What is Cyanobacteria in biology

what does cyanobacteria mean and what is cyanobacteria blue green algae, what are benefits cyanobacteria provide to the environment
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Published Date:02-08-2017
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Cyanobacteria CYANOPHYCEAE Morphology The Cyanophyceae or blue-green algae are, today, The simplest morphology in the cyanobacteria is usually referred to as the cyanobacteria (blue- that of unicells, free-living (see Figs. 2.19(c), 2.20) green bacteria). The term cyanobacteria acknowl- or enclosed within a mucilaginous envelope (Figs. edges that these prokaryotic algae are more 2.48, 2.56(a), (b)). Subsequent evolution resulted in closely related to the prokaryotic bacteria than to the formation of a row of cells called a trichome eukaryotic algae. For the last quarter century, (Fig. 2.16). When the trichome is surrounded by a cyanobacteria were thought to have evolved sheath, it is called a filament (Fig. 2.10). It is possi- about 3.5 billion years ago. These reports were ble to have more than one trichome in a filament based on interpretation of microfossils, difficult (Figs. 2.56(e), 2.58(b)). The most complex thallus at best with such small organisms. It now appears is the branched filament (Fig. 2.58(a)). Such a that these investigators selected specimens that branched filament can be uniseriate (composed of fit the assumptions of the authors, with most phy- a single row of cells) or multiseriate (composed of cologists now rejecting their claims. Based on one or more rows of cells). other reports, the actual time of evolution of cyanobacteria is thought to be closer to 2.7 billion Cell wall and gliding years ago (Buick, 1992; Brasier et al., 2002; Dalton, 2002). The cell wall of cyanobacteria is basically the Cyanobacteria have chlorophyll a (some also have chlorophyll b or d), phycobiliproteins, glyco- same as the cell wall of Gram-negative bacteria gen as a storage product, and cell walls containing (Fig. 2.1). A peptidoglycan layer is outside of the amino sugars and amino acids. cell membrane. The peptidoglycan is an enor- mous polymer composed of two sugar derivatives, At one time, theoccurrence of chlorophyll b in cyanobacteria was used as a criterion to N-acetylglucosamine and N-acetylmuramic acid, place the organisms in a separate group, the and several different amino acids (Fig. 2.2). Prochloro phyta.Modernnucleic-acid sequenc- Outside of the peptidoglycan is a periplasmic space, probably filled with a loose network of pep- ing, however, has shown that chlorophyll b evolved a number of times within the cyanobac- tidoglycan fibrils. An outer membrane surrounds teria and the term Prochlorophyta has been the periplasmic space. discarded (Palenik and Haselkorn, 1992; Urback Some cyanobacteria are capable of gliding, that is, the active movement of an organism on a solid et al., 1992).34 THE PROKARYOTIC ALGAE Fig. 2.1 Transmission electron micrographs of sections of the wall of the cyanobacterium Phormidium uncinatum.The cell wall to the long axis of the cell and is occasionally (CW) contains layers similar to those of a Gram-negative interrupted by reversals in direction. Gliding is bacterium, e.g., the cytoplasmic membrane (CM), peptidog lycan accompanied by a steady secretion of slime, layers (P), periplasmic space (PS) and outer membrane (OM). In which is left behind as a mucilaginous trail. addition, the cyanobacterium contains the additional two external layers typical of a motile cell, the serrated exter nal Some cyanobacteria (Phormidium, Oscillatoria) layer (EL) and hair-like fibers (F). (CJ) Circumferential junc tion; rotate during gliding while other cyanobacteria (JP) junctional pore. (From Hoiczyk and Baumeister, 1995.) (Anabaena) do not rotate. The cell wall of gliding bacteria has two addi- tional layers outside of the cell wall (Figs. 2.1, 2.3, substrate where there is neither a visible organ respon- 2.4). A serrated external layer (S-layer) and a layer sible for the movement nor a distinct change in the of hair-like fibers occur outside of the outer mem- shape of the organism (Jarosch, 1962). Gliding is a 1 brane of the cell wall of gliding cyanobacteria. slow uniform motion (up to 600 m s in The hair-like fibers of the outermost layer are com- Oscillatoria; Bhaya, 2004) at a direction parallel posed of a rod-like glycoprotein called oscillin (Hoiczyk and Baumeister, 1998; Hoiczyk, 2000). The cross walls of neighboring cells of gliding cyanobacteria contain junctional pores that are 15 nm in diameter and radiate outward from the cytoplasm at an angle of about 30–40° relative to the plane of each septum (Figs. 2.1, 2.5, 2.6). The number of rows of junctional pores around each side of the septum varies from one circumferential ring in Phormidium to several rows of pores that girdle the septum in Anabaena. The junctional pore is 70–80 nm long and spans the entire multi- layered cell wall. The junctional pore is composed of a tube-like base and an outer pore complex. Gliding occurs by slime secretion through the circumferential junctional pores