Page 243 - Algae Anatomy, Biochemistry, and Biotechnology

226 Algae: Anatomy, Biochemistry, and Biotechnology

The low biomass and continual depletion of many trace elements from the surface waters of the
open ocean by biogeochemical processes makes this water much cleaner, and therefore preferable
for culturing purposes. Seawater can be stored in polyethylene carboys, and should be stored in cool
dark conditions.
Artificial seawater, made by mixing various salts with deionized water, has the advantage of
being entirely defined from the chemical point of view, but it is very laborious to prepare, and
often does not support satisfactory algal growth. Trace contaminants in the salts used are at rather
high concentrations in artificial seawater because so much salt must be added to achieve the salinity
of full strength seawater. Commercial preparations are available, which consists of synthetic mixes of
the major salts present in natural sea water, such as Tropic Marine Sea Salts produced in Germany for
Quality Marine (USA) and Instant Ocean Sea Salts by Aquarium System (USA).

Nutrients, Trace Metals, and Chelators
The term ‘nutrient’ is colloquially applied to a chemical required in relatively large quantities, but
can be used for any element or compound necessary for algal growth.
The average concentrations of constituents of potential biological importance found in typical
seawater are summarized in Table 6.10. These nutrients can be divided into three groups, I, II, and
III; with decreasing concentration:

. Group I. Concentrations of these constituents exhibit essentially no variation in seawater,
and high algal biomass cannot deplete them in culture media. These constituents do not,
therefore, have to be added to culture media using natural seawater, but do need to be
added to deionized water when making artificial seawater media.
. Group II. Also have quite constant concentrations in seawater, or vary by a factor less than
5. Because microalgal biomass cannot deplete their concentrations significantly, they also
do not need to be added to natural seawater media. Standard artificial media (and some
natural seawater media) add molybdenum (as molybdate), an essential nutrient for algae,
selenium (as selenite), which has been demonstrated to be needed by some algae, as well
as strontium, bromide, and flouride, all of which occur at relatively high concentrations
in seawater, but none of which have been shown to be essential for microalgal growth.
. Group III. All known to be needed by microalgae (silicon is needed only by diatoms and
some chrysophytes, and nickel is only known to be needed to form urease when algae
are using urea as a nitrogen source). These nutrients are generally present at low concen-
trations in natural seawater, and because microalgae take up substantial amounts, concen-
trations vary widely (generally by a factor of 10 to 1000). All of these nutrients (except
silicon and nickel in some circumstances) generally need to be added to culture media in
order to generate significant microalgal biomass.

Nitrate is the nitrogen source most often used in culture media, but ammonium can also be used,
and indeed is the preferential form for many algae because it does not have to be reduced prior to
amino acid synthesis, the point of primary intracellular nitrogen assimilation into the organic
linkage. Ammonium concentrations greater than 25 mM are, however, often reported to be toxic
to phytoplankton, so concentrations should be kept low.
Inorganic (ortho)phosphate, the phosphorus form preferentially used by microalgae, is most
often added to culture media, but organic (glycero)phosphate is sometimes used, particularly
when precipitation of phosphate is anticipated (when nutrients are autoclaved in the culture
media rather than separately, e.g.). Most microalgae are capable of producing cell surface phospha-
tases, which allow them to utilize this and other forms of organic phosphate as a source of phosphorus.
The trace metals that are essential for microalgal growth are incorporated into essential
organic molecules, particularly a variety of coenzyme factors that enter into photosynthetic