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There are thousands of different eubacteria or ‘truebacteria’. They are characterised by the presence of arigid cell wall, and if motile, a flagellum. Thecyanobacteria (also referred to as blue-green algae)have chlorophyll a similar to green plants and arephotosynthetic autotrophs (Figure 2.2). Thecyanobacteria are unicellular, colonial or filamentous,freshwater/marine or terrestrial algae. The coloniesare generally surrounded by gelatinous sheath. Theyoften form blooms in polluted water bodies. Some ofthese organisms can fix atmospheric nitrogen inspecialised cells called heterocysts, e.g., Nostoc andAnabaena. Chemosynthetic autotrophic bacteriaoxidise various inorganic substances such asnitrates, nitrites and ammonia and use the releasedenergy for their ATP production. They play a great rolein recycling nutrients like nitrogen, phosphorous,iron and sulphur.