Autotrophs and Heterotrophs
Based how they get their energy, all organisms can be classified as either autotrophs or heterotrophs. Autotrophs produce their own energy and food. Photoautotrophs use sunlight and inorganic sources of carbon, carbon dioxide, while chemoautotrophs use energy released from chemical reactions and carbon dioxide to produce food and energy. They are the producers and the basis of all food-chains.
Heterotrophs cannot produce their own food, and get the organic compounds and energy they need by consuming other organisms. Herbivores feed directly on producers, carnivores feed on herbivores, omnivores feed on both producers and herbivores, and saprobes break down dead material. Photoheterotrophs use sunlight for energy and organic compounds to make their food.
Chemoheterotrophs are a small group of micro-organisms that use organic compounds but
get their energy from chemical reactions.
All organisms need energy to survive and function. While some organisms can produce their own energy sources, some are dependent on others (1).
Autotrophs are organisms which can produce energy and nutrients they need themselves. They produce the organic compounds they need by using inorganic elements (2, 3, 4). They are therefore also called producers (2, 3). They are the basis of all food-chains providing energy and nutrients for consumers (2, 5).
There are two types of autotrophs depending on the source of energy used. Carbon is the basis of all organic compounds, and autotrophs use carbon dioxide as their source (2, 4).
The most common are the photosynthetic autotrophs or photoautotrophs, who use energy from sunlight (2, 4). Carbon dioxide from the air is their source of carbon. In a process called photosynthesis, energy from the sun is used to combine carbon with water to give carbohydrates (4, 6). The first product is glucose, a simple sugar (4, 6). These sugars are later used to produce starch, cellulose, amino acids, and other organic compounds that the organisms need (4). The chemical reaction that occurs is,
CO2 + 6H2O → C6H12O6 + 6O2 (7 ).
Plants (moss, ferns, trees), algae (including phytoplankton) and certain kinds of bacteria called cyanobacteria are examples of photoautotrophs (2, 4, 6). All green plants are photoautotrophs. Even carnivorous pitcher plants are considered to be autotrophs as they get their energy and glucose from photosynthesis (1). The animals they consume provide necessary nutrients like nitrogen, potassium etc., which other plants normally get from the soil (1, 8).
Photoautotrophs are found on earth surface, in shallow water and even under ice where sunlight can reach them (7).
The chemosynthetic autotrophs, or chemotrophs use chemical energy as their energy source, through a process called chemosynthesis (2). Carbon dioxide is once again the source of carbon. The energy is released by chemical reactions, usually by oxidation of hydrogen sulphide or methane. Sometimes sulphur can also be reduced and this energy is used by the organisms (4, 7). The oxidation of hydrogen sulphide is as follows,
CO2+ 4H2S + O2 → CH20 + 4S + 3H2O (7).
Common examples of chemoautotrophs are bacteria that live in extreme environments where these compounds are found. On land surface they are found in places such as the hot springs in Yellowstone National Park (2, 7). Or in places where no sunlight reaches, as underground in volcano, hydrothermal vents and deep sea-beds in whale carcasses (1, 2, 7). Hydrogen sulphide is found in hydrothermal vents and deep seas (2). Deep seas are also the place where methane is present (2). In these dark places they are important producers and the basis of the food-chain (7).
Heterotrophs are organisms that cannot produce organic substances from inorganic compounds. So they consume sources that produce or have organic material (1, 9, 10).
All heterotrophs depend on other organisms for carbon source or organic material. Depending on the source of energy there are two types of heterotrophs (1, 11).
As their name indicates, they use light for energy, and their carbon source is ready organic compounds like carbohydrates, fatty acids and alcohol (1). Then similar to autotrophs these heterotrophs also manufacture their own food (2). Purple non sulphur bacteria, green non-sulphur bacteria and heliobacteria are examples of photoheterotrophs (1).
There are two types of chemoheterotrophs.
a) The heterotrophs who obtain both their energy and food by eating other organisms that already have organic compounds. These are therefore called consumers. All animals, some fungi and most of the pathogens belong to this group (1).
If the consumer feeds on plants or phytoplanktons the primary producers, they are called herbivores. Rabbits, deer, cow, sheep, goats, many insects and seed eating birds are herbivores. Small fish and crustaceans feeding on phytoplankton in seas are also herbivores (9, 10).
If the consumer eats herbivores, they are called carnivores, and they can be predators or scavengers. Tiger, lions, hawks or snakes, etc. which kill and feed on their prey are predators. Vultures feeding on dead animals are examples of scavengers (9, 10).
When consumers eat both plants and animals they are called omnivores (9, 10). Humans, and many birds and rodents are examples.
Some consumers absorb their nutrients after breaking down dead materials and are called saprobes. Some fungi and bacteria are saprophtic (9, 10).
b) Chemoheterotrophs, obtain their carbon from organic compounds, but get their energy by oxidation of organic compounds and reduced sulphur compounds (1, 12). Chemolithoheterotrophs are examples of this small little known group. Examples are some bacteria, like a few strains of Thiobacillius and Beggiatoa (12, 13).
In the foodchain, the first trophic level is made of autotrophs, the second trophic level has herbivores, and the third trophic level has omnivores, and carnivores (1, 2).
- Lengeler, Joseph W.; Drews, Gerhart; Schlegel, Hans Günter (1999). Biology of the Prokaryotes. Georg Thieme Verlag. p.238. ISBN 978-3-13-108411-8. (Table 10.3)
- Dworkin, Martin (2006). The Prokaryotes: Ecophysiology and biochemistry (3rd ed.). Springer. p. 989. ISBN 978-0-387-25492-0.