Earth Science: Single-celled Organisms – Autotrophic single-celled Organisms
In contrast to heterotrophic single-celled organisms, autotrophic single- celled organisms use the process of photosynthesis to produce their own organic molecules of nutrition from inorganic molecules.
Diatoms are a typical example of this group. They account for a major part of the world’s plankton and are therefore an important part of the marine food chain. Their cell wall consists of two halves (shells), often with complex patterns. Another example of an autotrophic single-celled organism is the genus Chlorella, a single-celled spherical-shaped genus that lives in fresh water.
They divide at very high rates and can create large amounts of biomass within short periods of time. Occasionally, Chlorella species are even cultivated for use in food production or the cosmetics industry. Some autotrophic single-celled organisms can switch to a heterotrophic lifestyle when light is scarce, for example, in the case of Euglena gracilis. This flagellated species is abundant in freshwater and is normally photosynthetically active.
But when these organisms are living in low light, they are able to adopt a heterotrophic lifestyle by eliminating chlorophyll from their bodies. As a result, their appearance changes from green to transparent. As soon as light becomes available again, even if several years have passed, the chloroplasts regain functionality and photosynthesis can resume.
Parasitic organisms
Certain single-celled organisms have adopted a parasitic lifestyle. Some species even carry diseases which may infect humans, for example Entameba histolytica, which causes tropical amoebic dysentery. When ingested through infested drinking water they can cause tissue damage with symptoms such as ulcers or other health issues that are potentially fatal if left untreated.
Pathogens causing malaria (Plasmodium spp.) are even more dangerous. This disease is responsible for the death of one to two million people annually. Sleeping sickness is another dreaded disease common in tropical regions where an estimated 500,000 people are affected. The disease is caused by species of the genus Trypanosoma and carried by tsetse flies (Glossina spp.).
THE FORMATION OF COLONIES
Some autotrophic single-celled organisms form colonies made up of independent cells connected by a gelatinous substance. In Pandonna and Eudonna species, 16 or 32 individual cells form a hollow sphere with their flagella to the outside.
The cells are connected by channels, causing the flagella to whip at the same time. In Volvox species, several thousand may com- bine, a small fraction of which reproduce while the remaining cells take over other functions such as photosynthesis and locomotion.
BASICS
IMMORTAL SINGLE- CELLED organisms reproduce by dividing into two equivalent daughter cells. Therefore these organisms are considered potentially immortal.
Cells that divide in this way will never die due to old age.