Cyanophyta

A clickable outline index of this document:
I. Precambrian Time
A. Archaean Era: Era of Archaea
B. Proterozoic Era: Era of Cyanobacteria
II. Cyanobacterial Cells
A. Cell Wall, Cell Membrane, Mesosomes
B. Chromatoplasm:Thylakoid Membranes, Cyanophycean Starch Grains
C. Centroplasm:Nucleoplasm, Ribosomes, Polyhedral Bodies, Cyanophycin Granules, Polyphosphate Granules, Lipid Droplets, Gas Vacuoles, Vacuole-like Inclusions
III. Division of Labor
Heterocysts, Akinetes, Branching, Asexual Reproduction, "Sexual" Reproduction
IV. Systematics of Division Cyanophyta
A. Class Cyanophyceae
     Orders Chroococcales, Pleurocapsales
           Oscillatoriales, Nostocales, Stigonematales
B. Class Prochlorophyceae
     Order Prochlorales

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We now embark on a study of the pathway leading to the evolution of higher plants. As our understanding of biology increases, in has become apparent that eukaryotic plants evolved in a series of steps from ancient forms. The endosymbiont theory for the evolution of eukaryotes is certainly a central feature in the process, and you already know that the ancestors of chloroplasts of plant cells were cyanobacteria. As you have already studied this theory in some depth in Organismal Biology and Cellular and Molecular Biology, I assume this is "old news" to you.

The Precambrian Time

I would like now to put a time frame to our discussion of evolution of plants on our planet. The 4.5 billion years since the origin of our planet can be divided into two Times. Multicellular eukaryotes became dominant organisms on our planet a little more than half-a-billion years ago. The time since that step is called the Phanerozoic Time. The 4 billion years before eukaryotes became dominant is called the Precambrian Time. To begin our study closer to the beginning of plant evolution, we must go back into the Precambrian Time: the age of the Kingdom Prokaryota. The Precambrian Time is also divided into two Eras: the Archaean Era and the Proterozoic Era as shown below.

Earth
Formed
First Life
Appears
Cyanobacteria
Appear
Oxygen
Appears
Eukaryotes
Appear
Eukaryotes
Dominant
4.5 billion 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 BYBP
Precambrian Time: The Time of Prokaryotes Phanerozoic Time
Archaean Era: Era of Archaea Proterozoic Era: Era of Cyanobacteria Eukaryotic
Time

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The Archaean Era

The Era of Archaea

The earth formed about 4.5 billion years ago; this is the beginning of the Precambrian Time and the Archaean Era. During the Archaean Era, earth's atmosphere was anaerobic (lacked oxygen gas, O2). The first organisms on our planet were probably most like what we know now as Archaea. These organisms are simple cells of the prokaryotic type: no nucleus, no chloroplast, no mitochondrion, no vacuole, etc. These first-cells evolved a range of amazing biochemistry pathways to derive energy, acquire carbon, obtain nitrogen, and so on. Today their distribution is limited to hot-springs, ocean-floor thermal vents and many other harsh environments. The oldest fossils in sedimentary rocks, over 3 billion years old, show the ancient ancestors of modern Archaea.

The fossilized Archaea appear in the form of microfossils and stromatolites. Stromatolites are layered structures started by filamentous archaea growing on the floor of an ancient sea. The archaeal filaments grew up from the sediment into the water and trapped sediment particles. As the organisms became buried, they grew up through the sediment and were again exposed in the water column. Because of cyclic periodic changes in sedimentation and archaeal growth rates, the stromatolites built up as layered structures. With time and mineralization, the stromatolites became fossilized and can now be sectioned and observed. Fragments from the layers can be dated to determine their age. The deepest layers in the stromatolites are the oldest. Because of the biochemistry of these organisms, Archaeal stromatolites are rare and limited in development. To date these organisms seem to have appeared about 3.5 billion years ago and provided stromatolites for about one-billion years.

