We reviewed the diversity of microscopic life forms.
In this article we will review the main types of microorganisms known today , analyzing their characteristics and functions within their ecosystem.
- We recommend: “Microbiota: what is it and what is it for?”
microscopic life
Since the advent of microscopy, science has maintained an interest in microscopic worlds, those that cannot be seen without the use of special viewing tools . The influence of this world and its inhabitants, invisible to the naked eye, has a great impact on nature, of which they are an integral part.
Depending on the type of microbe we are studying, we will see that its characteristics and functions differ to a greater or lesser degree from other types of organisms. We can find microscopic life in the most adverse environments, but also in practically all environments on the planet , from the seas to the interior of living organisms.
The study scale of these organisms ranges from nanometers, such as Mycoplasma genitalium -a typical pathogen of the primate bladder, about 200 nm in length, the smallest living organism known-, to “giant” bacteria such as Thiomargarita namibiensis , which They become visible to the naked eye due to their size close to a millimeter in length.
The 7 main varieties of microorganisms
The division of microorganisms is an increasingly complex issue. Some of them, due to their ability to share genetic material, complicate their phylogenetic study – that is, our ability to classify them according to their kinship. Other microorganisms have very similar morphologies and functions, despite having very little in common .
Although within each of the 7 main categories of microorganisms we find enormous diversity, each of these groups has certain morphological and genetic characteristics that distinguish them more or less precisely.
Among these groups of microbes we can find:
1. Bacteria
Bacteria are found in practically every corner of the planet earth, including the interior of living beings or even in the depths of the earth’s crust, more than 3 km deep -the so-called endoliths, which feed on surrounding minerals-.
This group of organisms differs from other forms of microscopic life mainly because of its genetic material, which is not organized in a nucleus , in addition to the presence of peptidoglycan in its cell wall, a large polymer formed from amino acids and sugars, which form an ordered crystalline structure that adds toughness to the cell wall.
Peptidoglycan is so characteristic of bacteria that it represents one of the largest bacterial antigens , that is, one of the substances of bacteria that initiate immune responses in the living beings they inhabit. Peptidoglycan disruption and metabolism are an excellent target for antibiotics.
Bacteria have very different functions depending on their characteristics . Some of them are parasites that can cause diseases -such as Salmonella enterica- , others collaborate with other organisms in a symbiotic way, such as nitrogen-fixing bacteria found in the roots of some plants.
One of the most influential types of bacteria are the cyanobacteria , also called blue algae, capable of photosynthesis and oxygen production. The appearance of cyanobacteria and the oxygen they generate was responsible for one of the largest mass extinctions that life on Earth went through. Oxygen was toxic to most organisms at the time, which gave way to the “Oxygen Catastrophe” that occurred more than 2 billion years ago.
2. Archaea
Until recently, archaea were considered another type of bacteria, but genetic analyzes have shown that these prokaryotic microorganisms are almost as different from bacteria as they are from eukaryotic organisms , which do have a genetic nucleus.
Archaea have a similar morphology to bacteria, but their metabolism is very different from theirs. This allows them to survive in environments where life is practically unthinkable, such as hot springs or salt lakes.
Although there is significant interest in the extremophile archaea – capable of surviving in these extreme environments – they can be found, like bacteria, in practically any corner of the world, including living beings.
3. Microscopic algae
Microscopic algae, like protozoa, are considered protists, eukaryotic living beings -which have a nucleus with genetic material-, different from plants, animals or fungi. Microscopic algae can be multicellular or unicellular, but all of them are photosynthetic.
Many organisms depend on the action of microscopic algae for their subsistence. Phytoplankton, the group of microscopic algae found in the water, represents a food source for many species, in addition to producing about 40% of all atmospheric oxygen .
Most of this oxygen is produced by diatoms , unicellular algae covered with a shell of silica, as if they were transparent glass domes within which the cell is protected.
4. Mushrooms
Most fungi are multicellular organisms, all of them have a nucleus. Its cell wall is made of chitin , the same component used by arthropods -such as insects, arachnids and crustaceans- to build their shells.
Their role in the ecosystem is mainly to decompose organic matter, but they also perform symbiotic functions -for example with algae, to form lichens- or parasitic, as is the case with some pathogenic fungi such as Candida albicans , responsible for candidiasis .
5. Protozoa
Protozoa, like single-celled algae, are considered protists , eukaryotes that don’t fit in with plants, animals, or fungi. They represent the base of the food chain in many environments, which depend on their presence to survive.
Protozoa have an enormous diversity of forms and species, which are usually divided classically according to their mechanisms of movement . Some propel themselves with multiple small appendages called cilia, others with large whip-like appendages called flagella, while others simply spread out and deform to spread in the desired direction.
Most protozoa are harmless, but some of them are parasites that can cause serious illness, such as Giarda lamblia , a protozoan that infects the intestines of mammals, such as humans.
6. Virus
Viruses are so rare that they are practically not considered organisms . They are at the limit of life, being unable to reproduce without parasitizing other cells, whether they are prokaryotes, as in the case of bacteriophages, or eukaryotes, such as any of the viruses that infect human beings.
Viruses are acellular, meaning they are not cells . They simply consist of a protein shell that protects and transports genetic material, either in the form of DNA or RNA. This genetic material remains inactive until it comes into contact with a cell, within which it will begin its cycle of infection and reproduction.
- It may interest you: “The 15 types of viruses: how they work and associated diseases”
7. Multicellular parasites (Helminths)
Helminths are parasites similar to worms , which live inside the organisms they parasitize, feeding on them and receiving protection from the environment. All of them are multicellular eukaryotic invertebrates, with long and usually flat bodies. They represent a serious health problem, especially in developing areas of the world, since their transmission often depends on contaminated water.
They are included in the microscopic world because some of their vital phases, such as the egg phase or their larval phases, occur in this microscopic environment. Despite this, in their adult phase they can usually be seen with the naked eye .
A terrifying example of a helminth is Dracunculus medinensis , also known as the guinea worm. This parasite inhabits the intestine of those affected, but little by little it burrows until it reaches the feet or leg, where it produces ulcers through which it expels its eggs.
Its removal requires a slow process, in which the part of the parasite that protrudes from the wound is rolled onto a support. This can last for days or weeks, as a rupture of the parasite can cause severe anaphylactic shock in the affected individual.
Bibliographic references
- Woese, C., Kandler, O., & Wheelis, M. (1990). Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings Of The National Academy Of Sciences, 87(12), 4576-4579. doi:10.1073/pnas.87.12.4576.
- Hamilton, T., Bryant, D., & Macalady, J. (2015). The role of biology in planetary evolution: cyanobacterial primary production in low-oxygen Proterozoic oceans. Environmental Microbiology, 18(2), 325-340. doi:10.1111/1462-2920.13118.
- Lyons, T., Reinhard, C., & Planavsky, N. (2014). The rise of oxygen in Earth’s early ocean and atmosphere. Nature, 506(7488), 307-315. doi:10.1038/nature13068.
- Cairncross, S., Tayeh, A., & Korkor, A. (2012). Why is dracunculiasis eradication taking so long?. Trends In Parasitology, 28(6), 225-230. doi:10.1016/j.pt.2012.03.003
To the classic question “what do you do?” I always answer “basically I am a psychologist”. In fact, my academic training has revolved around the psychology of development, education and community, a field of study influenced my volunteer activities, as well as my first work experiences in personal services.