Commercial algae production facilities employ both open and closed cultivation systems. Each of these has advantages and disadvantages, but both require high capital input (Pienkos and Damns 2009). Open ponds are much more cheaper than closed systems because it demands relatively high capital and O&M costs associated with installation and operation of PBRs (Benemann 2012; Buford et al. 2012).

Lower costs and the possibility to scale up to several hectares make open ponds the main choice for algae commercial production (Benemann 2012). However, open pond cultures suffer from many limitations that can disrupt algal productiv­ity during unexpected environmental events. Another challenge for this system includes having access to an adequate supply of water for growth due to continu­ing loss of water through evaporation. Therefore, open ponds must be in a geo­graphic setting that has a fairly near source of water and a relatively flat terrain to avoid costly earthworks (Buford et al. 2012). Moreover, the open systems are susceptible to wind-born biological agents that can affect the cultivation, such as grazers, infectious fungi, lytic bacteria, viruses, other algae, and also lower tem­peratures in colder climates (Benemann 2012).

These open pond limitations stimulate PBRs development; however, only a few commercial plants use closed PBRs systems, mainly due to high costs as men­tioned before. Nowadays, according to Benemann (2012), microalgae cannot be grown in PBRs for commodities and are not even successful for high-value prod­ucts. However, PBRs can be used for seed culture production, though only for ~0.1 % of the biomass. Closed PBRs are significantly more expensive to construct, but have not been engineered to the extent of other reactors in commercial prac­tice, and so there may be opportunities for significant cost reductions.

Neither open ponds nor closed PBRs are mature technologies. Therefore, until large-scale systems are built and operated over a number of years, many uncer­tainties will remain. Cultivation issues for both open and closed systems, such as reactor construction materials, mixing, optimal cultivation scale, heating/cool — ing, evaporation, O2 build-up, and CO2 administration, have been considered and explored to some degree, but more definitive answers await detailed and expansive scale-up evaluations (Pienkos and Darzins 2009).

Concerning the various algal species and strains, they vary from study to study, depending on location and culture techniques. For that reason, it is not yet possible to predict what species or strain will be the best suited for commercial biofuel pro­duction, but it is most likely that it will differ from case to case, depending on the location, cultivation techniques chosen, processing technologies available, nutri­ents source, local climacteric conditions, and among other potential factors.