large buffering capacity of
natural seawater due to its bicarbonate buffering system (refer to Chapter 4) it is quite easy to main-
tain the pH of marine culture media. The buffer system is overwhelmed only during autoclaving,
when high temperatures drive CO 2 out of solution and hence cause a shift in the bicarbonate
buffer system and an increase in pH, or in very dense cultures of microalgae, when enough CO 2
is taken up to produce a similar effect.
As culture medium cools after autoclaving, CO 2 reenters solution from the atmosphere, but
certain measures must be taken if normal pH is not fully restored:
. The pH of seawater may be lowered prior to autoclaving (adjustment to pH 7–7.5 with 1 M
HCl) to compensate for subsequent increases.
. Certain media recipes include additions of extra buffer, either as bicarbonate, Tris (Tris-
hydroxymethyl-aminomethane), or glycylglycine to supplement the natural buffering
system. Tris may also act as a Cu buffer, but has occasionally been cited for its toxic prop-
erties to microalgae such as Haematococcus sp. Glycylglycine is rapidly metabolized by
bacteria and hence can only be used with axenic cultures. These additions are generally
not necessary if media are filter sterilized, unless very high cell densities are expected.
. The problem of CO 2 depletion in dense cultures may be reduced by having a large surface
area of media exposed to the atmosphere relative to the volume of the culture, or by bub-
bling with either air (CO 2 concentration ca. 0.03%) or air with increased CO 2 concentrations
(0.5–5%). Unless there is a large amount of biomass taking up the CO 2 , the higher
concentrations could actually cause a significant decline in pH. When bubbling is
employed, the gas must first pass through an inline 0.2 mm filter unit (e.g., Millipore
Millex GS) to maintain sterile conditions. For many microalgal species, aeration is not
an option because the physical disturbance may inhibit growth or kill cells.
Some of the most commonly used marine media, defined and undefined, are listed in
Table 6.11–Table 6.17. Table 6.18 indicates the algal classes that have been successfully cultured
in the media included in this chapter.
For a full range of possible media refer to the catalog of strains from culture collections present
all over the world such as:
. SAG (Experimentelle Phykologie und Sammlung von Algenkulturen, University of
Gottigen, http://www.epsag.uni-goettingen.de/)
. UTEX (Culture Collection of Algae at the University of Texas at Austin, http://
www.bio.utexas.edu/research/utex/)
. CCAP (Culture Collection of Algae and Protozoa, Argyll, Scotland, http://www.ife.ac.uk/
ccap/)
. UTCC (University of Toronto Culture Collection of Algae and Cyanobacteria, http://
www.botany.utoronto.ca/utcc/)
. CCMP (The Provasoli-Guillard National Center for Culture of Marine Phytoplankton,
Maine, http://ccmp.bigelow.org/)