on one side of the septum (Hoiczyk and Baumeister, 1998; Hoiczyk, 2000). The slime passes along the surface of the oscillin fibers of the outer layer of the cell Fig. 2.2 The structure of a peptidoglycan molecule in the wall and onto the adjacent substrate, propelling cell wall of cyanobacteria. the filament forward. The orientation of theCYANOBACTERIA 35 Fig. 2.3 Cross sections of a wall of a cyanobacterium that is not capable of gliding and a cross section of a wall of a cyanobacterium that is capable of gliding. Cyanobacteria that can glide have an additional two wall layers on the outside. (From Hoiczyk and Baumeister, 1995.) Fig. 2.4 A model of the junctional pore complex of a cyanobacterium. Extrusion of slime through the oscillin fibers of the outer layer determines circumferentially arranged junctional pores on one side of whether the filament rotates during gliding. In the cross wall results in forward movement of the filament in contact with the substrate. The arrangement of the oscillin Anabaena, the spiral oscillin fibers produce a fibers in the outer layer of the cell wall determines whether clockwise rotation while in Oscillatoria princeps the filament rotates as it glides over the surface. In the and Lyngbya aeruginosa the oscillin fibers are drawing, the oscillin fibers are spiraled so the filament rotates spiraled in the opposite direction and produce as it glides. (Modified from Hoiczyk and Baumeister, 1998.) acounterclockwise rotation during gliding36 THE PROKARYOTIC ALGAE Fig. 2.5 Transmission electron micrographs of Phormidium uncinatum. (A) Isolated cell wall with the outer membrane Pili and twitching and external layer (EL) still attached. On both sides of the cross wall, the ring-shaped counterpart of the junctional Pili are proteinaceous appendages that project pores with their central pores are visible. (B) Negatively stained isolated wall showing the junctional pores (JP) filled from the surface of cyanobacterial cells (Fig. 2.8). with slime. (CW) cross wall. (C) Isolated outer membrane There are two types of pili in the unicellular with the ring-shaped parts of the junctional pores. (From cyanobacterium Synechocystis (Bhaya, 2004). The Hoiczyk and Baumeister, 1995.) cell is covered uniformly with a layer of thin- brush-like pili with an average diameter of 3–4 nm and a length of 1 m. Cells also have thick flexible (Hoiczyk and Baumeister, 1995) (Fig. 2.7). In pili with a diameter of 6–8 nm and length of 4–5 Phormidium, the oscillin fibers are not spiraled and m that often make connections with other cells. The pili are composed of 500 to 1000 units of the the filament does not rotate during gliding. The arrangement of hair-like fibers thus serves polypeptide pilin. Each pilin unit consists of as a passive screw as the slime passes over their between 145 and 170 amino acids (Bhaya et al., surface in gliding. Reversal of gliding occurs when 1999). The pilin molecule is similar to the oscillin molecule involved in gliding. Synechocystis is able slime stops coming out of the ring of junctional 1 pores on one side of the septum, and when slime to move across a surface at 1 to 2 m s using a begins coming out of the ring of junctional pores mechanism called twitching that utilizes change on the other side of the septum. in configuration of the pili (Wall and Kaiser, 1999). The pili probably move the cell body along a sur- Fig. 2.6 Structure of the organelles of the junctional-pore complex in Phormidium uncinatum. Transmission electron micrograph of a negatively stained isolated outer membrane patch showing the ring-shaped orifices of the junctional pores that would be circumferentially arranged in the cross wall in the cell. Inset shows anumber of superimposed images of junctional-pore complexes. (From Hoiczyk, 2000.)CYANOBACTERIA 37 Fig. 2.7 Rotation of the cyanobacterial filament depends on the orientation of the oscillin protein. Mucilage is Sheaths 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 A sheath (capsule or extracellular polymeric helically oriented. subs tances (EPS)) composed of mucilage and a small amount of cellulose is commonly present in cyanobacteria (Nobles et al., 2001) (Figs. 2.10, face by a reiterative process of pili extension, 2.11). The sheath protects cells from drying. adhesion, and retraction (Bhaya, 2004). Active growth appears necessary for sheath for- mation, a fact that may explain its sometimes Synechocystis exhibits both positive and neg ative phototaxis in blue light (450 nm wave- poor development around spores and akinetes. length) but not in red or far-red light (Terauchi The sheath of Gloeothece sp. is composed of poly - and Ohmori, 2004). Blue light stimulates the saccharides with neutral sugars and uronic acids including galactose, glucose, mannose, rham- production of cyclic adenosine monophosphate (cAMP), a common second messenger in biological nose, 2-O-methyl-D-xylose, glucuronic acid and systems (Fig. 2.9). 