eli5: How does the decomposition process work under water?


With composting, you usually want to work with a lot of air and oxygen, aerobic composting. But that is obviously difficult under the water where oxygen is more limited. So what exactly happens when large amounts of bio-matter breaks down in a lake or the sea as opposed to on land and with plenty of oxygen? And are there any man-made composting systems designed to work while fully submerged?

In: 2

In an oxygenated body of water, the reactions are pretty similar. Water can dissolve plenty of oxygen, which is why fish (which breathe oxygen just like you do, albeit with more efficient means of absorbing it) can live in it.

If there’s no oxygen, though, decomposition is very, *very* slow, taking centuries to millennia. This turns out to be pretty important to the Earth’s carbon cycle. Plants that grow in (anoxic = without oxygen and acidic) peat bogs end up sinking to the bottom and effectively never decaying. The resulting material, peat, is [one of the world’s largest carbon stores](https://en.wikipedia.org/wiki/Peat), even though it’s mostly found in tundra bogs near the poles and only covers a small part of the Earth’s surface.

Animals and humans who die in bogs can be preserved for ages ([this man](https://en.wikipedia.org/wiki/Bog_body#/media/File:Tollundmannen.jpg), for example, is still quite recognizable even though he died 2,500 years ago!), providing a very valuable source of information for anthropologists studying the history of ancient human civilizations.

Eh if you look at the chemical form of water H2O you will find there is indeed oxygen in water. A water molecule is composed of two hydrogen and one oxygen atom, covalently bonded.


When you put food in the trash and it starts to smell bad, that’s because tiny creatures called bacteria and fungi are eating the food and breaking it down. This is called decomposition.

When this happens under the water, like in a lake or the ocean, there isn’t as much air, so the tiny creatures that break down the food can’t use the air to help them eat. Instead, they use other things, like hydrogen sulfide, to help them break down the food.

This process makes bad-smelling gases, like methane, which can harm the environment. People have made special containers, called anaerobic digesters, that help break down food without air, but these containers aren’t for making compost for plants. They’re for cleaning up waste water.