Blue green algae, or Cyanobacteria, are remarkable in as much as they are somewhat of a hybrid between algae and bacteria. In some ways they resemble green plants and algae, and in others they are much closer to bacteria. The latter explains why antibiotics are sometimes used to rid the tank blue-greens.
Although this method results in their disappearance, at least for a while, the dangers associated with using them in an aquarium need to be stressed here.
Biological filters contain many bacteria themselves. These, too, can be affected and more than likely will be. This can lead to ammonia spikes in the tank due to the loss of the bacterial bed (partially or completely). Note that since live rock and live sand contains bacteria as well, the use of antibiotics will affect them as well.
Cyanobacteria are photosynthetic. They release oxygen and uptake carbon dioxide, depending on the photoperiod and lighting conditions. Cyanobacteria contain pigments. The prevalence of one particular pigment ultimately determines the color of the algae.
It should be noted that they are among the oldest form of organisms found on earth. Traces of cyanobacteria have been found in fossils dating from the early Precambrian period, some 3 billion years ago. They are extremely common, even today, and are found in fresh, brackish and salt water. They are present even on land in moist environments. Cyanobacteria, referred to as blue-green algae, come in various colors.
Do not let the name confuse you. They are not necessarily blue-green but can be blackish, greenish, blue greenish, yellowish, brownish and reddish. The latter color is perhaps the one most saltwater hobbyists are familiar with.
Note that your aquarium doubtlessly contains many more cyanobacteria than you are aware of. The main reason being that many of the various forms of cyanobacteria are present in any environment where photosynthesis takes place. Their numbers may be small and their size certainly is. You do, therefore, not notice them.
At times though they become really conspicuous because they agglomerate and form masses and patches. By themselves they are extremely small and are generally studied under an electron microscope, lest they would not be visible. Regular microscopes are not powerful enough to allow scientists to study them in detail.
The appearance of blue-green algae is always a disappointment to the hobbyist. Regardless of the type though their presence indicates a disturbance of the biological equilibrium in the tank (Fay, 1983). More on this later.
Note that blue greens are neither algae nor bacteria. They exhibit characteristics of both and can thus be considered an evolutionary link between the two. From the standpoint of cell structure they are clearly bacteria. From the perspective of their photosynthetic ability and the presence of pigments, they are algae. Confusing? Probably so, but of no real importance since we are not trying to come up with the ultimately correct nomenclature.
Various classification models have been put forth. Ongoing studies of Cyanobacteria result in frequent changes. A recent one I came across (Rippla et al.) and Fritsch (an older one) proposes the following principal groups (principal being the key word here):
Classification of Cyanobacteria Morphology Reproduction Order Names (general) Unicellular or Colonial Binary fission
=splitting in two
Unicellular or Colonial Budding
Multiple Fission =
splitting in more than two parts
Filamentous Trichome, which is
a chain of cells
Trichome fragmentation = splitting
of the chain of cells
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