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. thesheath. An excess of fixed carbon results in formation of a sheath (Otero and Vincenzini, 2004). sheath of Gloeothece contains only 2% protein and a trace of fatty acids and phosphate. The commer- Protoplasmic structure cial applications of cyanobacterial EPS have been reviewed by De Philippis and Vincenzini (1998). Many of the proto plasmic structures found in the Sheaths are often colored, with red sheaths found bacteria occur in the cyanobacteria. In the central in algae from highly acid soils and blue sheaths characteristic of algae from basic soils (Drouet, proto plasm are the circular fibrils of DNA which 1978). Yellow and brown sheaths are common in are not associated with basic pro teins (histones) (Figs. 2.11 and 2.14). The amount of DNA in specimens from habitats of high salt content, par- 9 unicellular cy anobacteria varies from 1.6 10 to ticularly after the algae dry out. 9 The sheath excludes India ink so the easiest 8.6 10 daltons. This is similar to the genome 9 9 size in bacteria (1.0 10 to 3.6 10 daltons) and way to visualize the sheath is to place a small is larger than the genome size in mycoplasmas amount of India ink in the water (Fig. 2.10). 9 9 (0.4 10 to 0.5 10 daltons) (Herdman et al., Production of a sheath is dependent on environ- mental conditions. A shortage of CO results in 1979). The peripheral proto plasm is composed 2 cessation of sheath production and release of principally of thylakoids and their associated structures, the phycobilisomes (on the thylak - oids, containing the phycobiliproteins) and glyco- gen granules. The 70S ribo somes are dis persed through out the cyanobacterial cell but are pre- sent in the highest density in the central region around the nucleo plasm (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 Fig. 2.10 A drawing of a filament of Hyella sp. in India ink. that consists of equimolar amounts of arginine This method clearly shows the sheath around the filament. and aspartic acid arranged as a polyaspartateCYANOBACTERIA 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 Fig. 2.13 Transmission electron micrograph of a section body; (PG) polyglucan granules; (Py) phycobilisomes; (R) of a cell of Plectonema boryanum showing cyanophycin bodies ribosomes; (S) sheath; (W) wall. (C). (From Lawry and Simon, 1982.) -carboxysomes and -carboxysomes, which differ in their protein composition. Cyanobacteria with -carboxysomes occur in environments where dissol ved carbon is not limiting (e.g., oligotro phic oceanic waters), whereas cyanobacteria with - Fig. 2.12 Cyanophycin is composed of equimolar amounts carboxysomes occur in environments where of arginine (Arg) and aspartic acid (Asp) arranged as a dissolved carbon is limiting (e.g., mats, films, estu- polyaspartate backbone. aries, and alkaline lakes with higher densities of photosynthetic organisms) (Badger et al., 2002). Carboxysomes also contain the enzyme car-  backbone (Fig. 2.12). Cyanophycin functions as a bonic anhydrase that converts HCO into carbon 3 temporary nitrogen reserve in nitrogen-fixing dioxide, the only form of carbon that is fixed by  cyanobacteria, accumulating during the tran - Rubisco (Fig. 2.15). Bicarbonate (HCO ) is trans- 3 sition from the exponential to the stationary ported into the cell and carboxysome. Carbonic  phase and disappearing when balanced growth anhydrase in the carboxysome converts HCO into 3 resumes. Nitrogen is stored in phycobilisomes in CO which is fixed by Rubisco into carbohydrates. 2 cyanobacteria that do not fix nitrogen (Li et al., The amount of a cell occupied by carboxysomes  2001a). increases as the inorganic carbon (HCO , CO ) 3 2 Carboxysomes (polyhedral bodies) (Fig. 2.14) in the medium decreases (Turpin et al., 1984). are similar to the carboxysomes in bacteria Heterocysts (Fig. 2.4) lack ribulose-1,5-bisphosphate and contain the carbon dioxide-fixing enzyme carbo xylase/ oxy genase and the ability to fix carbon ribulose-1,5-bisphosphate carboxylase/oxygenase dioxide. Heterocysts also lack carboxy somes (Rubisco). There are two types of 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 contain stored phosphate, the bodies being  HCO into CO which is fixed by ribulose-1,5-bisphosphate 3 2 absent in young growing cells or cells grown in a carboxylase/oxygenase into carbohydrates. phosphate-deficient medium, but present in older cells (Tischer, 1957). Polyphosphate bodies (metachromatic or Polyglucan granules (-granules) (Fig. 2.11) volutin granules) (Fig. 2.11) are spherical and are common in the space between the thylak oids appear similar to lipid bodies of eukaryotic cells in actively photo syn thesizing cells. These gran- in the electron microscope. Polyphosphate bodies ules contain a carbohydrate, composed of 14 to 16CYANOBACTERIA 41 Fig. 2.16 Light micrograph of Anabaena crassa showing vegetative cells, akinetes, and heterocysts. (From Li et al., the gas vesicle is quite rigid, with the gas inside 1997.) it at a pressure of 1 atm. The membrane is per - meable to gases, allowing the contained gas to glucose mole cules, that is similar to amylopectin equilibrate with gases in the surrounding solu - (Hough et al., 1952; Frederick, 1951). tion. The membrane must, however, be able to exclude water. It has been postulated