ABSTRACT
Microalgae is a collective term for tiny, multi-originated and oxygen evolving photosynthetic organisms that present one of most diverse and dynamic communities on the planet Earth. The origin and phylogeny of this microalgal world is still debatable but it is well established that most groups have the same ancestors i.e., cyanobacteria such as prokaryotes, which facilitates the evolution of the cyanobacteria and prochlorophytes. The primary endosymbiotic events between cyanobacteria and heterotrophic eukaryotes, give birth to primary plastids containing glaucophytes, red algae and green algae. Later subsequent secondary or possibly tertiary endosymbiotic events between autotrophic and heterotrophic eukaryotes induce the origin of cryptophytes, euglenoids, dinoflagellates and chromophytes.
Microalgae inhabit all the possible terrestrial, freshwater and marine ecosystems. Their diversity is evident from ice sheets to glaciers to deserts; snow to barren rocks to hot-springs; and poles to high mountains to oceanic environments. Their significant role as primary producers plays a distinctive role in nutrient cycling of major elements in the hydrosphere of the planet Earth. They are single unicellular to colonial to multi-cellular filamentous; non-flagellates to flagellates. In response to environmental conditions, microalgae develop some morphologically different structures such as heterocyst, hormogonia and akinetes that specialize mainly for nitrogen fixation and reproduction respectively. This chapter discusses the origin, distribution and morphology of microalgae.
