Blue-green algae make up the division Cyanophyta in the kingdom Monera, which is made up of about 1,500 species of prokaryotic organisms. There is disagreement on whether or not this division should be considered algae or bacteria. Cyanobacteria (Blue Green Algae) differ from other types of bacteria in that they have chlorophyll a, which other photosynthetic bacteria donít have. Another characteristic that supports the algae argument is the fact that free oxygen is given off in blue-green algae photosynthesis but it is not given off in the photosynthesis of other bacteria. Many bacteria split H2S instead of H2O as a source of electrons during their photosynthesis; this is why they donít produce free O2. These other bacteria have bacteriochlorophyll instead of chlorophyll a as their main photosynthetic pigment (Bold, 1985). Evidence supporting the bacteria argument has to do with blue-green algaeís cellular organization. They are prokaryotic (no membrane-bound organelles), they have only a haploid life cycle (while all algae life cycles have an alteration of generation), they reproduce through fission, they donít have cellulose in their cell walls, their DNA is not associated with histone proteins in their chromosomes (unlike algae and other plants) (Clark, 1998).
Cyanobacteria gets its common name from the blue-green pigment, phycocyanin, which along with chlorophyll a gives cyanobacteria a blue-green appearance. Phycocyanin is a protein that functions as the photosynthetic pigment in photosystem II, whereas in plants chlorophyll b is the pigment in photosystem II (Clark, 1998).
Cyanobacteria are responsible for life as we know it. It was cyanobacteria in the Archaean and Proterozoic Eras (2.5 billion years ago) that were responsible for creating our oxygen atmosphere. Before that time the atmosphere had a very different chemistry, unsuitable for life as we know it (Friend, 1999).
Cyanobacteria have a wide variety of habitats that range from frozen lakes, to acidic bogs, to deserts and volcanoes. They are most commonly found in alkaline aquatic environments (but also in aquatic environments ranging in salinity and acidity), they can also be found in soil, on rocks, and even in the atmosphere (Bold, 1985). There are a number of unique characteristics of cyanobacteria that are responsible for this wide variety of habitats. I will discuss these characteristics later in the paper.
Along with a wide variety of habitats, cyanobacteria also have a range of organization. They can range from unicellular, to filamentous, to colonial. Since cyanobacteria can inhabit some pretty extreme environments they are often the primary colonizers of a new area. As primary colonizers they have an important role of adding organic matter to the soil. The cells of a colony are undifferentiated from each other. Some colonial forms of cyanobacteria have been known to form mats down on the surface of the soil, which helps to prevent erosion. The filamentous organization is composed of a chain of cells and their enveloping sheath; this organization can be modified or lost as the environment changes (Bold, 1985).
Most cyanobacteria are photoautotrophic organisms [some are also photoheterotrophic, which means they use light to generate ATP but they must obtain carbon in organic form] that fix CO2 and release O2. Cyanobacteria have a special environmental adaptation for surviving in low CO2 concentrations, the CO2 concentrating mechanism (CCM). This mechanism actively transports and accumulates inorganic carbon (HCO3 and CO2) within the cell, creating a high CO2 concentration pool around the CO2-fixing enzyme, Rubisco (ribulose bisphosphate caroxylase-oxygenase). Rubisco is an enzyme that helps to convert carbon dioxide into sugars in cyanobacteria (Badger, 1998).
Another unique characteristic of some cyanobacteria is their ability to fix elemental (gaseous) nitrogen. They do not have to rely on other combined nitrogen sources. The enzyme complex responsible for this nitrogen fixation is called nitrogenase (Bold, 1985). In low N2 environments cyanobacteria will produce heterocysts, which are larger, thicker-walled cells that are better at fixing nitrogen (Clark, 1998). The ability of cyanobacteria to fix elemental nitrogen has made it a very important agricultural asset. They are used as nitrogen fertilizer in the cultivation of rice and beans (Bold, 1985).
As I mentioned earlier, unicellular cyanobacteria reproduce asexually by fission. The bacterial chromosome (whose DNA is not associated with the histone protein) is called a genophore. During fission this genophore attaches to the plasma membrane and replicates. Cell division then occurs, separating the bacterium into two identical cells. Because cyanobacteria are prokaryotic they have no nuclei, which means they can not undergo meiosis and fertilization. This limits genetic recombination among cyanobacteria. Yet they do have three ways of genetic exchange. They can exchange genetic material through a conjugation pilus, which physically connects two adjacent cells. Secondly, transformation can occur, which is the absorption of free DNA, which is then expressed in the host. And the third way in which genetic exchange occurs is through transduction, in which a virus enters the blue-green algae cell and uses its DNA to replace the DNA already in the cell. Mutations are also a common occurrence in cyanobacteria cells. All of these factors help cyanobacteria to overcome its genetic recombination limitations due to asexual reproduction (Clark, 1998). The colonial and filamentous cyanobacteria reproduce by fragmentation. In fragmentation segments of the parents break off and float away (they are motile). These fragments then grow into new cells (Bold, 1985).
