Most of the life you can see uses oxygen. But in waterlogged soil, the mud at the bottom of a pond, or deep in a compost pile, oxygen runs out. Life goes on there, using fermentation and other pathways, and that changes how matter cycles.
Key idea
Aerobic means with oxygen; anaerobic means without. In anaerobic places many microbes still release energy from food by fermentation or by passing electrons to something other than oxygen, such as nitrate. Some microbes cannot survive oxygen at all.
Why it works
Oxygen is the final electron acceptor in aerobic respiration, and it makes the most ATP. Without it, cells must use other acceptors, which give less energy, or ferment, which gives only 2 ATP per glucose. Water fills the spaces between soil grains and oxygen diffuses through water about 10,000 times more slowly than through air, so a flooded soil quickly runs out of it.
- Denitrification: in waterlogged soil, bacteria use nitrate instead of oxygen and release nitrogen gas, so wet soils lose nitrogen to the air.
- Methane: archaea in wetlands, rice paddies and the stomachs of cattle make methane (CH₄) as they break down matter without oxygen.
- Slow decay: without oxygen, dead plants break down slowly, so peat builds up in bogs. Over very long times, buried dead matter became coal and other fossil fuels.
Worked example: Two soils
A soil is flooded for weeks. What happens to its nitrate, and why?
- Water fills the air spaces, so oxygen runs out.
- Denitrifying bacteria then use nitrate in place of oxygen.
- They turn it into nitrogen gas, which escapes to the air.
Answer: The nitrate falls as it is turned into N₂ gas.
Worked example: Comparing energy
Per glucose, fermentation makes 2 ATP and aerobic respiration about 30. About how many times more ATP does aerobic respiration make?
- 30 ÷ 2 = 15.
Answer: About 15 times more.
Common mistake
People often think life stops without oxygen. Many microbes thrive without it, and some cannot live with it at all. Your own muscle cells can run briefly without oxygen by fermentation. Check the place: waterlogged soil, mud and the gut are full of anaerobic life.