The first cyanobacteria (Cyanophyta) appear in fossils about 2.8 billion years old. Their biochemistry evolved to be fundamentally different from the Archaea. They were the first dominant organisms to use oxygenic photosynthesis. Their evolutionary innovation was photosystem II. This photosystem has the power to split water and use its electrons and protons to drive photosynthesis. As a byproduct of this new reaction system, oxygen gas (O2) was produced for the first time in abundance. This was a fundamental change for Earth's atmosphere and its impact was observed in all surface layers. As cyanobacteria increased the oxygen in the atmosphere, the iron in surface sediments was oxidized into red ferric oxide. In ancient sedimentary rock, the transition to an aerobic atmosphere is marked by a shift in the color of the layers from gray to red. These cyanobacteria obviously marked the planet in a very permanent way and paved the way for the subsequent evolution of oxidative respiratory biochemistry. The change in rocks occurs at about 2.5 billion years ago. This change marks the end of the Archaean Era of the Precambrian Time.

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The Proterozoic Era

The Era of Cyanobacteria

The Proterozoic Era of Precambrian Time is often called the age of the cyanobacteria (Cyanophyta). These organisms changed the composition of the atmosphere as well as the color of earth's sediments. They grew into large, common stromatolites on the ocean bottoms that became fossilized. The cyanobacteria dominated the shallow seas until eukaryotes replaced them. This means that cyanobacteria were the dominant organisms on earth from about 2.5 billion to about 0.5 billion years ago. That covers 2 billion years of the planet's development (which is only 4.5 billion years total). Since life began 3.5 billion years ago, cyanobacteria obviously hold the record for dominance among all organisms...2 billion years is more than half of the time that life has existed on Earth.

Cyanobacteria, then, are amazing organisms that held sway for 2 billion years. They produced an atmosphere of about 20% oxygen gas which permitted the evolution of modern plants and animals. The ozone produced in the upper atmosphere by chemical reactions with oxygen from cyanobacterial photosynthesis provided all life forms with a mutation-stabilizing UV-light screen. Thus cyanobacteria prepared the earth for further diversification and stabilization of life. Moreover, cyanobacterial endosymbionts evolved to become the chloroplasts of eukaryotic algae and higher plants.

Obviously we need to know more about the biology of the progenitors of plants.

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Cyanobacterial Cells

If we remember that cyanobacteria are essentially prokarytoic cells and evolved into modern chloroplasts, their structure becomes quite predictable and memorable.

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Cell Wall

The cell wall is a four-layered structure. An apparently soft inner layer (colorless in diagram) faces the cell membrane. The second layer is the rigid layer (orange in diagram) composed of murein. Murein is a peptidoglycan: peptides attached to a polysaccharide made of alternating N-acetylglucosamine and N-acetylmuramic acid. The softer two outer layers (yellow in diagram) are made of lipopolysaccharides. Some species have an extensive sheath (gray in diagram) that holds the individual cyanobacterial cells into colonies or filaments.

The synthesis of the cell wall is susceptible to poisoning by penicillin, as is the case with the related gram-negative eubacteria. It can also be broken down with lysozyme. Cytokinesis in cyanobacteria is essentially a furrowing process initiated by the cell membrane, followed by progressive development of a murein ridge inside the wall progressively being synthesized and ultimately pinching the one cell into two. If one thinks of the closing of the iris diaphragm in the condenser of a light microscope, one has the idea of how the wall is synthesized during cytokinesis.

The cell wall is perforated with small (70 nm) pores allowing for secretion of a mucilaginous sheath made of complex polysaccharides. [note: we shall see later that plasmodesmata of plant cell walls are about 60 nm in diameter.] These pores also allow for movement of materials from one cell to another in filaments of certain species. It is further believed that close alignments of pores are responsible for fragmentation of some cyanobacterial filaments as a means of dispersal.

Precisely how motility is accomplished in cyanobacteria is still somewhat controversial. Clearly there are NO flagella. Instead cyanobacterial cells move by a kind of gliding motility. This was originally thought of as a propulsion by a jet-like secretion of a slimy mucilage from the pores but, while the mucilage is necessary for motility and motility cannot occur without mucilage on a solid substrate, it may be that small, spirally arranged fibrils in the two outer layers of the cell wall move in a wave-like pattern that accounts for the rotational movements of Oscillatoria as it glides. Whether this fibrillar wave motility using the viscosity of mucilage as a shear-force generator applies to other species is not yet resolved. The wall is clearly involved in motility in any case.

The motility is adaptive in moving a cyanobacterial cell into a good position for light exposure. When light is dim, cyanobacteria are positively phototactic; when light is bright the cells are negatively phototactic. The mechanism of how light is detected and movement is oriented during phototaxis is not precisely known at present.

