Algae Need Their Vitamins

Algae are simple photosynthetic eukaryotes which-owing to their colonization of the oceans-are responsible for up to 50% of the planet's atmospheric carbon fixation (22).They comprise a diverse group that can broadly be defined as unicellular (microalgae) or multicellular (macroalgae) photosynthetic organisms that lack roots, stems, leaves, conducting vessels, and complex sex organs (55).A single endosymbiotic event between a cyanobacterium-like organism and a nonphotosynthetic eukaryote is thought to have given rise to the three basal groups of algae: the chlorophyta (from which higher plants arose), the glaucocystophyta, and the rhodophyta (Fig. 1).The chlorophyta and the rhodophyta, via secondary and tertiary endosymbiotic events with different nonphotosynthetic eukaryotes, gave rise to algal groups with complex plastids (Fig. 1).Some groups, such as the apicomplexans (e.g., Plasmodium falciparum) subsequently lost the ability to photosynthesize, although they still retain plastids.The number of symbioses that have occurred during the evolution of algae has been heavily debated, and the details are discussed elsewhere (46), but there are likely to have been several events.Consequently, it is not surprising that the physiology and metabolism of algae are extremely varied.For example, while the majority of green algae contain a highly structured cell wall comprising glycoproteins (19), euglenophyta simply contain a protein layer (known as the pellicle) beneath the cell membrane, and the cell walls of diatoms are made from silica (24).Dinophyta, euglenophyta, and heterokontophyta contain members that are phagotrophic on bacterial prey, but this characteristic is absent from the groups with simple plastids.Furthermore, although most algae are regarded as free-living organisms, many dinoflagellates are closely associated with corals, and members of several algal groups live with fungi as lichens (13), providing photosynthate for their heterotrophic partner.