Aeration and mixing
Aeration is important for microalgal culture because:
1) air is a source of carbon (from CO2) for photosynthesis;
2) CO2 provides essential pH stabilization; and 3) physically
mixing the culture keeps nutrients and cells evenly
distributed, reduces self-shading and/or photoinhibition
(a decrease in photosynthesis due to excess light), and
avoids thermal stratification in outdoor systems. Air diffusers
(airstones) create small bubbles that maximize oxygen/
CO2 transfer, and they are frequently used for smallvolume
cultures. In larger culture containers, fine bubbles
from air diffusers create spray and foam that can promote
bacterial growth; larger bubbles (no airstones) actually do
a better job of mixing the culture with minimal foaming.
Common alternatives for mixing larger volume cultures
include jet pumps, paddle wheels, continuous recirculation,
and air-lift pumps (Persoone et al., 1980).
Carbon dioxide (CO2 ) source and pH control
Carbon dioxide plays a dual role in microalgal culture.
It provides a source of carbon to support photosynthesis,
and it helps maintain pH at optimum levels (7.5 to
8.2 for marine species). As culture density increases, more
carbon is consumed through photosynthesis, reducing
CO2 concentration and causing the pH to rise. At about
pH 10 some nutrients will precipitate, algal growth will be
retarded, and the culture could completely collapse. This
can be prevented if the pH is maintained by introducing
CO2 into the air delivery system. This can be done manually
(while the cultures are illuminated), pulsed intermittently
using a timer and solenoid valve, or, most effectively,
by using a pH monitor/controller.
Inoculum
Most hatcheries will culture several species of microalgae
to provide live feeds with different sizes and nutritional
characteristics, depending on the animal being
cultured and its life stage. The culture protocol for each
species will be dictated by the characteristics of the microalgae
(e.g., growth rate and environmental requirements),
harvest yields, and use requirements.