that the inner surface must be hydrophobic, thereby pre - Gas vacuoles venting condensa tion on it of water droplets, and restraining, by surface tension, water creeping A gas vacuole is composed of gas vesicles, or through the pores. At the same time these mole- hollow cylindrical tubes with conical ends, in cules must present a hydrophilic surface at the the cytoplasm of cyanobacteria (Figs. 2.11, 2.17) outer (water-facing) surface in order to mini mize (Walsby, 1994; Oliver, 1994). Gas vesicles do not the interfacial tension, which would oth er wise have true protein-lipid membranes, being com- result in the collapse of the gas vacuole. posed exclusively of protein ribs or spirals Cyanobacteria possessing gas vacuoles can be arranged similarly to the hoops on a barrel. It is divided into two physiological-ecological groups. In possible to collapse the gas vesicles by applying the first group are those algae having vacuoles only pressure to the cells, the collapsed vesicles having at certain stages of their life cycle, or only in certain the two halves stuck together. The membrane of types of cells. In Gloeotrichia ghosei and in certain (a)(b) Fig. 2.17 (a) Transmission electron micrograph of two cells of Oscillatoria redekei showing a cross wall separating areas of gas vacuoles (gv); (pg) lipid droplet. (From Whitton and Peat, 1969.) (b) Freeze-etch preparation of gas vacuoles. (From Jones and Jost, 1970.)42 THE PROKARYOTIC ALGAE species of Tolypothrix and Calothrix,gasvesicles Oscillatoria agardhii (Fig. 2.19(a)), buoyancyislost appear only in hormogonia. The hormogonia float through cessation of gas vesicle production and when they are released, and it is possible that the an increase in cell mass. In Microcystis aeruginosa buoy ancypro videdisofsignificanceindispersalof (Fig. 2.56(b)), loss of buoyancy can be due to entrap- these stages. The second group consists of plank- ment of whole colonies in a colloidal precipitate tonic cyanobacteria, including species of Anabaena composed of organic material and iron salts. The (Figs. 2.16, 2.18(d), 2.57(b)), Gloeotrichia (Fig. 2.18(a)), colloidal precipitate is formed in certain lakes Microcystis (Figs. 2.48, 2.56(b)), Aphanizomenon (Fig. when dissolved iron in the anoxic water of the 2.18(b)), Oscillatoria (Figs. 2.19(a), (b), 2.34(a), (b)), hypolimnion in stratified lakes becomes oxidized Trichodesmium (Figs. 2.31, 2.56(g)), and Phormidium on mixing with aerated water of the epilimnion (Figs. 2.18(c), 2.56(c)). These algae derive positive (Oliver et al., 1985). buoyancy from their gas vesicles, and as a conse- There is a direct relationship between buoyancy quence form blooms floating near the water sur- and light quantity in nitrogen-fixing cyano bacteria face. The loss of buoyancy, and subsequent sinking such as Anabaena flos-aquae (Fig. 2.18(d)) (Spencer of these algae in the water column, can be due toandKing,1985).Therela tion shipiscomplexand different factors. In Anabaena flos-aquae (Fig. 2.18(d)),alsoinvolvestheconcentra tionofammoniumions  the lossof buoyancy is caused by the loss of gas vesi- (NH )inthewater.Buoyancyin Anabaena flos-aquae 4 2 cles owing to increased turgor pressure, whereas in increases under low irradiance (less than 10 Em Fig. 2.18 (a) Gloeotrichia echinulata. (b) Aphanizomenon flos- (a) aquae. (c) Phormidium inundatum. (d) Anabaena flos-aquae. (A) Akinete; (H) heterocyst. (After Prescott,1962.) (b) (c) (d) (a) Fig. 2.19 (a) Oscillatoria agardhii. (b) O. limnetica. (c) Synechococcus aeruginosus. (d) Cylindrospermum (b) majus. (A) Akinete; (H) heterocyst. (e) Drawing of the fine structure (c) of Gloeobacter violaceus. (C) Cyanophycin granule; (P) (e) polyphosphate body; (Phy) probable layer of phycobiliproteins; (Pl) (d) plasmalemma. ((c),(d) after Prescott, 1962; (e) after Rippka et al., 1974.)CYANOBACTERIA 43 1  s ), absence of NH ,andlowCOconcentra tions. (Palenik and Haselkorn, 1992; Urback et al., 1992). 4 2 Suchcondi tionsoccurinmanystagnanteutrophic In contrast, Acaryochloris marina (Fig. 2.20), the lakes during the summer. In these lakes, rapid only cyanobacterium known to have chlorophyll  growth of algae has depleted the NH and CO . The d, appears to be only distantly related to other 4 2 water trans mitslittlelightbecauseofthelarge cyanobacteria (Miller et al., 2005; Miyashita et al., standingcropofalgae.Underthesecondi tions, A. 2003). The absorption spectrum of chlorophyll d flos-aquae and other nitrogen-fixing cyanobacteria (Fig. 1.19) is shifted toward far-red wavelengths havinggasvacuolesincreasetheirbuoy ancyand and A. marina exists in environments where there rise close to the surface of the water. Here they are is an abundance of these wavelengths of light abletoout competeotheralgaebecauseoftheirabil- (e.g., under red algae). itytofixnitro geninwaterthathaslittleavail able The carotenoids of the cyanobacteria differ nitro gen.Cyanobacteriathatdonotfixnitro gen from those of the eukaryotic algae in having echi- havereducedgrowth,andthere forereducedbuoy - neone (4-keto--carotene) and myxoxanthophyll, ancy, and sink in the water column. Once estab- which eukaryotic algae do not have; in lacking lished, a bloom of buoyant, nitrogen-fixing, lutein, the major xanthophyll of chloro plasts; and cyanobacteriatendstobe self-perpetuatingin in having much higher pro por tions of -carotene that increased mass of the bloom maintains the than are found in eukaryotic algae (Goodwin, reduced light and CO levels required for maximum 1974). 