Description of cyanobacteria
Cyanobacteria are a very diverse group of microorganisms that resemble the typical algae in many ways. Without close inspection they look somewhat alike. They are found in similar places and conditions. They are, as a rule, both green. However, what we want to demonstrate is that there are basic differences that set cyanobacteria apart from the typical algae.
1.) Internal Anatomy
Cyanobacteria are classified as prokaryotes. Prokaryotes (bacteria, for example) are organisms that are less complex than eukaryotes (plants and animals). In Greek, “pro-” means “before” and “karyon” means “nut” or “kernel” in contrast to “eu-” which means “well” or “true”. The prokaryotes are thought of as “primitive organisms” and eukaryotes are thought of as “complete organisms”. This is reflected in their internal structures, as well. Prokaryotes have fewer and simpler internal structures (sometimes called bacterial microcompartments) that are surrounded (bound) by layers of protein [Komárek, 2013]. Eukaryotes have a wide array of internal structures, called organelles, which are surrounded by lipid membranes. Examples of organelles are the nucleus or the chloroplast.
Though cyanobacteria do not have lipid membrane-bound organelles, there are observable structures in their cells. Inside many cyanobacteria are hollow structures called aerotopes (or gas vacuoles in older literature). The protein walls surrounding the aerotopes will allow gases to pass through them but not water [Oliver and Ganf, 2000]. As the aerotopes collect gas, they make the cell more buoyant in water so it can float to the surface. This moves the cell (or group of cells) into the light so it/they can photosynthesize. As the cells accumulate denser carbohydrate through photosynthesis, they become less buoyant and sink. As the cells consume the carbohydrate and produce carbon dioxide, water and energy as by-products, the cells become more buoyant and the cycle continues. Cyanobacteria are the only photosynthetic plankton that has aerotopes which makes possible the regulation of buoyancy and the resulting vertical migration though the water. There are many advantages to this vertical migration. There are often higher concentrations of chemical nutrients (such as nitrogen or phosphorous) in the lower water levels and less competition for them. There are also dangers closer to the surface of the water: predators are more numerous at the surface and high light intensity can damage pigments and cell structures.
A.) Vertical mobility.
Though almost all types of cyanobacteria can produce aerotopes, generally it is the ones that form surface blooms that produce the greatest numbers of aerotopes. These genera would be Microcystis, Dolichospermum and Aphanizomenon. These usually form surface films or scums when the water is calm and stagnant and the cells have lost their ability to regulate their buoyancy [Walsby, 1994]. There are other bloom-forming cyanobacteria that have aerotopes (Cylindrospermopsis and Planktothrix) that do not form surface scums.
Another use of the aerotopes is to collect the carbon dioxide produced by carbohydrate break down (metabolism) for reuse in photosynthesis [Rae, et al., 2013]. When aerotopes are present in great numbers in cells, they refract the light going through the cell which makes them appear dark brown or even black under a microscope.
There are other internal structures in cyanobacteria - some with direct roles in photosynthesis. One is another type of protein-walled bacterial microcompartment called a carboxysome [Rae, et al., 2013]. This structure acts to increase the efficiency of photosynthesis in cyanobacteria by increasing the concentration of carbon dioxide around a crucial enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase, abbreviated RuBisCO. This enzyme is involved in the first major step of photosynthesis: the process by which atmospheric carbon dioxide is incorporated into the series of chemical reactions that will eventually store energy in the form of carbohydrate. This key initial step is also referred to as carbon fixation: mobile, atmospheric inorganic carbon is pulled (fixed) into a more stable structural form in living things. It is suggested that cyanobacteria could not attain the environmental dominance that they sometimes achieve without this increased efficiency in photosynthesis [Rae, et al., 2013].
Also inside the cyanobacterial cell are extensions of the external membrane that fold back on themselves increasing surface area. These protrude into the protoplasm in a somewhat organized manner. It is on these surfaces, called thylakoids, that the photosynthetic pigment complexes are displayed to capture solar energy [Olive, et al., 1997]. The pigments in these complexes are phycocyanin (blue-green), allophycocyanin (blue), chlorophyll-a (green) and phycoerythrin (red) [Colyer, et al., 2005]. The phycocyanins are coupled with proteins to form visible structures called phycobilisomes. They are the primary light collecting structures for photosynthesis in cyanobacteria [Grossman, et al., 1993]. This is particularly true at low light levels. These structures also give the blue-green algae their characteristic color.
2.) External Anatomy
In addition to these internal variations, cyanobacteria also exhibit external differences in appearance. Their cells usually have relatively thick, gelatinous cell walls [Kangatharalingam, et al., 1992]. The level of organization of these cells ranges from simple, uniform, single cells to fairly complex groups or colonies of multiple cells with a limited degree of specialization.