Another way in which cyanobacteria are able to survive in extreme conditions is by producing akinetes, thick-walled cells that are able to resist desiccation (drying out) and freezing. They can remain dormant for long periods of time until conditions become right and they germinate (Clark, 1998).
Cyanobacteria do not have organs for movement such as flagella, but some filamentous blue-green algae do exhibit a gliding movement. It is thought by some to be a result of slime secretion along with contractile waves on the cells (Bold, 1985).
Blooms of cyanobacteria due to nitrogen-based runoff (either from agricultural fertilizer or waste) have been responsible for killing aquatic life in the Gulf of Mexico, off the coast of North Carolina, and elsewhere. These algal blooms are responsible for the hypoxia (low oxygen) occurring in these areas due to bacterial decomposition of the algae and the Zooplankton waste (Annin, 1999). Algal blooms such as this have raised concerns over the safety of cyanobacteria in drinking water, and recreational water. It is known that certain species of cyanobacteria produce biotoxins that could be potentially harmful to animals and people in the right concentrations. A study in Australia observed the effects of water containing different concentrations of cyanobacteria on people in recreational areas (Burch, 1997). In the study, the scientist would travel to various recreational areas between January and February (the peak season for algal blooms in Australia) and invite people over six years of age to participate. These participants filled out a questionnaire, which addressed health status, how long they spent in the water on the day of the interview and in the previous five days. The scientists would take water samples from these sites and determine the cyanobacteria concentration. The scientists then did phone follow-ups on the participants two and seven days after their exposure to the water. These participants were than asked if they suffered diarrhea, vomiting, flu-like symptoms, skin rashes, mouth ulcers, fevers or eye and ear infections lasting more than 24 hours since they had been in the water. The scientist found there was no significant difference between participants exposed to cyanobacteria and those not exposed after only two days. At seven days there was a significant trend toward an increasing occurrence of symptoms with increasing lengths of time in the water (duration of exposure). There was also a trend of increasing symptoms with increasing cyanobacteria cell count. Participants exposed to more then 5,000 cells per ml for more than an hour had significantly higher symptom occurrence rates than people in lower cyanobacteria concentration environments. They found that unhealthy symptoms due to cyanobacteria exposure increased with the time of exposure and the concentration of the blue-green algae (Burch, 1997).
Cyanobacteria are not all bad. They have lots of positive uses in todayís society; as nitrogen fertilizer, for example. Cyanobacteria may also have applications in the area of human health. For example, an extract of Arthrospira platensis (a filamentous blue-green algae) inhibited HIV-1 replication in human T-cell lines (peripheral blood mononuclear cells (PBMC), and Langerhans cells (LC)). A group of scientists at the Dana-Farber Cancer Institute injected these cells with varying concentrations of the extract to observe the effectiveness of different concentrations. What they found was that concentrations between 0.3 and 1.2 mg/ml reduce the viral production by 50% in PBMC cells. This cyanobacteria has antiretroviral activity that could in the future become part of medication for HIV patients (Ayehunie, 1998). Another positive use of cyanobacteria is as a dietary supplement; users say it enhances their mental clarity and energy. Cyanobacteria have many more uses and positive affects ranging from inhibiting the growth of tumors, to lowering cholesterol (Howe, 1997).
As you can see cyanobacteria are a very hot topic concerning both their potential health risks and health assets. This variety of seeing cyanobacteria as both a potential danger and a potential medicine corresponds with the wide variety of blue-green algae that make up the division Cyanophyta. When talking about cyanobacteria we are talking about a whole range of species that differ in habitat, organization, health potential, and nutrition, just to name a few. They all have unifying characteristics such as being prokaryotic, having chlorophyll a, and the liberation of O2 during photosynthesis, but along with all these similarities they have an equally long list of differences. Some secrete harmful biotoxins and others help to inhibit the growth of retroviruses. When talking about cyanobacteria in general you have to just look at the basic similarities that exist among the species, but at the same time you have to recognize that major differences also exist.
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