6 Cyanobacteria (Chloroxybacteria)

Though microscopic in size and simple in cellular structure, the cyanobacteria played dramatic roles in Earth’s remote past. As noted in Chapter 2, photosynthetic cyanobacteria produced Earth’s first oxygen atmosphere, which fostered the rise of eukaryotes. In addition, ancient cyanobacteria produced massive undersea carbonate deposits, thereby reducing atmospheric carbon dioxide levels and influencing global climate. The first plastids arose from cyanobacteria that forged endosymbiotic partnerships with early protists.

Cyanobacteria remain important today, having both positive and negative impacts from the human perspective. Recognized for their ability to occupy diverse aquatic and terrestrial habitats, cyanobacteria produce organic compounds used by other organisms, and they stabilize sediments and soils. Nitrogen-fixing cyanobacteria increase soil and water fertility and foster the growth of certain plants and fungi in symbiotic associations. Some cyanobacteria have potential biotechnological applications in the production of medicinally useful compounds or hydrogen-based energy (see Chapter 4). Yet the tendency of many cyanobacteria to produce toxins and form harmful blooms is an increasingly serious concern worldwide. This chapter provides a survey of cyanobacterial structure, physiology, ecology, and evolution, designed to be useful in understanding both beneficial and harmful aspects of cyanobacteria.