Page 231 - Algae Anatomy, Biochemistry, and Biotechnology

214 Algae: Anatomy, Biochemistry, and Biotechnology

collapse due to the disruption of many cellular processes can result from a failure to maintain an
acceptable pH. The latter is accomplished by aerating the culture. In the case of high-density
algal culture, the addition of carbon dioxide allows to correct for increased pH, which may
reach limiting values of up to pH 9 during algal growth.

SALINITY
Marine algae are extremely tolerant to changes in salinity. Most species grow best at a salinity that
is slightly lower than that of their native habitat, which is obtained by diluting sea water with tap
water. Salinities of 20–24 g l 21 are found to be optimal.

MIXING
Mixing is necessary to prevent sedimentation of the algae, to ensure that all cells of the population
are equally exposed to the light and nutrients, to avoid thermal stratification (e.g., in outdoor
cultures), and to improve gas exchange between the culture medium and the air. The latter is of
primary importance as the air contains the carbon source for photosynthesis in the form of
carbon dioxide. For very dense cultures, the CO 2 originating from the air (containing 0.03%
CO 2 ) bubbled through the culture is limiting the algal growth and pure carbon dioxide may be sup-
plemented to the air supply (e.g., at a rate of 1% of the volume of air). CO 2 addition furthermore
2
buffers the water against pH changes as a result of the CO 2 /HCO 3 balance.
Mixing of microalgal cultures may be necessary under certain circumstances: when cells must
be kept in suspension in order to grow (particularly important for heterotrophic dinoflagellates); in
concentrated cultures to prevent nutrient limitation effects due to stacking of cells and to increase
gas diffusion. It should be noted that in the ocean cells seldom experience turbulence, and hence
mixing should be gentle. Depending on the scale of the culture system, mixing is achieved by
stirring daily by hand (test-tubes, erlenmeyers), aerating (bags, tanks), or using paddle wheels and
jet pumps (ponds). Not all algal species can tolerate vigorous mixing. The following methods may
be used: bubbling with air (may damage cells); plankton wheel or roller table (about 1 r.p.m.);
and gentle manual swirling. Most cultures do well without mixing, particularly when not too
concentrated, but when possible, gentle manual swirling (once each day) is recommended.

CULTURE VESSELS

Culture vessels should have the following properties: non-toxic (chemically inert); reasonably
transparent to light; easily cleaned and sterilized; and provide a large surface to volume ratio
(depending on the organism).
Certain materials which could potentially be used for culture vessels may leach chemicals
which have a deleterious effect on algal growth into the medium. The use of chemically inert
materials is particularly important when culturing oceanic plankton and during isolation.
Recommended materials for culture vessels and media preparation include Teflon, polycarbonate,
polystyrene, and borosilicate glass.
Culture vessels are usually borosilicate glass conical flasks (narrow or wide mouth Erlenmeyer
flasks) of various volumes (from tens of milliliters to 3–5 l) or test-tubes for liquid culture, and test-
tubes for agar cultures. Borosilicate glass flasks and tubes, which have been shown to inhibit growth
of some species, can be replaced by more expensive transparent polycarbonate vessels, which offer
excellent clarity and good physical strength. Like borosilicate, polycarbonate is autoclavable, but is
expensive and becomes cloudy and cracks with repeated autoclaving, undergoing some loss of
mechanical strength. Teflon is very expensive and it is used only for media preparation, and
polystyrene, the cheaper alternative to Teflon and polycarbonate, is not autoclavable. Polystyrene