Cyanobacteria colonies have two primary shapes or forms: filamentous and coccoid. In the filamentous colony form, the cells are in linear chains called filaments or trichomes. The cells in filaments usually look square to rectangular. Many are disc-shaped so they look like stacked hockey pucks. When filament cells divide, the new cell wall is produced at a 90° angle between the two new cells. It is also perpendicular to the length or long axis of the filament. This extends the chain (like stacking blocks or pucks) and maintains the connections between the neighboring cells. On the other hand, the cells in the coccoid-form colonies are usually round to oval in shape. The term coccus is derived from the Greek word for a round berry. When coccoid cells divide, the new cell wall is not produced like the filamentous cells, but in seemingly random, multiple planes of division. Furthermore, the new cells will often separate from one another, so they don’t form linear chains. This occurs generation after generation, so coccoid colonies have a wide variety of shapes. Some coccoid colonies are orderly spheres, others are two-dimensional sheets, and still others are amorphous blobs. These coccoid colonies take on a wide variety of forms.
A.) Sheath
Many cyanobacterial cells and colonies can surround themselves in a jelly-like coating called a sheath. This sheath can envelop an individual cell, an entire colony or both. The sheath is made up of a combination of fat and starch called a lipopolysaccharide. There are other bacteria that secret this type of coating. Also like the other bacteria, the substances in the sheath have been observed to induce immunological (allergic) responses in humans and other mammals [Sivonen and Jones, 1999]. The sheath acts to protect the cyanobacteria cell or colony from predation, desiccation and UV radiation from the sun. It also provides structural integrity to the colony. Some sheathes are thin and watery, others are slimy and still others are stiff, thick gels. The sheath ranges in appearance from clear and colorless to densely dark yellows and browns - some are almost opaque. This is why cyanobacteria may or may not look blue-green when we see them in the field [Garcia-Pichel, et al., 1991]. The appearance depends on the organism and the environment in which it grows.
3.) Cellular Anatomy
Cyanobacterial cells vary in size, shape and complexity. Most cells are thin-walled vegetative cells. They are the primary structural unit of both filamentous and coccoid colonies. They can be smaller than 1 micron in diameter (one-millionth of a meter, or µm). It is difficult to see detail at this size with a light microscope. These cells can be as large as 400 µm (the size of a human hair) and is visible to the unaided human eye [Komárek, et al., 1999].
There are a wide variety of cell shapes among cyanobacteria. Filamentous cells generally appear square or rectangular, but there are also many other shapes. Cells in coccoid colonies tend to be spherical or oval, but again, there are many other cell shapes. It must be clearly stated that cell shapes are generally consistent for a specific organism in their environment. The cell shape of species X in colony 1 is very similar to the cell shape of species X in colony 2. But, there is always variability in biological organisms in terms of cell size, shape and complexity.
The changes in complexity that occur in cyanobacteria cells are often in response to unfavorable environmental conditions, such as seasonal dormancy/ replication or nutrient deficiency. Organisms and their cells can and must change in response to their circumstances. For cyanobacteria, it is usually the vegetative cells that divide and change into other types of cells. Depending on cyanobacteria and their conditions, the vegetative cells can generally form one of three types of complex or specialized cells: akinetes, apical cells or heterocysts.
A.) Akinetes are thick-walled cells that are specially adapted to dormancy. They are often produced in response to dropping temperatures or drying conditions. Akinetes are the most common of the dormant cells. They are also described in the Dormancy segment below.
B.) Apical cells are the terminal cell in a cyanobacteria filament. These often become specialized reproductive cells that look very different from the other cells in a filament. This also makes apical cells very useful for humans to tell apart one group of cyanobacteria from another. Both akinetes and apical cells will be discussed in greater detail in the Replication segment on the following page.
