Microalgae are a very heterogeneous group of microorganisms. The term microalgae includes prokaryotes and eukaryotes. Cyanobacteria (blue-green algae) are frequently unicel­lular, with some species forming filaments or aggregates. The internal organization of a cyanobacterial cell is prokaryotic, where a central region (nucleoplasm) is rich in DNA and a peripheral region (chromoplast) contains photosynthetic membranes. The sheets of the photosynthetic membranes are usually arranged in parallel, close to the cell surface. Eukary­otic autotrophic microorganisms are usually divided according to their light-harvesting pho­tosynthetic pigments: Rhodophyta (red algae), Chrysophyceae (golden algae), Phaeophyceae (brown algae), and Chlorophyta (green algae). Their photosynthetic apparatus are organized in special organelles, the chloroplasts, which contain alternating layers of lipoprotein mem­branes (thylakoids) and aqueous phases (Staehelin, 1986).

All photosynthetic organisms contain organic pigments for harvesting light energy. There are three major classes of pigments: chlorophylls (Chl), carotenoids, and phycobilins. The chlorophylls (green pigments) and carotenoids (yellow or orange pigments) are lipophilic and associated in ChI-protein complexes, while phycobilins are hydrophilic. Chlorophyll molecules consist of a tetrapyrrole ring (polar head, chromophore) containing a central magnesium atom and a long-chain terpenoid alcohol. Structurally, the various types of Chl molecules, designated a, b, c, and d, differ in their side-group substituent on the tetrapyr­role ring. All Chl have two major absorption bands: blue or blue-green (450-475 nm) and red (630-675 nm) (Niklas Engstrom, 2012). Chl a is present in all oxygenic photoautotrophs.

Photoautotrophic cultures seldom reach very high cell densities; they are more than an order of magnitude less productive than many heterotrophic microbial cultures, the reason that microalgal cultures are carried in very large volumes. However, the microalgal photosyn­thetic mechanism is simpler than that of higher plants, providing more efficient solar energy conversion. This makes microalgae the most important carbon-fixative group and oxygen producer on the planet. Microalgae cultures have some advantages over vascular plants (Benemann and Oswald, 1996): All physiological functions are carried out in a single cell, they do not differentiate into specialized cells, and they multiply much faster.