A closed photobioreactor system is essentially a series of plas- tic or glass containers for water and algae. Proponents of such bioreactors (compared to open ponds) say the growth environment can be more easily controlled; they prevent evaporation; and light penetrates through all sides of the container, which increases cell density. However, bioreactors suffer high materials and energy costs as well as mixing and gas-exchange inefficiencies; therefore scalability remains problematic.

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