Eukaryotic algae are a collection of extremely diverse, nonrelated organisms that perform photosynthesis in plastids, permanent organelles of green, brown, or bluish colors derived from endosymbiosis. In contrast to plants, algae do not form embryos.
Algae is a term of convenience and refers to a collection of highly diverse organisms that undertake photosynthesis and/or possess plastids (Keeling, 2004). Many authors even include the prokaryotic cyanobacteria into the algae, because they exhibit a life-style rather similar to their eukaryotic counterparts and often share the same habitat with eukaryotic algae. Cyanobacteria form the origin of plastids (for reviews see McFadden, 2001; Keeling, 2004; Palmer, 2003). Plastids are the organelles of plants and eukaryotic algae that harbor photosynthesis and synthesize many chemical compounds also important for other biochemical pathways (e.g., aromatic amino acids, heme, isoprenoids, and fatty acids);