An autotroph is an organism capable of synthesizing energy-bound organic molecules, such as sugars, using inorganic molecules and an environmental energy source. This can be directly compared to a heterotroph, which is incapable of synthesizing these molecules and must consume other organisms. An autotroph can use different energy sources, such as the sun or inorganic oxidation, to store the energy it will need for cellular reactions.
An autotroph can range widely in size and distribution. The smallest autotroph is like a cyanobacteria or other unicellular autotroph. The largest autotroph is a tree. In terms of height and girth, this title likely goes the Giant Sequoia Trees, though aspens and other species with multiple stems have been shown to have a greater mass.
These autotroph organisms are much larger than the largest heterotroph and get all of their energy from the sun. An autotroph is the opposite of a heterotroph, which must consume organic materials made by other organisms as food. An autotroph can either use photosynthesis or chemosynthesis to produce food. Each of these types of autotroph is discussed below.
Types of Autotroph
An autotroph using photosynthesis to survive is using the sun’s energy to bind carbon dioxide molecules into larger sugar molecules. This provides a source of food for the organism, which then survives by using the energy bound in the sugar molecules to drive other cellular reactions. This process also produces oxygen as a byproduct, which is convenient for us heterotrophic animals which rely on it.
There are several different kinds of photosynthetic autotroph and together they produce a large amount of the biological energy used by the rest of the organisms. The kingdom Plantae contains mainly autotrophic species. These organisms include all terrestrial plants, making up the basis of ecosystems from rainforests to grass plains. Smaller organisms can also be an autotroph, such as algae and cyanobacteria. These organisms are typically single-celled, but they photosynthesize all of the food they need. Other organisms feed on them by filtering them out of the water column. Thus, these autotrophs also form the basis of the marine ecosystem.
Life is not bound to sunlight, and the process of chemosynthesis allows an autotroph to obtain energy from sources other than the sun. There are many types of chemoautotroph, ranging from sulfur-using bacteria on hydrothermal vents deep in the ocean to organisms surviving in the extreme salinity, to organisms buried deep within the earth.
All of these organisms use different methods for obtaining energy, but the basic process is the same. Instead of getting energy from the sun’s rays and using it to create the molecules the cell needs, an autotroph using chemosynthesis will obtain the energy from a naturally occurring chemical reaction. Many natural reactions release energy, and the autotroph often has enzymes to help the process along. Just like the photoautotrophs, these organisms then store the energy in biomolecules. Like every other autotroph, these organisms start a food chain and can sustain entire communities.
Autotrophs in Ecology
An autotroph typically forms the base of any food web. This is because an autotroph is the only organism capable of producing and storing energy. Other organisms can only collect this energy by consuming an autotroph. Below is a typical food web. The autotrophic organisms (called producers in ecology), can be seen as the basis for all other life.
At the producer level, each autotroph takes in energy from the sun. These plants and small organisms (both terrestrial and aquatic) produce excess energy in the form of sugar, fats, and proteins. The tissues of an autotroph are consumed by organisms at the next level. These organisms are herbivores and feed only on the autotroph producers. Larger carnivores feed on the herbivores, and some omnivores feed on both. As you can see here, and in the types of autotroph above, an autotroph is always the basis of more complex ecosystems.
Kelp Forests and Sea Urchins
Kelp is an autotroph and a very advanced form of algae. These underwater plants can tower several meters off the seafloor. Gathering the energy of sunlight into edible materials, a kelp forest can be an incredibly productive ecosystem. The kelp not only provide food but shelter and even nesting grounds for many species.
The above picture shows a diver swimming in a kelp forest. The kelp in this picture is healthy and flourishing. This healthy ecosystem, based on the autotroph, can sustain a variety of life. However, human influences and natural diseases can easily wipe out these productive environments in a very short amount of time. For this to happen, a well-documented series of cascading events must take place.
Above is a sea urchin, one of the only real threats to the kelp. Sea urchins crawl along the sea floor eating algae which grows on the rocks and coral. To them, kelp is a delicious and nutritious prize and they can eat large amounts of it. While other fish eat the leaves of the kelp, these can be regrown and the autotroph will survive and continue to produce. Sea urchins destroy the one thing the autotroph needs to survive, the holdfast.
The holdfast is a small part on the base of the kelp which anchors it to the seafloor. Without the holdfast, the large kelp would wash ashore in the waves, quickly dry out, and die. Luckily for the kelp, the sea urchin has a few natural predators. Both sea otters and starfish prey on kelp, and in a healthy kelp forest they keep sea urchin numbers at bay. But, when these species falter, there can be disastrous consequences for the autotroph, as well as the ecosystem which is built around it.
Off the west coast of the United States, this battle is currently being won by the sea urchins. The fur-trade largely targeted sea otters, which protected the kelp. The starfish numbers increased, for a while. This helped displace the lost otters. However, when a deadly starfish virus struck the waters of the west coast, devastation came. Without the starfish or the otters, the sea urchins won.
Vast areas of the coastline are now called urchin barrens. Here, the only autotroph is the lonely single-celled algae, drifting in the water column. The large kelp forests have been mowed down by a thick army of sea urchins. With no more natural predators, a ravenous diet, and a high tolerance for starvation, the sea urchins practically insure that a large autotroph like the kelp will never become established.
The giant coastal redwood trees of California and Oregon are also an interesting autotroph. This autotroph not only provides food, shelter, and oxygen for a number of residents in the surrounding ecosystem, but it also plays a role in the atmosphere of the ecosystem.
Redwood trees are so large, and suck up so much water from the ground that they increase the humidity significantly in the canopy. Though hundreds of feet off the forest floor, the humidity increase also increases the likelihood that the atmosphere will become saturated with water and rain will fall. While a single autotroph adds very little, a vast forest of giants can produce striking results.
Because of the increased rainfall caused by the autotroph, the rest of the plants below thrive. This increases the diversity of autotrophic organisms, which in turn increased the amount of heterotrophs. This cycle of positive reinforcement can lead to healthy and productive ecosystems, which benefit humans greatly. Likewise, cutting a giant autotroph down leads to desertification. The humidity increase is lost, along with the increased rainfall. This supports fewer autotrophs, and in turn leads to a drier, less productive ecosystem.