The 4 types of protozoa (and their characteristics)

The development of the seven kingdoms of living beings has been one of the greatest achievements in the history of Biology. And it is that being able to encompass any of the 1.2 million species that we have identified in seven well-defined kingdoms is something that has greatly facilitated life in the field of biological sciences.

In any case, since the first conception of kingdoms devised by the Swedish naturalist and botanist Carlos Linnaeus in 1735, this form of classification has varied greatly, the latest revision and the most accepted being the one that dates from 2015. Seven are differentiated therein kingdoms (instead of the five that are described in the 1998 system), of which there are some that we all know well such as animals, plants, fungi or bacteria.

But there are three that, surely, for the general public are more unknown: the chromists, the archaea and the protozoa. And in today's article we will focus on this last kingdom. Protozoa. Unicellular eukaryotic organisms that, since the 1998 classification, make up their own kingdom. A kingdom with some 50,000 species that, incorrectly (they have nothing to do with the animal kingdom) but to understand it, are considered unicellular animals, since they feed by engulfing other organisms.

So, below and, as always, from the hand of the most prestigious scientific publications, we are going to analyze all the biology behind these organisms, seeing what their main characteristics are and, of course, seeing what kinds of protozoa exist and what are its peculiarities. Let us begin.

What are protozoa?

Protozoa are a kingdom of single-celled eukaryotic living things that are generally heterotrophic and feed on other organisms through phagocytosis., that is, through an absorption process. Protozoa "eat" other living beings and it is a group of organisms with some 50,000 species that make up their own kingdom within the taxonomy since the 1998 classification by Thomas Cavalier-Smith.

They are eukaryotic beings that, like animals, plants, fungi and chromists, have a delimited nucleus where DNA and cellular organs present in the cytoplasm are stored. In addition, it is important to emphasize that they are unicellular. Protozoa are made up of a single cell. There are never multicellular organisms in this kingdom. One cell, one individual.

The vast majority of protozoan species feed on organic matter (hence they are generally heterotrophs) and, furthermore, they do so through a process of phagocytosis, which is the mechanism that allows these cells to absorb and ingest other living beings through their membrane and thus carry out an internal digestion in its cytoplasm.

They are not plants because they do not photosynthesize (except for the group Euglena, which photosynthesizes in freshwater habitats). They are not animals because they are unicellular, and they are not fungi because they perform internal digestion. Therefore, it was clear that they had to form their own kingdom. And although in 1969 a classification was made that included them together with the chromists in the same kingdom (the protists), in 1998 this group separated and formed their own kingdom.

Most protozoa are aerobic, that is, they need oxygen to carry out all the metabolic reactions to obtain energy. In addition, they have the characteristic that its membrane does not have any rigid covering, as this would prevent phagocytosis from taking place. In fact, it was precisely this lack of cellular coverage that made them separate from the chromists, who do have this armor.

It should also be noted that, although most protozoa are free-living, there are pathogenic species that behave like parasites of humans, such as the famous brain-eating amoeba (Naegleria fowleri), the parasite responsible for malaria (Plasmodium), Giardia, Leishmania, Trypanosoma cruzi, etc. But be that as it may, both these parasites and the free-living ones have the characteristic of always living individually, without forming colonies.

We are facing organisms that, appearing between 2,500 and 3,000 million years ago in the context of the Great Oxidation, were the first eukaryotic organisms on Earth. This explains that, being beings with such a primitive origin, most protozoa reproduce asexually. The cell replicates its genetic material and divides in two (or by budding), thus giving rise to two clones. Sexual reproduction is rare in this kingdom. Most are asexual.

Due to their metabolism based on the intracellular digestion of organic matter, protozoa are considered "unicellular animals". This serves to understand it but it is incorrect, since they are totally different kingdoms. What is certain is that protozoa are able to move actively due to their mobility structures. And it is precisely in function of how they move and, above all, of what morphological structures they have for this that the classification that we will see is carried out.

To date, a total of about 50,000 species of protozoa have been identified (this is more than we have identified of fungi, whose number is 43,000; and of bacteria, which is 10,000), although it is believed that the diversity real could be much bigger. Of this, all require moisture to survive. And it is that coming from a time on Earth in which life was still linked to the oceans, all protozoa live in water or, at least, in soils with high humidity.

Their size varies enormously. But it is important to emphasize that, as they are unicellular organisms, they are microscopic in size and no species can be seen with the naked eye, although they are larger than bacteria. Now, beyond this, the diversity of shapes and sizes is immense. Most measure between 10 and 50 micrometers, but there are amoebas that can measure up to 130 micrometers. We are facing a very diverse group. And now, looking at the classification, it will be much clearer.

How are protozoa classified?

After this extensive (but necessary) introduction where we have summarized everything important about this relatively unknown kingdom of living beings, we are more than ready to dive deeply into the topic that has brought us together here today: the classification of protozoa. So, without further ado, let's see what kinds of protozoa exist and what their characteristics are.

1. Rhizopod protozoa

Rhizopod protozoa are those that base their mobility on pseudopods, protuberances of its cytoplasm and plasma membrane that project towards where the organism "wants" to advance. These same deformable appendages serve to capture food and introduce it, by phagocytosis, into the cytoplasm. They live in water or in soils with high humidity.

They are the simplest protozoa at a morphological level and there are about 200 described species. Be named, rhizopoda, means "root-shaped feet" and almost all species are free-living, divided into the groups of amoebids (the famous amoebas), foraminifera, radiolaria and heliozoa.

2. Flagellated protozoa

Flagellated protozoa are those that base their mobility on flagella, cellular organelles consisting of long, mobile, whip-shaped appendages that allow the protozoan to actively move. They may have one or more flagella, but the characteristic of this group is that they have these "tails" thanks to which they are propelled in the middle, which is always fresh or salt water.

Also known as mastigophorous protozoa, these protozoa present one or more flagella in all phases of their life cycle (because the rhizopods that we have seen can have but only in certain stages). Many species are free-living, but many others are parasites of animals, including humans.Trypanosoma cruzi, the parasite responsible for Chagas disease, is a clear example.

3. Ciliated protozoa

Ciliated protozoa are those that base their mobility on cilia, some organelles destined to displacement but much shorter than flagella. In addition, these protozoa have many of these extensions for most of their length, so it is not so much an idea of ​​"tail" or "Tails", but a kind of "hair" that covers its membrane. In addition, these cilia do not move as actively as flagella, but rather remove the environment in which the cell is located in order to move and, in turn, obtain nutrients.

Also known as ciliaphores, they are a group of protozoa with about 3,500 described species. They are found in practically all aquatic ecosystems (lakes, rivers, oceans, ponds ...) and also in humid soils. They tend to be relatively large organisms (compared to other protozoa) and tend to feed on bacteria, algae, or other protozoa.

4. Protozoa sporozoa

The sporozoa protozoa are those that do not have much mobility, a characteristic that has led them not to be able to develop as free life forms, so it is a group that stands out for being parasitic. These protozoa tend to act as internal parasites, remaining immobile within the organism they have infected.

The name "sporozoa" is due to the fact that they have a multiple division phase of asexual reproduction known as sporulation, a mechanism that consists of producing spores or endospores, resistant structures that end up giving rise to a clone. But be that as it may, we are dealing with protozoa that are obligate parasites of animals and fungi, which can cause diseases.