CYANOBACTERIA 37

Fig. 2.7 Rotation of the cyanobacterial filament depends on the orientation of the oscillin protein. Mucilage is secreted from the pores near the cross walls. The mucilage flows along the oscillin fibers causing rotation if the oscillin is helically oriented. There is no rotation if the oscillin is not helically oriented.

face by a reiterative process of pili extension, adhesion, and retraction (Bhaya, 2004).

Synechocystis exhibits both positive and negative phototaxis in blue light (450 nm wavelength) but not in red or far-red light (Terauchi and Ohmori, 2004). Blue light stimulates the production of cyclic adenosine monophosphate (cAMP), a common second messenger in biological systems (Fig. 2.9).

Sheaths

A sheath (capsule or extracellular polymeric substances (EPS)) composed of mucilage and a small amount of cellulose is commonly present in cyanobacteria (Nobles et al., 2001) (Figs. 2.10, 2.11). The sheath protects cells from drying. Active growth appears necessary for sheath formation, a fact that may explain its sometimes poor development around spores and akinetes. The sheath of Gloeothece sp. is composed of polysaccharides with neutral sugars and uronic acids including galactose, glucose, mannose, rhamnose, 2-O-methyl-D-xylose, glucuronic acid and galacturonic acids (Weckesser et al., 1987). The

Fig. 2.8 Transmission electron micrographs of negatively

stained whole cells of Synechocystis showing pili. (WT) Wild

type. (From Bhaya et al., 1999.)

38 THE PROKARYOTIC ALGAE

Fig. 2.9. The enzyme adenyl cyclase catalyzes the

formation of cAMP from ATP.

sheath of Gloeothece contains only 2% protein and a trace of fatty acids and phosphate. The commercial applications of cyanobacterial EPS have been reviewed by De Philippis and Vincenzini (1998). Sheaths are often colored, with red sheaths found in algae from highly acid soils and blue sheaths characteristic of algae from basic soils (Drouet, 1978). Yellow and brown sheaths are common in specimens from habitats of high salt content, particularly after the algae dry out.

The sheath excludes India ink so the easiest way to visualize the sheath is to place a small amount of India ink in the water (Fig. 2.10). Production of a sheath is dependent on environmental conditions. A shortage of CO2 results in cessation of sheath production and release of

Fig. 2.10 A drawing of a filament of Hyella sp. in India ink.

This method clearly shows the sheath around the filament.

the sheath. An excess of fixed carbon results in formation of a sheath (Otero and Vincenzini, 2004).

Protoplasmic structure

Many of the protoplasmic structures found in the bacteria occur in the cyanobacteria. In the central protoplasm are the circular fibrils of DNA which are not associated with basic proteins (histones) (Figs. 2.11 and 2.14). The amount of DNA in unicellular cyanobacteria varies from 1.6 109 to 8.6 109 daltons. This is similar to the genome size in bacteria (1.0 109 to 3.6 109 daltons) and is larger than the genome size in mycoplasmas (0.4 109 to 0.5 109 daltons) (Herdman et al., 1979). The peripheral protoplasm is composed principally of thylakoids and their associated structures, the phycobilisomes (on the thylakoids, containing the phycobiliproteins) and glycogen granules. The 70S ribosomes are dispersed throughout the cyanobacterial cell but are present in the highest density in the central region around the nucleoplasm (Allen, 1984).

Cyanophycin is a non-ribosomally synthesized protein-like polymer that occurs in the cytoplasm in structured granules that are not surrounded by a membrane (Fig. 2.13) (Aboulmagd et al., 2000; Sherman et al., 2000). Cyanophycin is a polymer that consists of equimolar amounts of arginine and aspartic acid arranged as a polyaspartate

CYANOBACTERIA 39

Fig. 2.11 Drawing of the fine-structural features of a cyanobacterial cell. (C) Cyanophycin body (structured granule); (Car) carboxysome (polyhedral body); (D) DNA fibrils; (G) gas vesicles; (P) plasmalemma; (PB) polyphosphate body; (PG) polyglucan granules; (Py) phycobilisomes; (R) ribosomes; (S) sheath; (W) wall.

Fig. 2.13 Transmission electron micrograph of a section of a cell of Plectonema boryanum showing cyanophycin bodies

(C). (From Lawry and Simon, 1982.)

Fig. 2.12 Cyanophycin is composed of equimolar amounts of arginine (Arg) and aspartic acid (Asp) arranged as a polyaspartate backbone.

backbone (Fig. 2.12). Cyanophycin functions as a temporary nitrogen reserve in nitrogen-fixing cyanobacteria, accumulating during the transition from the exponential to the stationary phase and disappearing when balanced growth resumes. Nitrogen is stored in phycobilisomes in cyanobacteria that do not fix nitrogen (Li et al., 2001a).

Carboxysomes (polyhedral bodies) (Fig. 2.14) are similar to the carboxysomes in bacteria and contain the carbon dioxide-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). There are two types of carboxysomes,

-carboxysomes and -carboxysomes, which differ in their protein composition. Cyanobacteria with-carboxysomes occur in environments where dissolved carbon is not limiting (e.g., oligotrophic oceanic waters), whereas cyanobacteria with - carboxysomes occur in environments where dissolved carbon is limiting (e.g., mats, films, estuaries, and alkaline lakes with higher densities of photosynthetic organisms) (Badger et al., 2002).

Carboxysomes also contain the enzyme carbonic anhydrase that converts HCO3 into carbon dioxide, the only form of carbon that is fixed by Rubisco (Fig. 2.15). Bicarbonate (HCO3 ) is transported into the cell and carboxysome. Carbonic anhydrase in the carboxysome converts HCO3 into CO2 which is fixed by Rubisco into carbohydrates. The amount of a cell occupied by carboxysomes increases as the inorganic carbon (HCO3 , CO2) in the medium decreases (Turpin et al., 1984). Heterocysts (Fig. 2.4) lack ribulose-1,5-bisphosphate carboxylase/oxygenase and the ability to fix carbon dioxide. Heterocysts also lack carboxysomes (Winkenbach and Wolk, 1973).

40 THE PROKARYOTIC ALGAE

Fig. 2.14 Transmission electron micrograph of a section of a dividing cell of Anacystis nidulans showing thylakoids in the peripheral cytoplasm, DNA microfibrils, ribosomes, and a long carboxysome in the central cytoplasm. Bar 0.5 m. (From Gantt and Conti, 1969.)

Fig. 2.15 Carboxysomes contain the enzymes carbonic anhydrase and ribulose-1,5-bisphosphate carboxylase/ oxygenase. Carbonic anhydrase in the carboxysome converts HCO3 into CO2 which is fixed by ribulose-1,5-bisphosphate carboxylase/oxygenase into carbohydrates.

Polyphosphate bodies (metachromatic or volutin granules) (Fig. 2.11) are spherical and appear similar to lipid bodies of eukaryotic cells in the electron microscope. Polyphosphate bodies

contain stored phosphate, the bodies being absent in young growing cells or cells grown in a phosphate-deficient medium, but present in older cells (Tischer, 1957).

Polyglucan granules ( -granules) (Fig. 2.11) are common in the space between the thylakoids in actively photosynthesizing cells. These granules contain a carbohydrate, composed of 14 to 16