The light spectrum and intensity are factors that directly affect the performance of phototrophic microalgal growth, both indoors and outdoors. In outdoor cultures, sunlight is the major energy source, whereas innovations in artificial lighting, such as light-emitting diodes (LED) and optical fiber, are interesting for indoor cultivation systems. In indoor cul­tures, the biggest challenge is the high cost of artificial lighting (Chen et al., 2011).

Regardless of the light source, its usage by microalgae occurs in the same way. In a photo­synthetic system, 8 photons of radiation are required to fix one CO2 molecule in the form of carbohydrate; this results in the maximum photosynthetic efficiency (Chini-Zittelli et al., 2006).

Multiproteic complexes, also called photosystems, catalyze the conversion reaction of light energy captured by excited molecules of chlorophyll into the form of usable energy. A photosystem consists of a center of photochemical reaction consisting of a protein com­plex, and molecules of chlorophyll that enable the conversion of light energy into chemical energy. This photosystem also has an antenna complex consisting of pigment molecules that capture light energy and feed the reaction center. The antenna complex is important for the capture of light. In chloroplasts, it consists of a cluster of hundreds of chlorophyll molecules held together by proteins that keep them firmly together on the thylakoid mem­brane (Alberts et al., 2008).

When a chlorophyll molecule from the antenna complex is excited, the energy is rapidly transmitted from one molecule to another through a resonance energy transfer process until it reaches a special pair of chlorophyll molecules from the center of the photochemical reaction. Each antenna complex acts like a funnel collecting light energy and directing it to a specific site where it can be used effectively (Alberts et al., 2008). One strategy to optimize the utili­zation of light is to reduce the size of the antenna, which makes the cells less opaque and facilitates the transmission of light (Chen et al., 2011).

Several studies have been developed to improve the efficiency of light utilization and re­duce the costs of systems with artificial lighting. The advantage of cultivation in a laboratory is that is uses fluorescent tubes. Although they consume high amounts of energy, that usage can be reduced by more than 50% with the use of LEDs. Many cultures use only solar energy as a light source, which has no cost. However, the performance of outdoor systems is lower than indoor ones, and they require large areas of land (Chen et al., 2011).