Scientists believe that these organisms were the first inhabitants of the Earth: archaea and bacteria. Despite their antiquity, they were discovered recently, with the invention of optical magnification of objects using a microscope (A. Levenguk). A unicellular microorganism consists, respectively, of a single cell, in contrast to a multicellular one. Such a simple structure has many bacteria, some fungi and algae. Thus, this category is unsystematic, including representatives of several kingdoms of wildlife.
Prokaryotes and Eukaryotes
The former do not have a clearly formed nucleus in the cell and other organs, for example, the nuclear membrane and membrane organoids. They appeared on the planet in ancient times, as they say, more than three billion years. The second - have a pronounced nucleus and protoplasm in the cell. According to hypotheses, they have already existed two billion years ago. Moreover, all organisms except archaea and bacteria are nuclear (eukaryotes).
Unicellular Habitats
The most numerous representatives of unicellular are bacteria and archaea. The question of where they live can be answered simply: everywhere! The main thing for these microorganisms is that the environment (or - the substrate) is suitable for life. And the main thing is the presence of sufficient moisture, the presence of water. By the way, if the conditions are unfavorable - it doesn’t matter: many of the bacteria can form spores, which are a special form of existence, "preserved". In what environments do unicellular bacteria live? In the atmosphere, in soil, in water, in other organisms. Let's consider in more detail.
In the soil
If we talk about the environments in which unicellular live, then in the soil they contain the largest number. In humus, chernozem and other soils, which are a favorable substrate, there is everything for optimal life: nutrients, a certain amount of liquid. There is also no direct sunlight, but a cozy temperature regime that allows single-celled animals to develop and multiply well. Most of these bacteria perform the functions of transforming dead organisms, decomposing them into its constituent parts - trace elements and compounds. Saprophages (the so-called bacteria that feed on the remains) are involved in the formation of fertile humus. However, among beneficial bacteria there are also pathogens. For example, causative agents of tetanus and botulism, which can enter the human body with various skin lesions.
In water
Speaking about the environments in which unicellular bacteria live, one cannot fail to mention the water open spaces that cover most of the planet Earth. Here they are washed away from the soil mainly. So, in water, especially stagnant, like in soil, there can be millions of bacteria (in a few grams of liquid). The community is not without malicious single-celled. Among them are the causative agents of typhoid, dysentery, cholera and other diseases that are severe for humans.
In the air
There are not so many unicellular representatives, but there are plenty of them. It is known that many infectious diseases are transmitted mainly by airborne droplets. And these unicellular ones enter the air with dust particles and micro droplets of water raised by air currents. Scientists have discovered that bacteria can "fly" in a similar way to the very borders of the Earth’s atmosphere. Not bad, some of the unicellular ones also feel in a vacuum, forming canned life forms - spores.
On the skin and within us
In what environments do single-celled microorganisms live? The mass of bacteria is on the skin of a person, in some of its areas: armpits, perineum, mucous membranes. About three kilograms of live weight of bacteria “live” inside the average citizen. A huge army of organisms (in the quantitative sense - billions), which can not be ignored. Basically, these are friendly neighbors that form the microflora of a healthy person. But pathogens are also encountered, which under certain conditions can cause great harm.