Studies on microalgae are preferably done under controlled conditions. Microalgae biore­actors are often designed differently from bioreactors used to grow other microorganisms. Two parameters are the most important in algae cultivation: efficiency of light utilization and availability of dissolved CO2.

Like any organism, microalgae have nutritional requirements: carbon sources, energy, water, and inorganic nutrients. In the case of microalgae, the carbon source can be CO2 and the energy comes from sunlight. As microalgae grow in aqueous suspension, the manip­ulation and control of culture conditions makes their cultivation feasible, thus the productiv­ity is limited mostly by the available of light. Responses by algal cells to nutrients and cultivation environments can be used to manipulate the processes to favor the production of algal biomass (Benemann et al., 2002).

The development of media for microalgae cultivation involves a sufficient carbon source (carbon is a part of all the organic molecules in the cell, making up as much as 50% of the algal biomass); salt concentration (depending on the original biotope of the alga); nitrogen (represents about 5-10% of microalgae dry weight); phosphorus (part of DNA, RNA, ATP, cell membrane); sulfur (constituent of amino acids, vitamins, sulfolipids and is involved in protein biosynthesis); potassium (cofactor for several enzymes and involved in protein syn­thesis and osmotic regulation); magnesium (the central atom of the chlorophyll molecule); iron (constituent of cytrochromes and important in nitrogen assimilation); pH of the medium; temperature; trace elements, and addition of organic compounds and growth promoters.

Carbon is important because it is the source of energy for many cellular events (such as metabolites production) and reproduction and is part of the physical structure of the cell. In conditions of low dissolved inorganic carbon (DIC), a DIC transport is induced in most microalgae (Matsuda and Colman, 1995), allowing normal cell growth.

Depending on the material used in cultivation of microalgae and the utilization of biomass, three different systems can be distinguished (Becker, 1994):

1. Systems in which a selected algal strain is grown in a so-called clean process, using fresh water, mineral nutrients, and carbon sources. The algae in such systems are intended to be utilized mainly as food supplements.

2. Systems using sewage or industrial wastewater as the culture medium. The cultivation of the microalgae involves secondary (BOD removal) and tertiary (nutrient removal) treatments and production of biomass-based products.

3. Cultivation of algae in enclosed systems under sunlight or artificial light, with cells preferably being grown in autotrophic media.

Microalgae are microorganisms that are capable of producing many different compounds of industrial interest, some with high and some with low aggregated value. The final value of the product and its destination directly influence the conditions of cultivation. Therapeutical compounds produced by microalgae, for example, must be produced through a totally con­trolled and clean process, whereas for the fuel industry residues can be used and the control of the process can be less accurate. The low culture concentration and the corresponding high downstream costs define production trends.

The utilization of complex media (those of which the composition is not determined, such as industrial residues) in the cultivation of microalgae is one alternative to make the produc­tion of some microalgal metabolites economically feasible. Associated with residue compo­sition and microalgae metabolism, knowledge of the needs of the microalgae might save time (and money) in the development of a process. It is very important to supply all microalgae chemical needs because it is known that variations in the chemical composition of phytoplankton are also tightly coupled to changes in growth rate (Goldman et al., 1979).