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Cell Membrane

The cell membrane is perhaps not fundamentally different from those of eukaryotes. It appears as a three-layered structure in the electron microscope. The outer and inner layers are made of extrinsic (aka peripheral) and the hydrophilic extensions of intrinsic (aka integral) membrane proteins (red in diagram). The middle layer is a phospholipid or glycosylglyceride bilayer traversed by the hydrophobic portions of intrinsic (aka integral) membrane proteins (white in diagram). Phospholipid-based membranes are the standard for eukaryotic cells, but chloroplast membranes consist almost exclusively of the glycosylglyceride bilayers.

Some Membrane Bilayer Components

phospholipid:choline+-PO4--glycerol-(C14-24)2
monogalactosyldiacylglycerol (MGDG):galactose-glycerol-(C14-24)2
digalactosyldiacylglycerol (DGDG):galactose-galactose-glycerol-(C14-24)2
sulfoquinovosyldiacylglycerol (SQDG):-SO3-galactose-glycerol-(C14-24)2

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Mesosomes

Perhaps the critical evolutionary innovation of the cell membrane in cyanobacteria is its ability to invaginate to create a space between the cell membrane and the cell wall. The invaginations (red in diagram) are sometimes called mesosomes. These areas of the cell membrane are rich in proteins involved in electron transfers. Indeed at one point in evolution of bacteria, these proteins developed a mechanism to pump hydrogen ions into the space between the membrane and wall. The gradient of hydrogen ions could then be collapsed back toward the cytosol by passing the hydrogen ions through a membrane-bound ATP synthase. This permitted the evolution of the electron transport system required for the light reactions of photosynthesis (photophosphorylation)...and later...the ETS for aerobic respiration (oxidative phosphorylation).

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The Chromatoplasm

Early in the evolution of cyanobacteria, the invaginations of the cell membrane formed thylakoid membranes. These sacs are packed in layers around the cell just inside the cell wall, somewhat like the layers of an onion. This intensely-pigmented peripheral part of the cell interior is sometimes called the chromatoplasm.

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Thylakoid Membranes

The membrane of the thylakoids (blue-gray in diagram) has the electron transport system needed for the light reactions of photosynthesis. This includes the important reaction center pigment, chlorophyll a, attached to its membrane-bound proteins. This reaction center harvests light of a broader range of wavelengths thanks to other membrane-bound accessory (aka antenna) pigments. These pigments include zeaxanthin, β-carotene, echinenone, canthaxanthin, and myxoxanthophyll bound to membrane proteins. Attached to the cytosol face of the thylakoid are phycobilisomes. These particles (blue-gray specks along thylakoid in diagram) also serve as a light-energy antenna for photosynthesis. Extending into the cytosol, the phycobilisomes consist of a cluster of phycobilin pigments including phycocyanin (blue) and phycoerythrin (red) attached by their phycobiliproteins. Phycobilisomes preferentially funnel light energy into photosystem II for the splitting of water and generation of oxygen. While many photosynthetic eubacteria posess photosystem I to oxidize H2S (for example), only cyanobacteria have photosystem II. The evolution of photosystem II apparently occured in cyanobacteria.

The Color of Cyanobacteria

The relative abundance of phycobilin pigments explain the color of cyanobacteria en masse. Microscopically, the phycocyanin (blue) pigment in combination with the chlorophyll a and the accessory pigments lead to a bluish-green color...hence the common name: blue-green algae. It is also true that different species of cyanobacteria have differing ratios of phyocyanin and phycoerythrin. Cyanobacteria such as Hammatoidea, Heterohormogonium, Albrightia, Scytonematopsis, Thalopophila, Myxocarcina and Colteronema give thermal springs and geyser pools some beautiful color patterns from red to purple and the complete visible spectrum of colors between. Frequently terrestrial "blooms" produce a gooey slime that is black in color; black because virtually all wavelengths of light are absorbed by the combination of chlorophyll and the accessory pigments. A disease of coral heads is caused by a cyanobacterium (Phormidium corallactinium) and is know as "black line disease." Moreover, the rocks in the supralittoral fringe (splash zone) of many tropical shores are covered with epilithic (Scytonema, Gleocapsa and Pleurocapsa) or impregnated with endolithic (Mastigocoleus) cyanobacteria . This zone is often called the "black" zone because of the color of these cyanobacteria.