2 buoy ancy. The Cyanophyceae have four phycobilipro- Structures other than gas vesicles can cause teins: C-phycocyanin (absorp tion maximum at a significant variation in cell density, and there forewave length  of 620 nm), allophycocyanin ( max buoyancy (K onopka et al., 1987). Polyphosphate at 650 nm), C-phycoerythrin ( at 565 nm), max 3 granules may have a density of 2 g cm or greater, and phycoerythrocyanin ( at 568 nm). All max and glycogen (which is accumulated under high cyanobacteria contain the first two, whereas 3 light intensities) has a density of about 1.5 cm . C-phycoerythrin and phycoerythrocyanin occur 3 Both have a higher density than water (1 g cm ) only in some species. The phycobiliproteins of and can cause cells to sink (Booker and Walsby, the cyanobacteria change in concentra tion in 1981; McCausland et al., 2005). response to light quality and growth condi tions. Cyanobacteria that produce the red phycoery- thrin and the blue phycocyanin in white light, Pigments and photosynthesis suppress phycoerythrin syn thesis in red light and phycocyanin syn thesis in green light (comple- The major components of the photo syn thetic mentary chromatic adapta tion; see Tandeau de light-harvesting system of the cyanobacteria are Marsac, 1977). chlorophyll a in the thylak oid membrane, and In the evolu tion of the cyanobacteria, the the phycobiliproteins, which are water-soluble thylak oids probably originated by invagina tions chromo proteins assembled into macro molecular of the plasmalemma; some cyanobacteria today aggregates (phycobi li somes) attached to the outer have thylak oids that are continu ous with the plas- surface of the thylakoid membranes. Some cyano - malemma. An example of the primitive condi tion bacteria contain chlorophyll b and the cyanobac- may be Gloeobacter violaceus (Fig. 2.19(e)), a uni - terium Acaryochloris marina (Fig. 2.20) contains cellular cyanobacteria that lacks thylak oids but chlorophyll d. At one time those cyanobacteria has chloro phyll a, caroten oids, and phycobilipro- containing chlorophyll b (Prochlorococcus (Fig. 2.20), teins. In this alga the pig ments, and pre sum ably Prochlorothrix, Prochloron) were thought to be a dis- photo syn thesis, are associated with the plas- tinct evolutionary group and they were placed in malemma (Rippka et al., 1974). the Prochlorophyta. Evolutionary trees based on Many cyanobacteria have the ability to photo - nucleic acid sequencing have shown that chloro- syn thesize under aerobic or anaerobic condi tions. phyll b arose a number of times and that these Under aerobic condi tions, elec trons for photo - cyanobacteria are spread throughout the group system I are derived from photo system II. Under44 THE PROKARYOTIC ALGAE Fig. 2.20 Transmission electron micrographs of a section of Prochlorococcus, a chlorophyll b containing cyanobacterium, and of Acaryochloris marina, a chlorophyll d containing photo system II (Fig. 2.28) and carries out photo syn - cyanobacterium. Prochlorococcus is the smallest known thesis aero bically. Thus O. limnet ica, by utilizing photosynthetic organism. (Ca) Carboxysome (polyhedral combined anoxygenic and oxy genic photo syn - body); (Cy) cytoplasm; (pb) polyhedral body; (th) thylakoids. thesis, is the dominant photo troph of the Solar (Arrow) channel-like structures perforating thlyakoids. Lake, with its fluctuating photo aero bic and photo - (Arrowheads) areas of accumulation of phycobiliproteins. anaerobic condi tions. The inter linking posi tion of (Prochlorococcus micrograph from Partensky, Hess and Vaulot, the cyanobacteria in the photo tropic world is 1999; Acaryochloris micrograph is from Marquardt et al., 2000.) compat ible with the fact that they are among the oldest organ isms, dating back to the Precambrian anaerobic condi tions, in the pres ence of sulfur, Period. Significantly, two of the sulfide-rich ecosys- elec trons are derived by the reduc tion of sulfur: tems containing high numbers of cyanobacteria – that is, hot sulfur springs and the marine littoral light sedi ments – may repre sent old ecosystems that CO 2H S → (CH O) H O S 2 2 2 2 may predate the oxidized bio sphere. chlorophyll sugar Photosynthesis in many cyanobacteria is These cyanobacteria are facultative photo trophic stimu lated by lowered oxygen concentra tion, the anaerobes and fill an important ecolog ical niche oxygen competing with carbon dioxide for the enzyme ribulose-1, 5-bisphosphate carboxylase/ in aquatic systems (Padan, 1979). Eukaryotic algae are restricted to photo aero bic habitats, whereas oxy genase (Fig. 2.21) (Stewart and Pearson, 1970; photo syn thetic bacteria are restricted to photo - Weller et al., 1975). This phenomenon probably anaerobic habitats. In habitats that fluctuate reflects an