Page 245 - Algae Anatomy, Biochemistry, and Biotechnology
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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/)
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