C.) Heterocyst formation is the most drastic change in cell shape and complexity by cyanobacteria. Heterocysts are larger than vegetative cells and have especially thick, double-walls. Heterocysts are formed in response to nitrogen deficiency. Although elemental nitrogen (N2) makes up about 79% of Earth’s atmosphere, it is in a chemical form that is not usable by most organisms. The bonds holding together the two nitrogen atoms are too strong for most organisms to break, so the N2 in the atmosphere is not usable. N2 must be changed into chemical compounds that organisms can absorb and use. These usable or reduced forms of nitrogen are compounds such as ammonia (NH3). This reduced form of nitrogen can be absorbed and used organisms to make proteins, nucleic acids and other structural components. This chemical transformation of N2 into NH3 is accomplished, in part, by the enzyme nitrogenase and some very complex chemistry. This enzyme is only found in a few groups of bacteria, one of which are the cyanobacteria. The cyanobacteria only form nitrogenase in heterocysts because the enzyme is destroyed in the presence of oxygen (O2). The thick, double-walled construction of heterocysts is required to keep O2 away from the nitrogenase enzyme. Within the heterocyst, elemental nitrogen N2 is reduced or ‘fixed’ into the form (NH3) usable by organisms. This capability has been utilized for centuries in rice cultivation by farmers adding the aquatic fern Azolla to rice paddies. The farmers didn’t know that the nitrogen-fixing cyanobacteria Anabaena lived inside Azolla which had the result of adding reduced nitrogen to the rice crop [Bocchi and Malgioglio, 2010].
4.) Replication of Cyanobacteria Cells
There are major differences in how cyanobacteria generate their next generation of cells as compared with plants and animals: both in the nature of the processes and in the end product. To emphasize this important distinction, the cyanobacterial process is referred to here as replication rather than reproduction. The most significant distinction is that the replication of bacterial cells is completely separate from any exchange of genetic material. The process of sexual reproduction has never been observed in any bacteria. They undergo simple cell division, also known as binary fission. The end product of binary fission is two genetically identical daughter cells. The advantage of this is that it can be very rapid. Under ideal conditions a new generation of cyanobacteria cells is produced in as little as 30 minutes. This rapid duplication contributes to bloom formation.
Changes in Genetic Material
The process that usually changes the genetic material of succeeding generations of plant and animal cells is sexual reproduction. This process is not available to cyanobacteria. But, there are at least three other mechanisms, collectively referred to as horizontal or lateral gene transfer, that can change the genetic makeup of cyanobacteria cells. They are conjugation, transformation and transduction.
a.) Conjugation is a process where direct, physical contact between two bacterial cells is established to transfer genetic material. The conjugation process can only be initiated by a bacterium that has a particular set of genes called an “F factor”. These bacteria are designated F+ while those without these genes are designated F-. F+ bacteria will only conjugate with F- bacteria. The F+ bacterium, or donor, initiates the conjugate process by making an appendage, called a pilus, which is used to establish the physical contact with the recipient cell. Eventually the pilus becomes a cell-to-cell tunnel which delivers the genetic material to the recipient cell. The genetic material transferred is in the form of a small, pre-packaged, circular strand of DNA called a plasmid. The DNA in the transferred plasmid may code for a number of genes. The genes on the plasmid usually include the F factor, but could also have other genes, such as those that give the ability to cause disease or antibiotic resistance [Gyles and Boerlin, 2014] or to synthesize cyanotoxins [Stüken and Jakobsen, 2010].
b.) Transformation is a process where bacteria can absorb plasmids that are floating free in the environment. The bacterial cell could then incorporate this foreign DNA into its own genetic material and express these genes.
c.) Transduction is a process where genetic material is transferred from one bacterial cell to another by viral infection. There are many different viruses, called cyanophages that infect cyanobacteria. Some infect a wide range of cyanobacteria genera and are being studied as anti-bloom pathogens as well as genetic engineering tools to incorporate genes into cyanobacteria [Deng and Hayes, 2008].
All three of these processes have the potential to build genetic diversity into the cyanobacterial genome. However, there are questions about how often this external genetic material is incorporated into the bacterial genome and the actual mechanics of that incorporation [Michod, et al, 2008]. Nevertheless, broad genetic changes in cyanobacteria have been and continue to be observed in nature. The robust adaptive power of these organisms to an ever-changing environment continues to be a scientific and societal challenge, as well as a marvel to behold for all students of nature.
B.) Colony Replication
The multiplication or replication of cyanobacterial colonies occurs mostly by fragmentation and the formation of hormogonia.
a.) Fragmentation is simply when colonies break apart into smaller groups or individual cells, which divide and grow into new colonies. This occurs in both coccoid and filamentous cyanobacteria. This can happen in a variety of ways. The most common is probably by physical processes, for example, wave action or predators (grazers) breaking up the colony. In some cases, cells in a filament will die to release a filament segment to form a new colony.
b.) Hormogonia formation is a little more complicated. Hormogonia are essentially immature filaments. Because of how they develop, hormogonia are better equipped to develop new filaments as compared to fragmentation products. The hormogonia are released from the filament by the death of the cell connecting it to the filament. Once released, the hormogonia drift away to grow into new colonies.