adapta tion to the absence of free oxygen in the atmosphere of Precambrian times between the above condi tions, cyanobacteria with facultative anaerobic photo syn thesis have a clear when the cyanobacteria first evolved. After the selective advantage. An example of this is the Solar evolu tion of the oxygen-evolving cyanobacteria, Lake, Elat, Israel, where in the winter high levels of the oxygen in the atmosphere gradually built up, creating a pro tective ozone (O ) layer in the atmos- sulfide are found in the anaerobic bottom layers of 3 water of the thermally stratified lake. Oscillatoria phere at the same time. The ozone layer removed limnet ica (Fig. 2.19(b)) occurs in these highly anaer- most of the harmful ultra vio let radia tion from the obic bottom layers, where sulfide func tions as an sun and allowed the evolu tion of more radiation- sensitive organ isms. The cyanobacteria are rela- elec tron donor for photo syn thesis. In the spring, the lake overtur ns, with all of the water becoming tively insensitive to radia tion, having a system that aerobic, Oscillatoria limnet ica activates aerobic repairs radia tion damage (Bhattacharjee, 1977).CYANOBACTERIA 45 vegetative cells. Akinetes have often been com- pared to endospores in Gram-positive bacteria. Akinetes, however, are neither as metabolically quiescent nor as resistant to various environmen- tal extremes. Akinetes only occur in cyanobacteria that form heterocysts. In Aphanizomenon (Fig. 2.18(b)) (Wildman et al., 1975), the development of akinetes from vegeta- tive cells involves an increase in cell size, the grad- ual disappearance of gas vacuoles, and an increase in cytoplasmic density and number of ribosomes and cyanophycin granules. Akinetes of Nostoc lose 90% of their photosynthetic and respiratory capa- bilities, as compared with vegetative cells. The loss occurs even though there is little change in phy- cocyanin and chlorophyll, the main photosyn- thetic pigments (Chauvat et al., 1982). Mature akinetes are usually considerably larger than veg- etative cells, contain protoplasm full of food reserves, and have a normal cell wall surrounded Fig. 2.21 Graph showing stimulation of photosynthesis (●-●) by a wide three-layered coat (Jensen and Clark, and respiration (■-■)bylowconcentrations of atmospheric O 1969; Cmiech et al., 1986) (Figs. 2.16, 2.18, 2.19(d), 2 in Anabaena flos-aquae. (After Stewart and Pearson, 1970.) 2.22, 2.23). Loss of flotation by an increase in cyto- plasmic density causes filaments with akinetes to sink and overwinter in bottom sediments. In Akinetes akinete germination, there is a reverse of the above events (Fig. 2.24). Akinetes are generally recognized by their larger A wide range of physicochem ical factors have size relative to vegetative cells and conspicuous been reported to stimulate akinete differentiation; granulation due to high concentrations of glyco- for example, phosphate deficiency, low tempera- ture, carbon limita tion and reduc tion in the avail - gen and cyanophycin (Figs. 2.22, 2.23) (Meeks et al., 2002). The most consistent property of akinetes is abil ity of light energy (Li et al., 1997; van Dok and their greater resistance to cold compared with Hart, 1995). Fig. 2.22 Light micrographs of akinetes in Raphidiopsis mediterranea (a),(b) and R. curvata (c). ((a),(b) from Watanabe et al., 2003; (c) from Li et al., 2001a.)46 THE PROKARYOTIC ALGAE Fig. 2.23 Electron micrograph of a mature akinete of Cylindrospermum sp. with a thick layered wall (f,l) and cytoplasm full of proteinaceous cyanophycin (structured) granules (s), polyhedral bodies, and ribosomes. (From Clark and Jensen, 1969.) Fig. 2.24 The germination of an akinete of Cylindrospermopsis raciborskii. (Modified from Moore et al., 2004.) akinetes appear full of storage products) (Figs. Heterocysts 2.16, 2.18(a), 2.19(d), 2.25, 2.35). Heterocysts are photosynthetically inactive, they do not fix CO ,nor do they produce O . They also exhibit Heterocysts are larger than vegetative cells and 2 2 appear empty in the light microscope (whereas ahigh rate of respiratory O consumption and are 2CYANOBACTERIA 47 surrounded by a thick, laminated cell wall that Commitment point 1. 2-oxoglutarate (-keto - limits ingress of atmospheric gases, including O . glutarate) (Fig. 2.26) is the substrate used for 2 The internal environment of heterocysts is, there- incorporation of ammonium in cyanobacteria. fore, virtually anoxic, which is ideal for nitroge- The absence of combined nitrogen leads to an nase, a notoriously O sensitive enzyme. increase in intracellular 2-oxoglutarate since 2 Heterocysts are formed at regular intervals cyanobacteria lack 2-oxoglutarate dehydrogenase from vegetative cells by the dissolution of stor- and the ability to breakdown 2-oxoglutarate. The age granules, the deposition of a multilayered increase in intracellular 2-oxoglutarate activates env elope outside of the cell wall, the breakdown the protein NtcA and results in a drop in the of photosynthetic thylakoids, and the formation amount of the calcium-binding protein CcbP, and 2 of new membranous structures (Fig. 2.25) a rise in Ca in the cells (Zhao et al., 2005). 