Photo- and chemoautotrophs at Higher Level: An HL-only statement. Most food webs start with light, but some start with chemicals. You will meet deep-sea vents, a cave sealed for millions of years, and the bacteria that feed them.
Practise this as you read
- Distinguish the two energy sources, naming each process.
- Explain why chemoautotrophs are the main producers where there is no light.
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Same job, different power supply: Every producer is an autotroph (2.2.21). What differs is where it gets the energy to do it: light, or a chemical reaction.
The points to remember
- All producers are autotrophs: they make carbon compounds from carbon dioxide.
- They differ in their external energy source.
- Photoautotrophs use light energy in photosynthesis: plants, algae, phytoplankton.
- Chemoautotrophs use energy from exothermic inorganic chemical reactions in chemosynthesis.
- Chemoautotrophs are bacteria (and similar microbes); they need no sunlight.
Remember it as: Photo = light. Chemo = chemicals.
Photoautotrophs
- Energy: light
- Process: photosynthesis
- Examples: plants, algae, phytoplankton
- Where: anywhere with light
Chemoautotrophs
- Energy: chemical reactions
- Process: chemosynthesis
- Examples: sulfur and nitrifying bacteria
- Where: often dark places
Real example: in the Southern Ocean, phytoplankton photosynthesise in the sunlit surface water and feed Antarctic krill, which feed Adelie penguins.
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Chemosynthesis runs on exothermic reactions between inorganic chemicals. The bacteria oxidise such a chemical and trap the energy released.
How chemosynthesis works
- The bacteria oxidise an inorganic chemical, such as hydrogen sulfide, ammonia, iron or methane.
- The reaction is exothermic: it releases energy.
- That energy is used to turn carbon dioxide into sugars: new biomass.
- So the chemical, not the Sun, is the energy source at the base of the food chain.
In words: carbon dioxide + hydrogen sulfide + oxygen → sugar + sulfur + water
Photosynthesis (2.2.4) uses the same carbon dioxide to make sugar; only the energy source is different.
Say 'chemical energy', not 'heat': Vent bacteria do not use the heat of the vent. They use the energy released by chemical reactions, such as oxidising hydrogen sulfide.
Real example: at deep-sea vents, bacteria oxidise the hydrogen sulfide in the vent water. In soil, nitrifying bacteria oxidise ammonia in the same way.
In 1977 the submarine Alvin found animals crowded around a hydrothermal vent on the Galapagos Rift, 2,500 m down, where no sunlight ever reaches. Their food web runs on chemosynthesis.
The points to remember
- No light reaches the deep sea floor, so photosynthesis is impossible there.
- Vent water is rich in hydrogen sulfide; bacteria oxidise it for energy.
- These chemoautotrophs are the producers: the principal source of energy for the food web.
- Tube worms, mussels and limpets feed on the bacteria; crabs, fish and octopus feed on them.
- Stop the vent and the whole food web starves.
Remember it as: No light, no plants: the bacteria are the base.
Real example: giant tube worms have no mouth or gut. Chemosynthetic bacteria live inside their bodies and feed them, a symbiosis.
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Vents are not the only place. Chemoautotrophs are found wherever the right chemicals are, but they matter most where light is missing.
The points to remember
- Chemoautotrophs live in many ecosystems: sea floor, caves, hot springs, soil.
- Where there is little or no light, they are the main producers.
- Where there is light, photoautotrophs supply almost all the energy.
- In soil, nitrifying bacteria are chemoautotrophs too, but plants still feed the food web.
| Ecosystem | Light? | Main energy source for the food web |
|---|---|---|
| Deep-sea vent or cold seep | None | Chemoautotrophs |
| Sealed cave | None | Chemoautotrophs |
| Sunlit ocean surface | Plenty | Photoautotrophs (phytoplankton) |
| Meadow soil | Above ground | Photoautotrophs (plants) |
Real example: Movile Cave in Romania was sealed off for about 5.5 million years. Bacteria oxidise hydrogen sulfide and methane there, and they feed spiders, centipedes and a water scorpion found nowhere else. On the floor of the Gulf of Mexico, cold seeps support mussels in the same way.
Link the ecosystem to the light: Say why chemoautotrophs dominate: no light, so no photosynthesis, so chemical energy is the only supply.
How this comes up: Paper 2, Section A: a food web from a place with no light. Distinguish between the energy sources of its producers and those of a sunlit ecosystem [2].
Figure 1 shows a simplified food web at hydrothermal vents on the Galapagos Rift, 2,500 m below the surface.
Distinguish between the way the producers in Figure 1 obtain energy and the way phytoplankton in the sunlit surface ocean obtain energy.
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