A photobioreactor - or PBR - is a bioreactor that incorporates some form of light source in order to provide photonic energy input into the reactor. While an open pond could be considered to be one form of a photobioreactor, usually the term photobioreactor only refers to "closed systems", which are PBRs that are closed to the environment in order to prevent the direct exchange of ambient gases and contaminants that might otherwise adversely affect the growth processes intended to occur within the PBR. Photobioreactors are particularly useful and of great importance to the nascent biofuels and biomass production? industry, where the photoautotrophic? growth of algae is seen as one of the most versatile and truly sustainable energy technologies of the future.

There is a growing interest in and development of extremophilic organisms that can be grown in open ponds. However, there are many more "ambient-sensitive" and thus PBR-requiring microalgae that are promising for the production of an enormous variety of high-value compounds. Therefore, maximum optimization of PBR processes is often required in order to fulfill the economic needs of a particular end-product's market. The cultivation of these algae and their products then require the maintenance of a monoculture within enclosed photobioreactors.

A photobioreactor can be described as an enclosed, illuminated culture vessel designed for controlled biomass production of phototrophic? liquid cell suspension cultures. Photobioreactors, despite their costs, have several major advantages over open systems, because they can:

* prevent or minimize contamination, thereby permitting axenic algal cultivation? of cultivating monocultures
* offer better control over biocultural conditions such as pH, light, carbon dioxide, temperature
* prevent water evaporation and subsequent loss of energy
* lower carbon dioxide losses due to out-gassing
* permit higher and/or more optimum cell concentrations
* allow capture of high-value products, such as hydrogen and oxygen?

On the other hand, certain requirements of photobioreactors such as cooling, mixing, control of oxygen accumulation and biofouling?, make these systems more expensive to build and operate than open ponds. New, cheaper innovative systems are being designed and waste streams? are used to make the production of microalgae commercially attractive. An overview of production systems can be found at the research page of Wageningen UR¹.

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¹ Wageningen UR