The world production of seaweeds was some 8 million tons in 2003 (McHugh 2003). Seaweeds are used in the production of food, feed, chemicals, cosmetics, and phar­maceutical products.

Most microalgae are strictly photosynthetic, i. e., they need light and carbon diox­ide as energy and carbon sources. This culture mode is usually called photoau­totrophic. Some algae species, however, are capable of growing in darkness and of using organic carbons (such as glucose or acetate) as energy and carbon sources. This culture mode is termed heterotrophic.

Microalgae cultivation using sunlight energy can be carried out in open or cov­ered ponds or closed photobioreactors, based on tubular, flat plate, or other designs. Closed systems are much more expensive than ponds, present significant operating challenges (overheating, fouling), and, due to, among other things, gas exchange limitations, cannot be scaled up much beyond approx. 100 m2 for an individual growth unit.

The concept of using microalgae as a source of fuel is older than most people realize. The idea of producing methane gas from algae was proposed in the early 1950s. Currently there are three types of industrial reactors used for algal culture: (1) photobioreactors, (2) open ponds, and (3) closed and hybrid systems.

Photobioreactors are different types of tanks or closed systems in which algae are cultivated. Open-pond systems are shallow ponds in which algae are cultivated. Nutrients can be provided through runoff water from nearby land areas or by chan­neling the water from sewage/water treatment plants. Technical and biological lim­itations of these open systems have given rise to the development of enclosed pho­toreactors. Microalgae cultivation using sunlight energy can be carried out in open or covered ponds or closed photobioreactors, based on tubular, flat plate, or other de­signs. A few open systems are presented for which particularly reliable results are available. Emphasis is then put on closed systems, which have been considered to be capital intensive and are justified only when a fine chemical is to be produced. Mi­croalgae production in closed photobioreactors is highly expensive. Closed systems are much more expensive than ponds. However, closed systems require much less light and agricultural land to grow algae. High oil species of microalgae cultured in growth-optimized conditions of photobioreactors have the potential to yield 19,000 to 57,000 L of microalgal oil per acre per year. The yield of oil from algae is over 200 times the yield from the best-performing plant/vegetable oils (Chisti 2007).

Large-scale production of microalgal biomass generally uses continuous culture during daylight. In this method of operation, fresh culture medium is fed at a con­stant rate and the same quantity of microalgal broth is withdrawn continuously (Molina Grima et al. 1999). Feeding ceases during the night, but the mixing of broth must continue to prevent settling of the biomass (Molina Grima et al. 1999). As much as 25% of the biomass produced during daylight may be lost during the night due to respiration. The extent of this loss depends on the light level under which the biomass was grown, the growth temperature, and the temperature at night (Chisti 2007).

Algal cultures consist of a single or several specific strains optimized for pro­ducing the desired product. Water, necessary nutrients, and CO2 are provided in a controlled way, while oxygen has to be removed (Carlsson et al. 2007). Algae receive sunlight either directly through the transparent container walls or via light fibers or tubes that channel it from sunlight collectors. A great amount of devel­opmental work to optimize different photobioreactor systems for algae cultivation has been carried out and is reviewed in Janssen et al. (2003), Choi et al. (2003), Carvalho et al. (2006), and Hankamer et al. (2007).

Bioreactors are the preferred method for scientific researchers, and recently for some newer, innovative production designs. These systems are more expensive to build and operate; however, they allow for a very controlled environment. This means that gas levels, temperature, pH, mixing, media concentration, and light can be optimized for maximum production (Chisti 2007). Unlike open ponds, bioreac­tors can ensure a single alga species is grown without interference or competition (Campbell 2008).