Figure 1. A light micrograph of Synechocystis sp. - a common bloom-forming marine cyanobacterium. These cells are only about 2 um. Photo used with permission from owners: Jeff Johansen (John Carroll University), Mark Schneegurt (Wichita State University) and Cyanosite (http://www-cyanosite.bio.purdue.edu/index.html).
Cyanobacteria - also known as "blue-green algae" - are the most ancient life form known to inhabit earth, with a fossil record of over 3.5 billion years. They are procaryotic cells, so do not have organelles the way eucaryotic cells do. In fact, eucaryotic plants are the product of endosymbiosis between ancient relatives of extant cyanobacteria ("cyanocyte" below) and non-photosynthetic cells ("phagocyte" below), which engulfed or phagocytosed them.
Figure 2. Endosymbiosis occurs between a phagocyte, a cell
that incorporates another cell into its cytosol, and a cyanocyte,
a hypothetical relative of the extant cyanobacteria.
(Movie file size: 120 kb)
Cyanobacteria can be found as solitary cells, as shown in the image above, or in long filaments. Often, the filamentous forms can produce large mats which can be attached to the benthos or float in the water. If you have a filament, look for the occasional heterocyst cell, which is larger than the simple vegetative cells. Heterocysts are the specialized cells of nitrogen fixing cyanobacteria that convert diatomic atmospheric nitrogen (N 2 ) into biologically available forms, nitrate and nitrite.
Figure 3. Thick-walled nitrogen-fixing heterocyst,
flanked by vegetative cells.
Cyanobacteria are very important members of aquatic ecosystems. To find out more about them, go to Cyanosite, a website devoted to the fascinating world of cyanobacteria.
Cell size: ~ 2 m m
Cell wall: peptidoglycan
Chloroplasts: none, single thylakoid membrane
Photo-pigments: chlorophyll a, phycobilins, carotenoids
Reproduction: simple cell division, filament fragmentation, spores
Ecological roles: nitrogen fixation, toxin producers, blooms
Common genera: Oscillatoria, Synechococcus, Spirulina
© 2001 Caren E. Braby