However the ratio of phycocyanin and phycoerythrin (red) can be environmentally altered. Cyanobacteria grown in green light typically develop more phycoerythrin and become red en masse. These same organisms grown in red light become bluish-green en masse. This reciprocal change in color has been named 'chromatic adaptation.'

During a "bloom" of cyanobacteria the appearance of a body of water can be drastically changed. Color is also not always due to pigments alone. Lakes in the Swiss Alps have been know to be turned blood-red by blooms of Oscillatoria rubescens because they have refractive pseudovacuoles (not bounded by a tonoplast membrane) rather than by excessive phycoerythrin. The Red Sea got its name from periodic blooms of Trichodesmium erythraceum for similar reasons. One aquatic disaster, red tide, is not caused by cyanobacteria, but instead by dinoflagellates (Pyrrophyta).

The lumen of these thylakoid sacs is the location of hydrogen ion accumulation. The collapse of the concentration gradient through a thylakoid-bound ATPsynthase provides ATP and NADPH for driving the carbon fixation reactions (the Calvin cycle) in the cytosol. It is not surprising polyhedral bodies (carboxysome), crystalline reserves of ribulose-bis-phosphate-carboxylase-oxygenase (aka: Rubisco, RuBP carboxylase), are found near the chromatoplasm to assist in the Calvin cycle.

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Cyanophycean Starch Grains

The cytosol between the thylakoids is also loaded with small grains (silver-gray in diagram) of cyanophycean starch. This is the polymerized carbohydrate product of photosynthesis. It is α-1,4-linked glucan, with frequent α-1,6-linked branching, somewhat like the amylopectin fraction of starch in higher plants and glycogyen in animals. This part of plant starch is not detectible with iodine, so glycogen and cyanophycean starch similarly do not respond to iodine with a blue-black color. Instead they accumulate iodine only to an amber-brown color. The grains are small and the failure to react to iodine makes them not visible to light microscopy. [Long-chain α-1,4-linked glucan is called amylose, and its helical structure accumulates much iodine resulting in the blue-black color observed with higher-plant starch.]

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Centroplasm

The central region of the cyanobacterial cell is less colorful than the chromatoplasm; it is called the centroplasm. This region includes the nucleoid or nucleoplasm but should not be confused with a true nucleus. There is no nuclear envelope and no true nucleus in any cyanobacterium; all species are strictly prokaryotic. The region is less colorful because of the lack of thylakoids, but it is certainly not empty.

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Nucleoplasm

Indeed the central region features the naked (no histone proteins), circular DNA genome in the nucleoplasm. Upon exposure to chromosome stains, stained bodies appear in this region of the cell (lavender in diagram). These have been interpreted as "chromosomes." Indeed cyanobacteria are host to a range of small, circular DNA molecules called plasmids.

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Ribosomes

As with any DNA, transcription and translation is expected and achieved in a living cell. For cyanobacteria the process appears to be identical to that in other Eubacteria. The translation process is accomplished by the assistance of abundant 70S ribosomes in the centroplasm (brown in diagram). These are smaller than their counterparts in the eukaryotic cytoplasm, but are typical of all prokaryotes, mitochondria, and chloroplasts. They provide an essential piece of the evidence for the endosymbiont theory of eukaryotic evolution.

Cladistic Analysis of rRNA

In the case of cyanobacterial ribosomes, the three ribosomal RNA units (23S, 16S, and 5S) have been isolated and sequenced (2900, 1540, 120 bases long). Cladistic analysis of these sequences have provided phylogentic trees for the cyanobacteria. These indicate that the Nostocales, Stigonematales, and Pleurocapsales are likely single clades (natural taxa), but that the Chroococcales and Oscillatoriales are multiple-clades (artificial) groupings. More importantly for us, these analyses have included Prochlorophytes and plant chloroplast rRNA. The three extant Prochlorophytes appear to be aligned in different clades but clearly belong with the various cyanobacteria as a single taxon. The chloroplast rRNA data show them to belong to the same clade within the cyanobacterial assemblage.

