Cyanophyceae

(botany) A class of photosynthetic monerans distinguished by their algalike biology and bacteriumlike cell organization.

A class of prokaryotic organisms coextensive with the division Cyanophycota of the kingdom Monera. Because these organisms have chlorophyll a and carry out oxygen-evolving photosynthesis, they have traditionally been aligned with algae and, with regard for their characteristic color, called blue-green algae. Microbiologists have emphasized the prokaryotic structure of these organisms and aligned them with bacteria, as the Cyanobacteria. Other names applied to these organisms include Cyanophyta at the level of division and Myxophyceae or Schizophyceae at the level of class. Blue-greens range in form from unicells 1–2 micrometers in diameter to filaments 10 cm (4 in.) long. See also Cyanobacteria.

Unicellular forms, which may aggregate in colonies or loosely constructed filaments and which reproduce by binary fission or spores, constitute the order Chroococcales, from which two additional orders, Chamaesiphonales and Pleurocapsales, are sometimes segregated. Filamentous forms, which reproduce by hormogonia, constitute the order Nostocales (= Hormogonales or Oscillatoriales). The Nostocales may be restricted to unbranched or falsely branched forms (in which the ends of a trichome adjacent to a rupture grow out as a pair of branches), while those forms in which cells divide in more than one plane (true branching) constitute the order Stigonematales.

Geographically and ecologically, blue-green algae are nearly as ubiquitous as bacteria. They are especially abundant in the plankton of neutral or alkaline eutrophic fresh waters and tropical seas, often forming blooms. Habitats for benthic forms include hot springs, snow and ice, soil, rocks, tree trunks, and buildings. Cyanophyceae live symbiotically with a large variety of animals and plants. They constitute the phycobiont of many lichens.

In addition to contributing to food chains, blue-green algae play specific beneficial roles. Nitrogen-fixing forms greatly enrich rice paddies. On the other hand, blue-green algae are often a nuisance. They clog filters, impart undesirable tastes and odors to domestic water supplies, and make unusable or at least unattractive many swimming pools, aquariums, and fountains. Cyanophycean blooms are often toxic to fish, birds, and livestock.

Blue-green algae were pioneers on Earth and are known from rocks at least as old as 2.3 billion years. They are believed to have been responsible for the accumulation of oxygen in the primeval atmosphere and to have been involved in the formation of laminated reeflike structures called stromatolites. See also Algae; Stromatolite.