Deep-sea life without sunlight
Compare photosynthesis with chemosynthesis, which uses energy from chemical reactions to make food. Explain how vent bacteria support giant vent tube worms, clams and crabs. Distinguish food production without light from independence from photosynthesis. Discuss vent-origin hypotheses and NASA research into possible life in icy moons' oceans.
What a learner can do afterwards
- Compare the energy sources used in photosynthesis and chemosynthesis
- Explain how food-making vent bacteria support animals, including adult giant vent tube worms
- Distinguish food production without light from independence from oxygen made by photosynthesis
- Explain why scientists study vents when investigating life's origins and possible life on icy moons, without treating hypotheses as proof
1 · Read
You have met deep-sea vents before. They release hot, chemical-rich water from cracks in the ocean floor. Sunlight does not reach these deep sites. Plants and algae use photosynthesis: light energy helps them make sugar from carbon dioxide and water. Some vent bacteria make food using chemical energy.
Chemosynthesis makes food using energy from chemical reactions. Some vent bacteria react hydrogen sulphide with oxygen. The released energy lets them build sugars from carbon dioxide. These chemoautotrophic bacteria make their own food and can form the producer base of a food web. Sulphide supplies energy; carbon dioxide supplies carbon for food.
Adult giant vent tube worms have no mouth or stomach. Bacteria inside them supply food. Some large clams at vents house bacteria in their gills. Some vent crabs eat bacterial mats or smaller animals. To predict a food-web change, follow who feeds on whom. If bacteria provide less food, animals depending on them may have less to eat. Predators of those animals may then have less prey. Other food sources can change this outcome.
Making food without light does not make a whole vent community independent of photosynthesis. Vent animals and many vent bacteria need oxygen. Much of that oxygen was made by photosynthesis near the ocean surface and carried down by moving seawater. Food from surface waters can also reach the deep sea.
One hypothesis is that early life began near hydrothermal vents. Water, chemicals and reactions with rocks could have helped the first living systems form. This is not proved. Today's vent communities do not show where life began. Microbes use different chemical reactions, and some can live without oxygen.
NASA researchers study Earth's vents to test ideas about life elsewhere. Europa orbits Jupiter, and Enceladus orbits Saturn. Evidence indicates oceans beneath their ice. Scientists ask whether water and chemical energy there could support life without sunlight reaching the oceans. Earth's vents do not prove that either moon has life or that both have active vents.
Vent bacteria can make food using chemical energy. Many vent organisms still use oxygen made by photosynthesis. Vents help scientists investigate early life and possible life on icy moons.
"Extreme life" adapts California Academy of Sciences exhibit material supplied through the academy corpus. The other three readings were independently written for this dataset. The supplemental TED-Ed video is "There may be extraterrestrial life in our solar system" by Augusto Carballido.
More to read
Extreme life
In 1977, researchers discovered deep-sea hydrothermal vent communities. Vents release hot water containing dissolved chemicals. No sunlight reaches these deep sites, but some bacteria use chemical reaction energy to make food.
The bacteria can live in mats or inside animals. Adult giant vent tube worms have no mouth or gut. Bacteria inside them supply their food. Other animals can graze on bacteria or eat animals supported by them.
Black smokers are one kind of hydrothermal vent, not another name for every vent. Vent communities show that food production can begin with chemical energy. This does not mean all vent organisms are independent of oxygen or other materials arriving from surface waters.
Evidence of life and its possible beginnings
An ancient trace of life can tell us that organisms existed at a particular time. It does not necessarily tell us where life first began. Some layered rocks preserve structures built by communities of ancient microbes.
Scientists investigate several possible starting places for life, including hydrothermal vents and hot springs. They study whether water, minerals and chemical reactions in these places could help produce materials needed by living cells.
A useful experiment may support part of an explanation without proving the whole explanation. Making a cell ingredient is different from making a living cell. Finding organisms at a vent today is also different from finding evidence that the first organisms formed there.
A vent origin remains a hypothesis. Researchers compare chemistry, experiments and geological evidence to test it.
Energy and ingredients for food
Food production needs both material and energy. These have different jobs.
During photosynthesis, a green plant uses carbon dioxide and water to make sugars. Light supplies energy for the process. Oxygen is released. Algae and some bacteria also carry out photosynthesis. Aquatic organisms can obtain water directly from their surroundings; they do not all need roots in soil.
Some vent bacteria use a different energy source. A reaction involving hydrogen sulphide and oxygen releases energy. The bacteria can use that energy to build food from carbon dioxide. This food-making process is chemosynthesis.
Carbon dioxide supplies carbon atoms for the food. Light or chemical reactions supply energy needed to build it. Energy does not become the atoms in sugar.
When another organism uses food made by bacteria, it receives some of the energy stored in that food.
A dark habitat can depend on sunlit water
Long before forests existed, microbes called cyanobacteria carried out photosynthesis and released oxygen. Over time, oxygen accumulated in Earth's atmosphere. Its presence allowed many oxygen-using forms of life to develop.
Oxygen also connects sunlit ocean waters with deep habitats. Photosynthetic organisms make oxygen near the surface. Moving seawater can transport oxygen into deeper water, where animals and microbes use it.
At a vent, chemical reactions can supply the energy for making food. Many organisms there still need oxygen. Saying that their food production begins without light does not mean that every part of their lives is independent of photosynthesis.
Scientists studying possible life in an icy moon's ocean must consider these needs separately. Water and chemical energy are reasons to investigate a habitat. They are not evidence that living organisms have already been found there.
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