Cyanobacteria (Blue Green Algae)

Cyanobacteria, popularly called Blue Green Algae or Cyanophyceae, are ancient, photosynthetic, prokaryotic organisms. Their evolutionary age is written into the fossil record as stromatolites - deposits formed when cyanobacterial colonies bind with calcium carbonate - some 2.7 billion years old, and their abundance in that record indicates they were major contributors to raising the level of free oxygen in Earth's early atmosphere. Most cyanobacteria live in fresh water, with only a few marine forms (e.g. Trichodesmium, which can turn stretches of sea red through a bloom). Several genera (Nostoc, Anabaena) form nitrogen-fixing symbiotic relationships, living endophytically in the coralloid roots of Cycas, the leaves of the aquatic fern Azolla, and the thallus of hornworts like Anthoceros; others (Gloeocapsa, Nostoc, Scytonema) live as the algal partner (phycobiont) inside lichens. Structurally, cyanobacteria are prokaryotes lacking any motile reproductive structure, though a gliding movement is seen in some (e.g. Oscillatoria); their thallus form ranges from unicellular (Chroococcus) through colonial (Gloeocapsa) to filamentous/trichome (Nostoc). Their protoplasm is differentiated into a central centroplasm and a peripheral, pigment-bearing chromoplasm, and their photosynthetic pigments include c-phycocyanin and c-phycoerythrin alongside myxoxanthin and myxoxanthophyll; their reserve food is cyanophycean starch. Some filaments carry a large, colourless cell called a heterocyst (terminal or intercalary in position) that is specialised for nitrogen fixation. Reproduction is purely vegetative (no sexual reproduction at all), through akinetes (thick-walled dormant cells formed from a vegetative cell), hormogonia (a detached filament fragment that reproduces by cell division), fission and endospores. A characteristic mucilage sheath surrounds the thallus - the reas …