Page 230 - Algae Anatomy, Biochemistry, and Biotechnology

Algal Culturing 213

maintenance of flagellates, but may result in decreased cell size in diatom stock cultures. Stock
cultures are maintained for about a month and then transferred to create a new culture line.

CULTURE PARAMETERS

A culture has three distinct components: a culture medium contained in a suitable vessel; the algal
cells growing in the medium; air, to allow exchange of carbon dioxide between medium and
atmosphere.
For an entirely autotrophic alga, all that is needed for growth is light, CO 2 , water, nutrients, and
trace elements. By means of photosynthesis the alga will be able to synthesize all the biochemical
compounds necessary for growth. Only a minority of algae is, however, entirely autotrophic; many
are unable to synthesize certain biochemical compounds (certain vitamins, e.g.) and will require
these to be present in the medium (obligate mixotropy condition, see Chapter 1).
The most important parameters regulating algal growth are nutrient quantity and quality, light,
pH, turbulence, salinity, and temperature. The most optimal parameters as well as the tolerated
ranges are species specific and the various factors may be interdependent and a parameter that is
optimal for one set of conditions is not necessarily optimal for another.

TEMPERATURE
The temperature at which cultures are maintained should ideally be as close as possible to the temp-
erature at which the organisms were collected; polar organisms (,108C); temperate (10–258C);
tropical (.208C). Most commonly cultured species of microalgae tolerate temperatures between
16 and 278C, although this may vary with the composition of the culture medium, the species,
and strain cultured. An intermediate value of 18–208C is most often employed. Temperature-
controlled incubators usually use constant temperature (transfers to different temperatures should
be conducted in steps of 28C per week), although some models permit temperature cycling.
Temperatures lower than 168C will slow down growth, whereas those higher than 358C are
lethal for a number of species.

LIGHT
As for plants, light is the source of energy which drives photosynthetic reactions in algae and in this
regard intensity, spectral quality, and photoperiod need to be considered. Light intensity plays an
important role, but the requirements greatly vary with the culture depth and the density of the algal
culture: at higher depths and cell concentrations the light intensity must be increased to penetrate
through the culture. Too high light intensity (e.g., direct sunlight, small container close to artificial
light) may result in photoinhibition. Most often employed light intensities range between 100 and
200 mE sec 21 m 22 , which corresponds to about 5–10% of full daylight (2000 mE sec 21 m 22 ).
Moreover, overheating due to both natural and artificial illumination should be avoided. Light
may be natural or supplied by fluorescent tubes emitting either in the blue or the red light spectrum,
as these are the most active portions of the light spectrum for photosynthesis. Light intensity and
quality can be manipulated with filters. Many microalgal species do not grow well under constant
illumination, although cultivated phytoplankton develop normally under constant illumination, and
hence a light/dark (LD) cycle is used (maximum 16:8 LD, usually 14:10 or 12:12).

PH
The pH range for most cultured algal species is between 7 and 9, with the optimum range being
8.2–8.7, though there are species that dwell in more acid/basic environments. Complete culture