2 (Kulasooriya et al., 1972). Heterocysts have 10 times more Ca than vegeta- 2 The differentiation of heterocysts in Anabaena tive cells. The rise in Ca up-regulates the hetR is triggered by nitrogen deprivation (ammonia, gene that forms hetR, a serine-type protease, nitrate, nitrite) in two steps: the first step is which induces the vegetative cell to change into reversible and the second step is irreversible a heterocyst. The hetR protein is considered the (Adams, 2000; El-Shehawy and Kleiner, 2003). “master switch” in heterocyst development. The pro- duction of hetR protein constitutes commitment point 1. A lack of combined nitrogen over a period of about 10 hours results in continued produc- tion of hetR protein and the cell becoming a pro- heterocyst, which under the light microscope appears less granulated than vegetative cells but still lacks a thick cell wall. The hetR protein also induces the production of an oligopeptide called PatS. This oligopeptide diffuses to adjacent cells where it prohibits the formation of hetR protein in the cells and assures that the adjacent cells do not transform into proheterocysts. This sets up the spacing seen in Anabaena where hetero- cysts are formed equidistant from each other. Commitment point 1 is reversible. A prohetero- cyst will revert back to a vegetative cell if com- bined nitrogen is added to the medium, causing a switch “off” of the hetR gene and formation of hetR protein. Differentiation of a vegetative cell to a proheterocyst also involves loss of photo - system II activity, thus eliminating photosyn- thetic generation of O (which inhibits nitrogen 2 fixation). Commitment point 2. The second stage can not be reversed by the addition of combined nitro- gen and comprises the transformation of a pro- heterocyst into a mature heterocyst. This involves the formation of a thick cell wall containing gly- Fig. 2.25 Three-dimensional view of a heterocyst. The colipids and polysaccharides to reduce diffusion of envelope has homogeneous (H), fibrous (F), and laminated O . The nitrogen-fixing enzyme nitrogenase is acti- 2 (L) layers. (M) Membranes; (P) pore channel; (Pl) vated when 11 kilobases are removed between plasmalemma; (W) cell wall. (After Lang and Fay, 1971.) repeat sequences flanking nitrogenase (nif ) genes.48 THE PROKARYOTIC ALGAE Fig. 2.26 Summation of events leading to the formation of heterocysts. cyanobacteria, akinetes contain glycolipids char- acteristic of heterocysts, and the cell wall of hete- rocysts is identical to that of akinetes (and unlike This process of heterocyst development is interesting in that it may represent one of the ear- that of vegetative cells). Heterocyst differentiation liest examples of pattern formation in evolution. is a terminal event and is a basic form of pro- It is probable that akinetes were the evolutionary grammed cell death or apotosis. Apartfromtheexcep tionalcasesofgermina - precursors in heterocysts (Meeks et al., 2002). Akinetes are only seen in heterocyst-formingtion,hetero cystsareunabletodivide.HeterocystsCYANOBACTERIA 49 Fig. 2.27 The chemistry of nitrogen fixation and the subsequent incorporation of the fixed nitrogen into isms are prokaryotes. In nitrogen fixation, N 2 glutamate and glutamine. from the atmosphere is fixed by the enzyme nitro- genase into ammonium using ATP as a source of havealimitedperiodofphysio log icalactiv ityand energy (Fig. 2.27). The process is one of the most metabolically expensive processes in biology, appeartohavealimitedlife.Senescenthetero cysts undergovacuola tionandusuallybreakofffromthe requiring 16 ATP for each molecule of N fixed. 2 filament,causingfrag menta tionofthefilament. The amount of biologically fixed nitrogen pro- 13 1 Heterocysts are dependent on a supply of sub - duced is in excess of 2  10 g year . In contrast, lightning discharge, the primary abiotic source of strates from adjacent vegetative cells through 12 1 cyto plasmic connec tions (microplasmodesmata). fixed nitrogen, accounts for 5  10 g year These cytoplasmic connections probably convey (Raymond et al., 2004). nitro gen fixed in the form of glutamine (Fig. 2.27) The ammonium fixed by nitrogen fixation is added to 2-oxoglutarate (from the citric acid by the heterocysts to vegetative cells. The vegeta- tive cells transfer photosynthate to the hetero- cycle) by the enzyme glutamate dehydrogenase to cysts since the heterocysts are incapable of carbon form glutamate (glutamic acid) (Fig. 2.27). fixation. Addition of a second ammonium to glutamate produces glutamine, the molecule that is trans- ferred from one cyanobacterial cell to another. Nitrogen fixation In bacteria, nitrogenase is composed of two components, dinitrogenase reductase (iron pro- Cyanobacteria are diazotrophs (able to fix atmos - tein) and dinitrogenase (molybdenum-iron pro- pheric nitrogen). All known nitrogen-fixing organ- tein) encoded by the nif HDK operon (Henson et al.,50 THE PROKARYOTIC ALGAE 2004). The situation is probably similar in the nitrogen fixation. Heterocysts also have a form cyanobacteria. of myoglobin called cyanoglobin that Hydrogen gas