Encyclopedic Entry

Kelp, hard at work supporting an entire marine ecosystem.

Photograph by Mike Schwo, MyShot

Photoheterotrophs
Some varieties of bacteria use light to create their own food, just like organisms that use photosynthesis. However, these bacteria are not autotrophs, because they must rely on chemicals besides carbon dioxide for carbon. These strange bacteria are called photoheterotrophs.

Radiotrophs
Some fungi use gamma radiation and a natural pigment called melanin to create energy for growth. Gamma radiation is a high-frequency band of light that is invisible to people and can cause damage to human tissues when encountered in large doses. These strange, rare fungi are called radiotrophs. They are found inside and around the abandoned Chernobyl Nuclear Power Plant in Ukraine.

An autotroph is an organism that can produce its own food using light, water, carbon dioxide, or other chemicals. Because autotrophs produce their own food, they are sometimes called producers.

Plants are the most familiar type of autotroph, but there are many different kinds of autotrophic organisms. Algae, which live in water and whose larger forms are known as seaweed, is autotrophic. Phytoplankton, tiny organisms that live in the ocean, are autotrophs. Some types of bacteria are autotrophs.

Most autotrophs use a process called photosynthesis to make their food. In photosynthesis, autotrophs use energy from the sun to convert water from the soil and carbon dioxide from the air into a nutrient called glucose. Glucose is a type of sugar. The glucose gives plants energy. Plants also use glucose to make cellulose, a substance they use to grow and build cell walls.

All plants with green leaves, from the tiniest mosses to towering fir trees, synthesize, or create, their own food through photosynthesis. Algae, phytoplankton, and some bacteria also perform photosynthesis.

Some rare autotrophs produce food through a process called chemosynthesis, rather than through photosynthesis. Autotrophs that perform chemosynthesis do not use energy from the sun to produce food. Instead, they make food using energy from chemical reactions, often combining hydrogen sulfide or methane with oxygen.

Organisms that use chemosynthesis live in extreme environments, where the toxic chemicals needed for oxidation are found. For example, bacteria living in active volcanoes oxidize sulfur to produce their own food. At Yellowstone National Park in the U.S. states of Wyoming, Idaho, and Montana, bacteria capable of chemosynthesis have been found in hot springs.

Bacteria that live in the deep ocean, near hydrothermal vents, also produce food through chemosynthesis. A hydrothermal vent is a narrow crack in the seafloor. Seawater seeps down through the crack into hot, partly melted rock below. The boiling-hot water then circulates back up into the ocean, loaded with minerals from the hot rock. These minerals include hydrogen sulfide, which the bacteria use in chemosynthesis.

Autotrophic bacteria that produce food through chemosynthesis have also been found at places on the seafloor called cold seeps. At cold seeps, hydrogen sulfide and methane seep up from beneath the seafloor and mix with the ocean water and dissolved carbon dioxide. The autotrophic bacteria oxidize these chemicals to produce energy.


Autotrophs in the Food Chain

To explain a food chain—a description of which organisms eat which other organisms in the wild—scientists group organisms into trophic, or nutritional, levels. There are three trophic levels. Because autotrophs do not consume other organisms, they are the first trophic level.

Autotrophs are eaten by herbivores, organisms that consume plants. Herbivores are the second trophic level. Carnivores, creatures that eat meat, and omnivores, creatures that eat all types of organisms, are the third trophic level.

Herbivores, carnivores, and omnivores are all consumers—they consume nutrients rather than making their own. Herbivores are primary consumers. Carnivores and omnivores are secondary consumers.

All food chains start with some type of autotroph (producer). For example, autotrophs such as grasses grow in the Rocky Mountains. Mule deer are herbivores (primary consumers), which feed on the autotrophic grasses. Carnivores (secondary consumers) such as mountain lions hunt and consume the deer.

In hydrothermal vents, the food chain’s producer is autotrophic bacteria. Primary consumers such as snails and mussels consume the autotrophs. Carnivores such as octopus consume the snails and mussels.

An increase in the number of autotrophs will usually lead to an increase in the number of animals that eat them. However, a decrease in the number and variety of autotrophs in an area can devastate the entire food chain. If a wooded area burns in a forest fire or is cleared to build a shopping mall, herbivores such as rabbits can no longer find food. Some of the rabbits may move to a better habitat, and some may die. Without the rabbits, foxes and other meat-eaters that feed on them also lose their food source. They, too, must move to survive.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

algae

Plural Noun

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

autotroph

Noun

organism that can produce its own food and nutrients from chemicals in the atmosphere, usually through photosynthesis or chemosynthesis.

