Cyanobacteria

Cyanobacteria constitute a group of evolutionarily related, ancient, morphologically diverse, and ecologically important bacteria. The majority of strains carry out oxygenic photosynthesis (water-oxidizing, oxygen-evolving, plant-like photosynthesis). With few exceptions, they synthesize chlorophyll a as the major photosynthetic pigment and phycobiliproteins as light-harvesting pigments. They fix CO₂ as the sole source of carbon using primarily the reductive pentose phosphate pathway. Their chemoorganotrophic potential generally is restricted to the mobilization of reserve polymers during dark periods. They display some of the most sophisticated morphological differentiation among bacteria, and many species are truly multicellular organisms. Cyanobacteria have left fossil remains as old as 2000-3500 million years, are ultimately responsible for the oxygenation of Earth’s atmosphere, and constitute the evolutionary ancestry of the photosynthesizing plastids of algae and higher plants. Today, with a global biomass estimated to exceed 10¹⁵ g, they contribute importantly to the global primary production of the oceans and become locally dominant in many extreme environments. Blooms of cyanobacteria are important features for the ecology and management of many eutrophic fresh and brackish water bodies. The aerobic nitrogen-fixing capacity of some makes them important players in the biogeochemical nitrogen cycle of tropical oceans, terrestrial environments, and in some agricultural practices.