A large field of science is called taxonomy. This science is the field of nomenclature or how to name living things. At first, this science was developed by Carl Linnaeus (1707-1778), and is based on the classification of living organisms. For the most part, this classification is basically done on the basis of physical characteristics.

This whole system of nomenclature has seven levels of classification, including kingdom, phylum, class, order, family, genus, and species. With the help of this naming system, the whole description of a certain organism is enclosed in its name, whether it is a bacterium, a fungus, a flower or an animal. However, in common practice many of the living organisms are named according to their genus and species.

For instance, the monarch butterfly is named as Danaus plexippus, a dog is called Canis familiaris, a wolf is called Canis lupus, a man is known as Homo sapiens and the blue - green algae is known as Aphanizomenon flosaquae. As we know that this system of taxonomy is mainly based on physical characteristics, plants or animals with same characteristics have same names.

For example, the insects with eight legs and a two-segment body are known as Arachnidae or spiders. A scorpion is named as Scorpionida arachnida and a general spider is Araneae arachnida. A fruit with a big pit is called Prunus. An apricot is Prunus armeniaca and, similarly, a peach is Prunus persica. Likewise, a beautiful plant blooming in water was formerly known as Aphanizomenon flos-aquae, or "invisible flower of water."

Aphanizomenon is a genus of water plants, which is characterized on the bases of their filamentous colony-forming organization with heterocysts that is able to fix atmospheric nitrogen. Aphanizomenon carried the dissimilar green color of chlorophyll and to the first observers it looked same as a plant like algae.

It was commonly called blue - green algae because it had the distinctive blue pigment phycocyanin. More technically, it was also called cyanophyta, which means "blue plant." But after the invention of the microscope, AFA was observed through it and scientists observed that it did not comprise a nucleus. This was the same characteristic shared by all bacteria. However, it did contain phycocyanin, it was later known as cyanobacteria.

At that time AFA was defined both as a plant (as it contains chlorophyll content) and a bacterium (as it has cytoplasmic DNA) on the basis of these observations. However, later on, another development made the story yet a little more complex with the advancement in biochemistry.

It was observed by the scientists that AFA contained a molecule in its membrane that was same as glycogen, a polysaccharide made by animal cells. On the basis of this characteristic, one may classify AFA partially as an animal. So AFA is a bacterium or a plant -- and to a certain level an animal -- and the name one chooses to give it mainly relies on the point of view from which one wants to look at it.

It is just a part of the nature of human beings that we classify and name things because this way we can easily talk about those things. It is a very vital part of relating among ourselves. But there is a requirement to classify things because when we classify objects, we then relate more to the classification than to the objects themselves.

Sometimes people have shown deep concern because AFA is a bacterium and we are well aware of the fact that there are a number of bacteria that can bring very serious diseases with them. If we think of AFA as a pathogen because it is a bacterium would be the same thing as we think about a house cat as dangerous because both fearsome tigers and domestic cats are felines. In this context, a penguin should fly because it belongs to the family of birds.

As you have observed, too much generalization within any taxonomy can become a cause of serious misunderstandings!

There are a large number of bacteria, which are very beneficial for human health. Instead, there are many which are essential to health. Likewise, AFA is an advantageous microorganism no matter if AFA is a plant or a bacterium. It is merely a matter of classification. As a result, we are all correct at our places when we name AFA a cyanobacterium (blue bacterium) OR a cyanophyta (blue plant) OR cyanophycea (blue seaweed).