cyanobacteria

cyanobacteria A large and varied group of bacteria which possess chlorophyll a and which carry out photosynthesis in the presence of light and air with concomitant production of oxygen. They were formerly regarded as algae (division Cyanophyta) and were called ‘blue-green algae’. Fossil cyanobacteria have been found in rocks almost 3000 Ma old and they are common as stromatolite colonies in rocks 2300 Ma old. They are believed to have been the first oxygen-producing organisms and to have been responsible for generating the oxygen in the atmosphere, thus profoundly influencing the subsequent course of evolution. The organisms may be single-celled or filamentous, and may or may not be colonial. Some are capable of a gliding motility when in contact with a solid surface. There are no chloroplasts: chlorophyll is carried on specialized membranes (thykaloids) within the cells. Many species can carry out the fixation of atmospheric nitrogen. Cyanobacteria are widely distributed and are found in freshwater and marine environments, on soil, on rocks, and on plants as epiphytes or as symbionts. The taxonomy of the cyanobacteria is confused. Most of the phycological genera and species (defined when they were regarded as algae) are now known to be based on unreliable characteristics (e.g. characteristics that may depend on conditions of growth) and have been redefined or abandoned according to bacteriological criteria. Five ‘sections’ of cyanobacteria have been recognized. Section I includes unicellular cyanobacteria which reproduce by binary fission only. Section II includes unicellular species in which the cells are enclosed by an external, fibrous layer; reproduction occurs by multiple fission (see BAEOCYTE). Section III includes organisms that form unbranched, single (uniseriate) chains of vegetative cells only (no akinetes or heterocysts). Section IV includes organisms forming unbranched, single chains of cells which may include akinetes and heterocysts. Section V includes the most complex cyanobacteria; they are filamentous but cell division may occur in more than one plane, resulting in filaments which are partly multiseriate.