C.) Dormancy
Under adverse environmental conditions, cyanobacteria can produce cellular structures that can lay dormant for extended periods then ‘germinate’ into live cells. This is not reproduction in the strictest sense, but rather a defense mechanism to ensure the continuation of the organism through threatening circumstances. The most common form of resting cell, as we saw before, is an akinete.
a.) Akinetes are thick-walled, dormant cells which develop from vegetative cells when exposed to unfavorable environments. The conversion process to an akinete removes or reduces all non-essential functions and increases food reserves within the cell, not unlike a seed. Akinetes have been documented to be viable after 70 years of dry storage [Adams and Carr, 1981]. The genera Anabaena and Nostoc are good examples of organisms that produce akinetes.
5.) Habitat
Cyanobacteria are exceedingly plentiful and varied organisms. They inhabit virtually all aquatic and terrestrial habitats across all latitudes. Though they can survive many extreme environments, they generally grow best in slow moving/low mixing, nutrient-dense river and lake waters during the brighter/warmer months of the year. Under these summer conditions, they are often the dominant organism until the water cools later in the season. Under bloom conditions, the cyanobacteria are often the dominant organism in that body of water. This condition occurs when light, temperature, and nutrient parameters favor a particular species. The official US EPA definition of an algal bloom is 20,000 cells per milliliter, but cell densities up to several million cells per milliliter have been observed [Carmichael 1994].
A.) Vertical mobility
A characteristic that gives free-floating or planktonic cyanobacteria an advantage over other algae in slow moving/low mixing waters is the capacity to move up and down (vertically) through the water column [Hoehn and Long, 2008]. This is important because of the vertical structure and properties of rivers and lakes. The upper part of the water column is brighter and warmer; it is best for photosynthesis. However, the lower part of the water column has higher concentrations of the nutrients needed by algae, like phosphates. Thus, the ability to move up and down through the water column gives cyanobacteria greater access to both light and nutrients. This ability to move vertically is governed by the buoyancy of the cyanobacterial cell and can be changed or regulated in a cyclical way. The cycle could start with the buoyant cell in the upper, brighter part of the water column earlier in the day. As the cell photosynthesizes, it pulls carbon dioxide out of the aerotopes (which shrink) and converts it to heavier carbohydrate. As the aerotopes shrink and the cell accumulates carbohydrate, the cell becomes heavier, less buoyant, and slowly sinks. In the lower, darker part of the water column, photosynthesis tapers off and the cell begins to pull carbohydrate out of storage to consume it. One of the end products of carbohydrate consumption (respiration) is carbon dioxide. The CO2 is accumulated in the aerotopes, which makes them grow. With the consumption of carbohydrate and the inflation of aerotopes, the cell becomes more buoyant and rises into the upper portion of the water column to complete the cycle. This buoyancy is also influenced by time of day, weather and other variables. It is also why cyanobacteria congregate at the surface as pond scum.
B.) Viability through flexibility
Cyanobacteria are opportunistic organisms. They often thrive under the hot and dry conditions that stress their competitors (other algae) the most. Up to a point, the more hot and stagnant the body of water becomes, the greater the advantage for the cyanobacteria over the other algae. This is not unlike crabgrass in a hot and dry summer lawn - the greater the stress on the other lawn grass, the more the crabgrass can flourish.
The aquatic habitats where cyanobacteria are found include fresh, ocean and hypersaline waters, plus highly mineralized, acidic, alkaline, polluted, stagnant and high temperature waters. Within these waters there are cyanobacteria that are free-floating planktonic organisms, many that grow on or among aquatic vascular plants (cattails) or algae. Still others grow in mats on damp or submerged surfaces. Of the wide variety of habitats that cyanobacteria occupy, the primary focus here will be on freshwater, aquatic habitats.
Terrestrial cyanobacteria live in damp soils, mostly dry desert surfaces, bare rock and even within rocks. Studies in Antarctica have found cyanobacteria growing on and penetrating granite rock substrates in full exposure to the harsh Antarctic climate [de los Ríos, et al., 2007, Singh and Elster, 2007, Vincent, 2007].
There are even cyanobacteria that live inside other organisms. Lichens are a symbiotic cohabitation where an alga lives within a fungal matrix. They live on tree trunks, bare rocks and other “exposed” habitats. About 10% of lichens have cyanobacteria living within the fungus. The cyanobacteria provide glucose for the fungus, while the fungus provides protection and moisture for the alga. There are a variety of protozoa, sponges, liverworts, ferns and cycads that form symbiotic relationships with cyanobacteria. The blue-greens provide glucose and/or reduced nitrogen to their partner (symbiont). Some cyanobacteria are found living in the fur of sloths (for camouflage) and polar bears (in zoos with warm, muggy climates). To summarize: cyanobacteria can be found almost anywhere on Earth.