Closed systems offer a much better control of cultivation variables than open systems (Table 2) as there is less influence from
outside; for example, contamination with bacteria, algae, or grazers
becomes less likely in a closed system compared to open ponds
[15]. Areal biomass productivity varies a lot between different
cultivation systems but is generally higher in closed systems [15].
Photobioreactors, on the other hand, have some challenges
that are less problematic in open systems such as gas exchange
and need for cooling. Gas exchange, getting enough CO 2 to the
algae, is critically important for satisfying the carbon needed for
running photosynthesis and also for maintaining the pH. CO 2
dissolved in water is a weak acid and biological uptake of CO 2 by
algae will increase the pH. In dense cultures, the pH may increase
to critical levels, preventing further growth without proper gas
exchange. Another critical component is removal of O 2 , which
rapidly builds up to supersaturated concentrations, which will also
slow down or stop growth. The temperature in a closed system
rapidly increases when exposed to the sun and most algae will stop
growing and start to decay in temperatures above 30–35
C. Proper
cooling is therefore vital and there are several technical solutions
ranging from spraying the cultivation unit with cooling water or by
having water cooling units connected to the cultivation system.
Fig. 3 Closed photobioreactor with a plate design. Similar to the tubular reactor in Fig. 2 the algal culture is
pumped from a gas exchange unit to the panel unit where the algae collects light. Picture: IGV Biotech,
Wikipedia, CC BY-SA 3.0
Large Scale Cultivation of Microalgae
5
outside; for example, contamination with bacteria, algae, or grazers
becomes less likely in a closed system compared to open ponds
[15]. Areal biomass productivity varies a lot between different
cultivation systems but is generally higher in closed systems [15].
Photobioreactors, on the other hand, have some challenges
that are less problematic in open systems such as gas exchange
and need for cooling. Gas exchange, getting enough CO 2 to the
algae, is critically important for satisfying the carbon needed for
running photosynthesis and also for maintaining the pH. CO 2
dissolved in water is a weak acid and biological uptake of CO 2 by
algae will increase the pH. In dense cultures, the pH may increase
to critical levels, preventing further growth without proper gas
exchange. Another critical component is removal of O 2 , which
rapidly builds up to supersaturated concentrations, which will also
slow down or stop growth. The temperature in a closed system
rapidly increases when exposed to the sun and most algae will stop
growing and start to decay in temperatures above 30–35
C. Proper
cooling is therefore vital and there are several technical solutions
ranging from spraying the cultivation unit with cooling water or by
having water cooling units connected to the cultivation system.
Fig. 3 Closed photobioreactor with a plate design. Similar to the tubular reactor in Fig. 2 the algal culture is
pumped from a gas exchange unit to the panel unit where the algae collects light. Picture: IGV Biotech,
Wikipedia, CC BY-SA 3.0
Large Scale Cultivation of Microalgae
5