What that means is that we have strong evidence of chloroplasts as descendants of cyanobacteria lacking phycobilisomes. It means that the Prochlorophytes are cyanobacteria that lack phycobilisomes. It does not mean that prochlorophytes are the "link" to chloroplasts, however. It is possible that a prochlorophyte may be found which belongs to the chloroplast clade, but at present the extant prochlorophytes are descendants of unrelated cyanobacteria. That prochlorophytes exist in three different clades of cyanobacteria means that loss of phycobilisomes is not a particularly difficult step in evolution toward the chloroplast.

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Polyhedral Bodies

Some of the proteins translated by these ribosomes are of interest. Some of course are the carbon fixation enzyme, Rubisco, produced in sufficient abundance to form the polyhedral bodies (yellow in diagram) that are usually found closer to the chromatoplasm. Of course other soluble enzymes of carbon fixation and membrane proteins of electron transport are translated by these ribosomes in the centroplasm.

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Cyanophycin Granules

Another interesting protein that accumulates in sufficient abundance to form a visible structure is called cyanophycin. The 500 nm cyanophycin granules (green in diagram) are large enough to be observed sometimes with light microscopy! This protein is a polymer composed of just two amino acids: arginine and asparagine. These granules are considered to be an adaptation for accumulating and sequestering nitrogen for future use. They explain why these organisms can often grow nicely in areas with nitrogen-depleted waters.

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Polyphosphate Granules

Another product that accumulates to form granules is highly polymerized phosphate:

H2O2PO-[PHO2-]n-PO2H2

This material, likely polymerized by ribosome-translated enzymes, represents a storage material for phosphates (blue in diagram). Again these are considered an adaptation for accumulating and sequestering phosphate for future use. Again, they explain why cyanobacteria can often grow nicely in areas with phosphate-depleted waters.

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Lipid Droplets

Another storage inclusion in the cells are lipid droplets (various size black blobs in diagram). These are usually found in the chromatoplasm and perhaps the lipids are produced by processes catalyzed by reactions in the cytosol between the thylakoids. The enzymes for lipid synthesis are doubtless translated by ribosomes nearer the chromatoplasm.

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Gas Vacuoles

Perhaps a most-interesting translation product is the protein layer that surrounds gas vacuoles. These structures are not membrane-bound, but instead consist of laterally-connected cylindrical tubes of protein (honeycomb shape in diagram) that are permeable to gas but not to water. As dissolved gas accumulates in the cytosol, some of it equilibrates inside the tubes and collects as a bubble of free gas. Not surprisingly, the change in refractive index of the accumulating bubble makes gas vacuoles quite visible in the light microscope. They appear as gray bodies in the centroplasm. Gas vacuoles are involved with cell buoyancy. As gas accumulates, the cell moves up in the water column; as gas is used up, the cell sinks. A reverse situation may occur with cyanophycean starch grains as ballast. These two systems for buoyancy may cooperate to allow the cell to be positioned optimally in the water column for its photosynthesis or nutrient uptake processes.

Cyanobacteria can be found in many places on earth, they are both terrestrial and aquatic. If terrestrial, they generally grow near water. On the other hand, they can tolerate considerable desiccation for long periods thanks to resistant cells called akinetes. Some species live in hot springs (see above) and others (Romera elegans var nivicola) in banks of snow, sometimes changing them in color. If aquatic, the gas vacuole allows cyanobacterial cells to survive in two distinctly different environments. When gas vacuoles are missing or empty, the cyanobacteria sink in the water column to the bottom and sometimes even attach to the substrate. Such bottom-dwelling cyanobacteria are termed benthic. Species with functional gas vacuoles are often pelagic or planktonic. The gas vacuole allows these forms to maintain a favorable position in the water column. Planktonic cyanobacteria are often unicellular and very small in cell size; this facilitates buoyancy since flagella are not possible. It is estimated that 20% of the photosynthetic production in the ocean is accomplished by planktonic cyanobacteria.

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Vacuole-like Inclusions

The centroplasm often also contains a vacuole-like inclusion. This structure is not bound by a lipid-based membrane and so its structure and function are not clear (black border with white interior in diagram).

One last enzyme produced by translation in the centroplasm is nitrogenase. This interesting enzyme is responsible for nitrogen fixation; a process of global importance. Almost all cyanobacteria are capable of this process. As this enzyme is inhibited by molecular oxygen (O2), cells lacking special adaptations must be in an anaerobic or microaerobic environment.