is also produced in nitrogen scavenges oxygen, preventing inhibition of fixation (Fig. 2.27) and has drawn interest as a nitrogenase (Potts et al., 1992). Under renewable energy source as hydrogen fuel-cell anaerobic conditions, in an atmosphere of technology for motor vehicles becomes more of a nitrogen and carbon dioxide, both vegetative practical reality (Schutz et al., 2004). cells and heterocysts can fix nitrogen. Nitrogenase, the nitrogen-fixing enzyme, is 2 Non-filamentous cyanobacteria that fix nitrogen in very sensitive to inactivation by oxygen. Cyano - the dark but not in the light: These cyanobacteria bacteria have evolved three different mechanisms fix nitrogen in the dark when photosynthesis designed to exclude oxygen from the area of the is not producing nitrogenase-inhibiting cells containing nitrogenase: oxygen. If Synechococcus is grown under a 12- hour light: 12-hour dark cycle, most of the 1 Heterocystous cyanobacteria: These cyanobacteria nitrogen fixation occurs during the dark occur primarily in fresh and brackish water period (Fig. 2.30). If the cells are subjected and fix nitrogen in heterocysts. Heterocysts are to continuous illumination, an endogenous surrounded by a glycolipid layer which is timing cycle entrained by cell division impermeable to O (Staal et al., 2003). 2 continues to alternate the level of Heterocysts lack photosystem II (Fig. 2.28) and, photosynthesis and nitrogen fixation therefore, the ability to evolve O . Heterocysts 2 (Mitsui et al., 1986; Chen et al., 1996). do have cyclic photophosphorulation (Fig. Nitrogenase activity peaks at the same time 2.29) and can produce the ATP necessary for that it had in the previous dark period Fig. 2.28 Non-cyclic photophosphorulation. Heterocysts in cyanobacteria lack photosystem II and do not produce oxygen.CYANOBACTERIA 51 Fig. 2.29 Cyclic photophos - phorulation. Heterocysts in cyanobacteria produce ATP by cyclic photophosphorulation. Fig. 2.30 Illustration of the relationship between photosynthesis and nitrogen fixation in a culture of Synechococcus sp. growing under a 12-hour light: 12-hour dark photoperiod. Under continuous illumination, an endogenous clock maintains the cycles. (After Mitsui et al., 1986.) (Fig. 2.30). At the beginning of the next light beginning of the next dark period for nitrogen period, oxygen is produced by photosynthesis fixation to occur. and the nitrogenase is inactivated. New 3 Trichodesmium (Figs. 2.31, 2.56(g)) and nitrogenase must be synthesized at the Katagnymene (Fig. 2.31): These cyanobacteria52 THE PROKARYOTIC ALGAE Fig. 2.31 Some nitrogen-fixing cyanobacteria that lack Circadian rhythms heterocysts. The cyanobacteria have circadian rhythms in pho- tosynthesis, nitrogen fixation, and cell division similar to those in eukaryotic organisms (Fig. 2.30). are the major bloom-forming, nitrogen-fixing, The requirements of a circadian rhythm are: (1) organisms in the oceans, responsible for fixing one-quarter of the total nitrogen in the oceans approximate 24-hour cycles in biological processes of the world (Bergman and Carpenter, 1991). even in the absence of an environmental cycle; (2) synchronization with the environment through These filamentous cyanobacteria do not have light or environmental cues; and (3) maintenance heterocysts yet fix nitrogen in the light under aerobic conditions (Bergman et al., 1997). of a nearly constant period over a range of physio- Within the filaments, 10 to 15% of the cells logically relevant temperatures (Golden, 2003). The details of the cyanobacterial circadian (called diazocytes) are specialized to fix rhythm have been elucidated in Synechococcus elon- nitrogen, while the others do not (Lundgren et al., 2001). Cells that fix nitrogen are gatus (Williams et al., 2002). The key components adjacent to one another and have a denser of the timekeeping complex are the proteins KaiA, KaiB, and KaiC coded by the genes KaiA, KaiB, and thylakoid network with fewer gas vacuoles KaiC (Figs. 2.32, 2.33). Interactions between these and cyanophycin granules (Fredriksson and Bergman, 1997). The tropical seas where three proteins maintain the rhythm at about 24 Trichodesmium and Katagnymene live are hours. In the real world, the clock is reset daily by cycles of light and darkness, temperature and relatively low in dissolved oxygen and this may humidity. The major portal into the clock is the assist the nitrogen-fixing cells in maintaining anaerobic conditions in the protoplasm where protein kinase CikA. Environmental change to nitrogenase is present (Staal, Meysman and CikA changes the configuration of the protein KaiA which causes phosphorulation of the protein Stal, 2003). KaiC (Fig. 2.32), resetting the clock. The output Trichodesmium and Katagnymene represent the most ancient type of nitrogen- that sets cycles of nitrogen fixation, photosynthe- fixing cyanobacteria (Berman-Frank et al., sis, etc. begins when KaiC interacts with the pro- tein SasA (Synechococcus adaptive sensor). This 2001). results in information being relayed downstream to produce the observed effect.

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