The Cyanobacteria

The cyanobacteria are a diverse group of more than 60 genera of Gram-negative bacteria. They inhabit a wide range of environments, including freshwater and marine habitats, soils, and the surfaces of rocks. In addition to being photosynthetic, many are able to convert nitrogen gas (N2) to ammonia, which can then be incorporated into cell material. This process, called nitrogen fixation, is an exclusive ability of prokaryotes. â–  nitrogen fixation, p. 775

General Characteristics of Cyanobacteria

Cyanobacteria are morphologically diverse. Some genera are unicellular, with typical prokaryotic shapes such as cocci, rods, and spirals. Others form filamentous multicellular associations called trichomes that may or may not be enclosed within a sheath, a tube that holds and surrounds a chain of cells (figure 11.7)

Motile trichomes glide as a unit. Cyanobacteria that inhabit aquatic environments often have gas vesicles, enabling them to move vertically within the water column. When large numbers of buoyant cyanobacteria accumulate in stagnant lakes or other freshwater habitats, they may form mats on the surface. In the bright, hot conditions of summer, these cells lyse and decay, creating an odiferous scum called a nuisance bloom (figure 11.8).

The photosynthetic systems of the cyanobacteria are like those contained within the chloroplasts of algae and plants. This is not surprising in light of the genetic evidence indicating chloroplasts evolved from a species of cyanobacteria that once resided as an endosymbiont within a eukaryotic cell (see Perspective 3.1). In addition to light-harvesting chlorophyll pigments, cyanobacteria have phycobiliproteins. These pigments absorb energy from wavelengths of light that are not well absorbed by chlorophyll. They contribute to the blue-green, or sometimes reddish, color of the cyanobacteria.

Nitrogen-Fixing Cyanobacteria

Nitrogen-fixing cyanobacteria are critically important ecologically. Because they can incorporate both N2 and CO2 into organic material, they generate a form of these nutrients that can then be used by other organisms. Thus, their activities can ultimately support the growth of a wide range of organisms in environments that would otherwise be devoid of usable nitrogen and carbon. As an example, nitrogen-fixing cyanobacteria that inhabit the oceans are essential primary producers that support other sea life. Also, like all cyanobacteria, they help control atmospheric carbon dioxide buildup by utilizing the gas as a carbon source.

Nitrogenase, the enzyme complex that mediates the process of nitrogen fixation, is destroyed by O2; therefore, nitrogen-fixing cyanobacteria must protect the enzyme from the O2 they generate. Species of Anttbttena,, which are filamentous, isolate nitrogenase by confining the process of nitrogen fixation to a specialized thick-walled cell called a heterocyst

15 mm

15 mm

Nitrogen Fixation Enzyme

Figure 11.8 Nuisance Bloom Excessive growth of cyanobacteria causes buoyant masses to rise to the surface.The eventual death and decay of the cells creates objectionable odors.

Figure 11.7 Cyanobacteria (a)The spiral trichome of Spirulina species. (b) Differential interference contrast photomicrograph of a species of Oscillatoria. Note the arrangement of the individual cells in the trichome.

Figure 11.8 Nuisance Bloom Excessive growth of cyanobacteria causes buoyant masses to rise to the surface.The eventual death and decay of the cells creates objectionable odors.

Nitrogen Fixation Azolla
Figure 11.9 Heterocyst of an Anabaena Species Nitrogen fixation occurs within these specialized cells.

(figure 11.9). Heterocysts are not photosynthetic and, consequently, do not generate O2. The heterocysts of some species form at very regular intervals within the filament, reflecting the ability of cells within a trichome to communicate. One species of Anabaena, A. azollae, forms an intimate relationship with the water fern Azolla. The bacterium grows and fixes nitrogen within the protected environment of a special sac in the fern, providing Azolla with a source of available nitrogen. Synechococcus species, which are unicellular, fix nitrogen only in the dark. Consequently, nitrogen fixation and photosynthesis are temporally separated. Remarkably, members of the genus Trichodesmium fix nitrogen even while they are photosynthesiz-ing. How they protect their nitrogenase enzyme from O2 is still not understood.

Other Notable Characteristics of Cyanobacteria

Cyanobacteria have various other notable characteristics—some are beneficial, others damaging. Filamentous cyanobacteria appear to be responsible for maintaining the structure and productivity of soils in cold desert areas such as the Colorado Plateau. Their sheaths persist in soil, creating a sticky fibrous network that prevents erosion. In addition, these bacteria provide an important source of nitrogen and organic carbon in otherwise nutrient-poor soils. Cyanobacteria growing in freshwater lakes and reservoirs used as a source of drinking water can impart an undesirable taste to the water. This is due to their production of a compound called geosmin, which has a distinctive "earthy" odor. Finally, some aquatic species such as Microcystis aeruginosa can produce toxins. These can be deadly to animals that consume them.

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