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Division of Labor

Cyanobacteria are essentially unicellular organisms. Most cells operate independently from each other and can be fragmented to disperse the organism in the environment. This idea must be tempered by some basic ideas that you learned above.

Cytokinesis is not complete in some species, so a filament of attached cells can be produced. Cell wall pores allow cells in a filament to communicate with each other, so neighboring cells are not completely separated. In other cases, the mucilagenous sheath (gray borders in diagram) often holds cells of unicellular species together to produce what are essentially colonial forms.

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Heterocysts

Moreover, in filamentous species, not all cyanobacterial cells are identical in structure or function. The Stigonematales and Nostocales produce heterocysts. These are cells of the filament that are specially adapted for nitrogen fixation in an aerobic environment. Nitrogenase accumulates in these cells but storage granules and gas vacuoles do not. The chromatoplasm area appears yellowish at best, but the cells are almost colorless. The loss of phycobilisomes means, of course, that the cell lacks photosystem II function and therefore does not generate oxygen gas internally. Photosystem I activity remains and generates ATP and reducing power for nitrogen fixation. Additional ATP is generated by respiration of any permeating oxygen. The walls are quite thick sometimes with an additional knob of cell wall material at the ends of the cell. These walls make the heterocyst permeable to nitrogen gas but essentially impermeable to oxygen gas. The wall does have some very narrow pores with cytoplasmic connections to adjacent vegetative cells. The carbon source is probably imported through these pores as a sugar, and the product of nitrogen fixation is probably glutamine returned to the adjacent cells through the pores. The nitrogen is ultimately stored as cyanophycin.

It is interesting that heterocysts are produced when ammonia (NH4+) is depeleted in the environment. Precisely how the vegetative cells determine this, how they "produce" a heterocyst as a result, is an interesting mechanism awaiting elucidation! Regardless, one interesting observation results from this process...if an environment has enhanced phosphates, a bloom of cyanobacteria is likely. With sufficient phosphate, light, water, and gases, a cyanobacterium with heterocysts can make as much nitrogen as it needs to grow very rapidly. For this very reason phosphates were banned from laundry detergents in the 1970s.

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Akinetes

A second cell type produced is called an akinete. As the name implies, it looks like a cell that has grown without dividing. It has filled itself with much reserve: cyanophycean starch and lipid droplets for energy and cyanophycin for nitrogen. They usually lack polyphosphate granules, and akinete formation may be triggered by a deficiency of phosphate in the environment. On the other hand our knowledge of why and how these cells form is quite sketchy. What we do know is that these akinetes can remain in sediments for literally many years, enduring very harsh conditions, and remain viable. When suitable conditions for vegetative growth are restored, the akinete germinates into new vegetative cells. Thus the akinete appears to be a "resting" stage somewhat akin to hypnospores.

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Branching

Another kind of "division" of labor in filamentous cyanobacteria is branching. Branching can be of two different types. In the Nostocales and Oscillatoriales any branching is called false branching. In these families, the chains of cells (called trichomes) are held together by a shared sheath to form the filament. A break in the chain of cells in the trichome results in a branch. The broken trichome grows out at some angle inside a branch of sheathing material. The break makes this a false branch as it is attached only by means of the containing sheath. In the Stigonematales, true branching occurs. Here a single cell divides in two directions forming a connected branch.

Cell Division

A range of cell types can also be made for dispersal which are generally called spores but should not be confused with sexual spores in eukaryotic organisms. Cyanobacterial spores are all produced vegetatively by cell division processes. In cyanobacteria the genomic replication process is called fission and must NOT be confused with mitosis. Mitosis occurs only in eukaryotic cells!

The nuclear material is replicated by fission and the cytoplasms are divided by progressive constriction of the cell membrane and cell wall. Thus cytokinesis is by means of a process akin to furrowing. This feature is also observed in the proliferation of mitochondria and chloroplasts in plant cells; further evidence of the endosymbiont theory.

