This chapter will look at different types of microorganisms. It will discuss their cell structure and functions. It will also discuss the position of microbes in food chains and their role in the biosphere.
Archaea were once thought to be a type of bacteria. After extensive research of their DNA and membrane structure, some scientists decided to put them into a separate group - Archaea. Another group of scientists is still not convinced and refers to them as Archaeabacteria.
Archaea are unicellular prokaryotes which make them bacteria-like organisms. Their DNA structure resembles the DNA of eukaryotic cells. Also, the cell walls of Archaebacteria are structurally different from the bacterial ones. Archaea live in environments that would not be suitable for most life forms. They can be found around hot geysers, volcanic vents, very salty lakes and on the ocean floor. Because Archaea thrive in such extreme conditions, they are sometimes called 'organism - extremists' or 'extremophiles. Archaeans can survive these extreme conditions due to their physiology. They synthesise different enzymes that keep their cells from being destroyed by high temperatures, salty or acidic water.
Because of their 'fondness' for extreme environmental conditions, Archaeans are believed to be the first living forms that appeared on Earth when it was still waterless, airless and a scorching hot planet.
Based on their habitat, all Archaeans can be divided into the following groups:
- Methanogens - methane-producing organisms;
- Halophiles - Archaeans that live in salty environments;
- Thermophiles - Archaeans that live at extremely hot temperatures;
- Psychrophiles - cold-temperature Archaeans.
Archaeans use different energy sources like hydrogen gas, carbon dioxide and sulphur. Some of them use sunlight to make energy, but not the same way as plants do. These creatures absorb sunlight using their membrane pigment, bacteriorhodopsin. This reacts with light, leading to the formation of the energy molecule, of adenosine triphosphate (ATP).
Compared with bacteria, Archaebacterial cell walls are composed of different polysaccharides and proteins, with no peptidoglycan. See image 1.
Cyanobacteria are aquatic, photosynthetic organisms. They can be unicellular or colonial. Cyanobacteria are sometimes called blue-green algae. Like other plants, they make their energy through photosynthesis, but besides that, they do not have anything in common with plants. Millions of years ago, the atmospheric oxygen that we depend on was generated by cyanobacteria. Most cyanobacteria live in water, damp soil and rocks.
Green land plants originated from cyanobacteia. It is believed that millions of years ago a plant's organelle, chloroplast, used to be free-living cyanobacteria. Some time in the late Proterozoic period cyanobacteria began to take up residence within some eukaryotic cells. These cyanobacteria 'payed their rent' by generating an energy source for the host cell. This process is called endosymbiosis. The endosymbiotic theory is supported by various structural and genetic similarities between chloroplasts and cyanobacteria. See image 2.
See animation 1.
All bacteria are unicellular prokaryotes, meaning they do not have a defined cellular nucleus. Their genetic information is in their nucleoid, - single, circular tightly- packed DNA molecule. According to their shape, all bacteria are divided into three groups:
- spirilla (with a spiral body shape);
- cocci (with a spherical body shape);
- bacillus ( with a rod (stick) shaped body).
Some types of bacteria live on their own and others form colonies. Some bacteria are quite mobile and others 'stay put' for their whole life. Bacteria move using their cytoplasmic tail - flagella, or by secreting slimy substances that allow them to slide along surfaces.
The cell walls of most bacteria contain a polysaccharide called peptidoglycan. Differences in their cell wall structure is a major feature used in classifying these organisms. The staining abilities of bacteria are also based on their cell wall structure. According to the way they stain, bacteria can be classified as either Gram - positive or Gram - negative.
Based on their response to gaseous oxygen, all bacteria can be divided into the following groups:
Aerobic - living in the presence of oxygen;
Anaerobic - living without oxygen;
Facultative anaerobes - can live in both environments.
According to the way they obtain energy, bacteria are classified as heterotrophs or autotrophs. Autotrophs make their own food by using the energy of sunlight or chemical reactions, in which case they are called chemoautotrophs. Heterotrophs obtain their energy by consuming other organisms. Bacteria that use decaying life forms as a source of energy are called saprophytes. See image 3.
Protozoa is a subkingdom of unicellular, mostly aerobic, eukaryotic organisms. Sometimes they are also called protists. They are neither plants nor animals. They make up the largest group of organisms in the world in terms of numbers and biomass. Some protozoans, like Euglena, have chloroplasts like plants and make their own food, which makes them autotrophs. Others, like amoeba, are heterotrophs. Protozoans can be free-living or parasitic, unicellular or colonial. Some parasitic protozoans can cause diseases in humans. Protozoans move around using their flagella or pseudopodia - cytoplasmic temporary 'feet'.
Because heterotrophic protozans consume bacteria, they play a very important role in controlling biomass. Biomass is the total weight of living organimsms in a given area. See image 4.
Fungi are saprophytic (feed on decaying organic matter) and parasitic organisms. Fungi include moulds, rusts, mildews, smuts, mushrooms and yeast. By breaking down dead organic material, they continue the cycle of nutrients through ecosystems. Some plants have a symbiotic relationship with fungi. Symbiosis is a mutually beneficial co-existence of dissimilar organisms. For example, there are mushrooms that live near tree roots and supply them with essential nutrients.
All fungi are made of eukaryotic cells. Fungican be single-celled or with cells arranged in filaments called hyphae. Yeasts are unicellular fungi. Masses of hyphae are called mycelia. Mycelia can be well structured, as in a mushroom, or tangled and unstructured, as in moulds. Some fungi can exist in the form of yeast and hyphae. These types of fungi are called dimorphic.
All fungi are heterotrophic, meaning that they obtain their energy and carbon compounds from organic nutrients. None of the fungi are photosynthetic. Some fungi are parasites and can cause diseases in humans, animals and plants. Some fungi are used in the food industry and pharmaceutics (antibiotic production). See image 5.
Although viruses are not considered living organisms, they are sometimes classified as microorganisms. Viruses are much smaller than common microbes. They are made of a DNA molecule covered with a protein shell called a capsid. Retroviruses are made of an RNA molecule covered with a capsid. Capsids can take many shapes. Viruses cannot reproduce outside the host cell, but they cannot be called parasites either. Scientists still argue today about whether viruses are true living forms because they are not cells and they cannot metabolise on their own.
Viruses can infest prokaryotic and eukaryotic cells, often causing diseases in organisms. A virus that infects bacteria is known as a bacteriophage. See image 6.