Algae are organisms that have plastids, or organisms that are derived from cells whose ancestors possessed plastids. Until 1994, it was thought that the apicomplexa did not have plastids (and consequently were not covered in phycology textbooks). Then it was shown that a known organelle in many apicomplexa was actually a reduced colorless plastid called an apicoplast (Fig. 8.1) (Wilson, 1993; Wilson et al., 1994). Molecular studies have shown that the apicoplast and dinoflagellate plastids originated from red algae by a single endosymbiotic event that occurred relatively early in eukaryotic evolution (Fast et al., 2001).

The discovery of the apicoplast generated considerable interest since most apicomplexans are unicellular endoparasites that cause some of the most significant tropical diseases (Foth and McFadden, 2003). Malaria in humans is produced by the apicomplexan Plasmodium. About 300 million people are infected with malaria, leading to one million deaths annually (Ralph et al., 2004). Apicomplexans cause other serious diseases in livestock and humans, such as cryptosporidiosis, babesiosis (Texas cattle fever), theileriosis (East Coast fever), and toxoplasmosis. The realization that these endoparasites were once algae raised hopes that the apicoplast might be a drug target for two reasons. The first is that the apicoplast is essential for the survival of Plasmodium and Toxoplasma. The second is that drugs effective against prokaryotic organisms might be effective against the apicoplast since all plastids originally evolved from endosymbiotic prokaryotic cyanobacteria.