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Asexual Reproduction

If a normal-sized vegetative cell divides repeatedly without increasing its volume, the cells contained in the original wall are very dwarf and are called nannocytes. In other species, the vegetative cell grows extensively before the cytoplasm is divided up into several normal-size spores called endospores. When the outer wall breaks down the contained nannocytes or endospores are released all at the same time. In yet other species, the end of a filament or a vegetative cell divides repeatedly and releases spores one at a time into the environment; such spores are called exospores.

Filaments often have a structurally-designed weakness to break apart along the trichome somewhere. The multicellular fragment of a trichome is released from the sheath as a hormogonium. Sometimes this is accomplished by certain distributions of pores in the wall, by positions of heterocysts, or positions of akinetes.

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"Sexual" Reproduction

Well, that heading sure sounds misplaced when considering prokaryotic cells. There is no such thing as meiosis or syngamy in prokaryotes. There are "trick" questions on GRE Biology exams focused on this misconception...watch out! Anyway, bacteria in general and cyanobacteria in particular do appear to have some forms of genetic recombination possible. These are divided into two categories: transformation and conjugation.

Transformation occurs when DNA is shed by one cell into the environment and is taken up by another cell and incorporated into its genome. The ability to be transformed by external DNA is species-specific and typically requires special environmental conditions. Biologists have learned to harness this process to engineer bacterial genomes to accomplish scientific goals.

Conjugation is a "parasexual" process in which one of the partners develops a conjugation tube that connects to the recipient cell. Typically a plasmid (a small circle of DNA) from the donor cell passes through the conjugation tube. It is thought that genes for gas vauolation, antibiotic resistance, and toxin production are carried on plasmids in cyanobacteria.

To date the evidence for transformation in cyanobacteria is developing strongly. Evidence for conjugation is expected but has not be produced to date.

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Systematics of Division Cyanophyta

Based on modern methods, the division (phylum) Cyanophyta should be thought of as including the former division Prochlorophyta, so I will take that approach here. This division, therefore includes all oxygenic photosynthetic prokaryotes and can be defined in that way.

The cyanophytes can be further divided into two classes. The Cyanophyceae includes all cyanobacteria which possess phycobilisomes and phycobilin pigments. The Prochlorophyceae include cyanobacteria which lack phycobilisomes and phycobilin pigments. This division into two classes is probably artificial and therefore subject to further revision as more members of the Prochlorophyceae are found.

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Class Cyanophyceae

The class, Cyanophyceae, can be divided into five orders: Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales, Stigonematales.

Order Chroococcales

The Chroococcales contains unicellular cyanobacteria. In some species the cells are held together by a sticky mucilaginous sheath to form colonies of various forms. Dispersal is by cell division, formation of nannocytes, and by budding. Included genera are: Cyanothece, Aphanothee, Merismopedia, Chroococcus, Gloeocapsa, Microcystis, Chemaesiphon, and Eucapsis.

Order Pleurocapsales

The Pleurocapsales includes unicellular or small colonial forms. Some may even consist of a pad of parenchymatous cells with short attached branched or unbranched filaments. Dispersal is by cell division and endospores. Included genera are: Cyanocystis Chamaesiphon and Pleurocapsa.

Order Oscillatoriales

The Oscillatoriales includes filamentous cyanobacteria that disperse primarily by formation of hormogonia. Any branching in the filament is false, and neither heterocysts nor akinetes are produced. Included genera are: Oscillatoria, Lyngbya, Microcoleus, Phormidium, Arthrospira, and Spirulina.

Order Nostocales

The Nostocales includes filamentous cyanobacteria that disperse primarily by formation of hormogonia. Any branching is false, and both heterocysts and akinetes can be produced. Included genera are: Nostoc, Anabaena, Cylindrospermum, Aphanizomenon, Scytonema, Gloeotrichia, and Rivularia.

Order Stigonematales

The Stigonematales includes filamentous cyanobacteria that disperse mainly by formation of hormogonia. Branching is true and heterocysts and akinetes can both be produced. Included genera are: Stigonema, Hapalosiphon, and Fisherella.

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Class Prochlorophyceae

The class, Prochlorophyceae, is divided into one order: Prochloroales.

Order Prochloroales

The Prochloroales includes cyanobacteria that lack phycobilisomes, have both chlorophylls a and b, and therefore are grass-green in color. The storage polysaccharide is starch-like. Cyanophycin, a storage peptide, is absent. Included genera are: Prochloron, Prochlorococcus, and Prochlorothrix.