Microbes, otherwise known as microorganisms, are a broad grouping of single or multi-celled organisms which are too small to see with the naked eye. There are approximately 1.7 million known microorganisms and it is estimated that there are millions more that people have yet to discover on the planet. As a result of so many different kinds of these life forms, scientists have been trying to find ways to classify microorganisms into distinct groups. The current method of microbe classification is through genetic sequencing and this allows for microorganisms to be divided into three groups.
While the group eukaryotes includes many visible organisms and even includes humans, it also contains many microbes which are not visible to the naked eye. The eukaryotic microbes may be unicellular (remains a single cell organism for the rest of its lifecycle) or multicellular and include several different groupings. What sets this group apart from the others is the presence of organelles inside the cells such as the cell nucleus and mitochondria. Theses features are not present in the remaining groups.
The eukaryotes can be divided into the following subcategories: animal, plant, fungus, and protists. While many animals are visible to the naked eye, there are eukaryotic animals which are too small to be seen and still multicellular. Microscopic animals may include arthropods like dust mites and crustaceans like nematodes. These microanimals may reproduce sexually or asexually and can survive harsh environments.
The plants subcategory is primarily made up of green algae species. The green algae use photosynthesis and can grow in both fresh and salt water as well as on trees, rocks, and in the soil. Basically any place that is moist enough will allow for the growth of green algae. While green algae are usually helpful, should they die off in significant numbers, their decay will remove dissolved oxygen from the water, inevitably affecting other microbes and even the macroorganisms in the water.
Most people know what fungus is and while it was initially grouped with plants, scientists found that fungi behave similar to animals at a cellular level but still have much in common with planet so were deserving of a separate category. These microorganisms can grow on just about any surface in low moisture. Fungi come in so many different shapes and even sizes. They can include multicellular structures like mushrooms and molds in addition to a single cellular microorganism like yeast. The size can vary to just a microscopic grouping to large clusters and even huge chains of the same type of fungus that can stretch for miles under the surface. Though a stationary organism, fungi typically spread through spores which are carried away or through branching out into a chain structure.
The protist subcategory is probably the least understood of the all eukaryotes. They include a group of diverse eukaryotes that would not fit into another category. Most are unicellular but some are multicellular as well. They inhabit just about any environment that contains water and include types of algae and protozoa.
The unicellular category of bacteria is probably the most complex group among all the microorganisms. There are thousands of different bacteria that either have a stick/rod, spherical, or helical shape to them. Bacteria are present in soil, air, water, and in other larger organisms. They can survive extreme conditions and may exist as an individual, a pair, a chain, or a group of similar organisms. These organisms reproduce by binary fission and have several different methods of moving around or may stay in one place. They have very important roles on the planet as they are responsible for the breakdown of other organisms as well as many other functions, some of which occur inside the human body. Similar to plants, bacteria have cell walls and may or may not use photosynthesis as well as produce oxygen as plants do.
The group archaea used to be included with bacteria until sequencing was used in the 1970s and genetic differences were discovered. One example of the differences between archaea and bacteria has to do with the cell membrane and wall. While they look similar they are quite chemically different. While originally thought to only inhabit extreme environments, archaea have been found in many other environments as well. Genetic sequencing is usually the only method to tell most archaea apart, or tell them apart from bacteria, as they tend to look very similar. The archaea were also once thought to be relatively rare as they were only discovered as a group in the 1970s but since then it has been shown that they exist in many environments on the planet and may be globally distributed.