Encyclopedic Entry: autotroph

bacteria

Plural Noun

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

carbon dioxide

Noun

greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

carnivore

Noun

organism that eats meat.

Encyclopedic Entry: carnivore

cellulose

Noun

complex carbohydrate that forms the tough, rigid cell wall of most plants and is necessary for such products as paper and textiles.

cell wall

Noun

tough, rigid, and non-living barrier surrounding the soft cells of most autotrophs, such as plants.

chemosynthesis

Noun

process by which some microbes turn carbon dioxide and water into carbohydrates using energy obtained from inorganic chemical reactions.

circulate

Verb

to move around, often in a pattern.

cold seep

Noun

marine environment where hydrogen sulfide and methane seep up from beneath the seafloor and mix with the ocean water.

consume

Verb

to use up.

consumer

Noun

organism on the food chain that depends on autotrophs (producers) or other consumers for food, nutrition, and energy.

convert

Verb

to change from one thing to another.

decrease

Verb

to lower.

devastate

Verb

to destroy.

energy

Noun

capacity to do work.

fir

Noun

variety of pine tree.

food

Noun

material, usually of plant or animal origin, that living organisms use to obtain nutrients.

Encyclopedic Entry: food

food chain

Noun

group of organisms linked in order of the food they eat, from producers to consumers, and from prey, predators, scavengers, and decomposers.

Encyclopedic Entry: food chain

forest

Noun

ecosystem filled with trees and underbrush.

fox

Noun

type of mammal related to a dog with a thin muzzle and thick tail.

glucose

Noun

"simple sugar" chemical produced by many plants during photosynthesis.

grass

Noun

type of plant with narrow leaves.

habitat

Noun

environment where an organism lives throughout the year or for shorter periods of time.

Encyclopedic Entry: habitat

herbivore

Noun

organism that eats mainly plants.

Encyclopedic Entry: herbivore

hot spring

Noun

small flow of water flowing naturally from an underground water source heated by hot or molten rock.

hydrogen sulfide

Noun

chemical compound gas responsible for the foul odor of rotten eggs.

hydrothermal

Adjective

related to hot water, especially water heated by the Earth's internal temperature.

increase

Verb

to add or become larger.

methane

Noun

chemical compound that is the basic ingredient of natural gas.

mineral

Noun

inorganic material that has a characteristic chemical composition and specific crystal structure.

moss

Noun

tiny plant usually found in moist, shady areas.

mountain lion

Noun

large cat native to North and South America. Also called a cougar, puma, catamount, and panther.

mule deer

Noun

large deer (mammal) with long ears native to North America.

mussel

Noun

aquatic animal with two shells that can open and close for food or defense.

nutrient

Noun

substance an organism needs for energy, growth, and life.

Encyclopedic Entry: nutrient

octopus

Noun

marine animal (mollusk) with a soft body and eight arms.

omnivore

Noun

organism that eats a variety of organisms, including plants, animals, and fungi.

Encyclopedic Entry: omnivore

organism

Noun

living or once-living thing.

oxidation

Noun

chemical process of a substance combining with oxygen to change the substance's physical and molecular structure.

photosynthesis

Noun

process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

phytoplankton

Noun

microscopic organism that lives in the ocean and can produce its own food through photosynthesis.

plant

Noun

organism that produces its own food through photosynthesis and whose cells have walls.

primary consumer

Noun

organism that eats plants or other autotrophs.

producer

Noun

organism on the food chain that can produce its own energy and nutrients. Also called an autotroph.

rabbit

Noun

mammal with long ears that hops on strong hind legs.

Rocky Mountains

Noun

mountain range in the western United States and Canada.

seafloor

Noun

surface layer of the bottom of the ocean.

seaweed

Noun

marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria.

secondary consumer

Noun

organism that eats meat.

seep

Verb

to slowly flow through a border.

shrub

Noun

type of plant, smaller than a tree but having woody branches.

snail

Noun

marine or terrestrial animal (mollusk) with a shell and one foot on which it glides.

soil

Noun

top layer of the Earth's surface where plants can grow.

sugar

Noun

type of chemical compound that is sweet-tasting and in some form essential to life.

sulfur

Noun

chemical element with the symbol S.

toxic

Adjective

poisonous.

trophic level

Noun

one of three positions on the food chain: autotrophs (first), herbivores (second), and carnivores and omnivores (third).

volcano

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.


For Further Exploration

Websites

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writers

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrators

Tim Gunther
Mary Crooks, National Geographic Society

Editors

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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