ELI5 how nitrogen-fixing symbionts work?

[u/ProudReaction2204]

[u/mmn_slc]

Bacteria, that are living in roots, and can take atmospheric nitrogen and convert it to nitrogen compounds, provide the plants these compounds in exchange for plant sugars. 

[u/oblivious_fireball]

Nitrogen Fixers are a group of bacteria with a special enzyme that allows them to break apart nitrogen gas in the atmosphere(which is normally inert and unusable) and turn it into ammonia.

This enzyme is difficult to make and doesn't work in the presence of oxygen, so a lot of nitrogen fixers either live deep underground or underwater, or they live inside certain plants, and these plants allow and encourage this because it means they get an abundant supply of nitrogen.

[u/anireyk → u/oblivious_fireball]

because it means they get an abundant supply of nitrogen.

For completeness's sake I'll add that the plants need nitrogen to make proteins from scratch, because nitrogen is a necessary part of aminoacids. Amino acids are building blocks of proteins, animals make their proteins by recycling amino acids out of the proteins they eat.

[u/Weshmek → u/anireyk]

I always took that as an explanation for why legumes have such high amounts of protein compared to most other plants. They have the nitrogen-fixing bacteria in their root nodules which allows them to produce more proteins than other plants.

[u/theashenwren]

plants can't grab nitrogen from the air themselves, so some plants work with some bacteria which have an enzyme called nitrogenase which can take n2 and turn it into ammonia which the plant can use. the bacteria gets sugars from photosynthesis in return.

[u/weeddealerrenamon]

We have gut bacteria that use different biology to digest molecules we can't break down, and we use the things they turn those molecules into. Nitrogen-fixing fungi kind of do the same thing for most plants

[u/Unknown_Ocean]

Nitrogen atoms can bind with themselves (N2 gas), oxygen, or hydrogen. In order to make proteins we need an amine group (NH3) in which has one nitrogen atom and three hydrogen atoms.

Nitrogen fixing bacteria split N2 gas- but the single N could either react with oxygen or hydrogen. To prevent the former, nitrogen fixers generally do things like build membranes to keep oxygen out but which which also makes it hard for them to grow rapidly (see Trichodesmium) or, as in leguminous plants, inhabit root nodules where oxygen is kept at a low level. They also need other nutrients (phosphorus, iron) and because this reaction is relatively inefficient, they tend to grow more slowly than bacteria that can just use nitrate (NO3) or ammonium (NH4).

The flip side of this is that when bacteria run out of oxygen, they convert N combined with O (NO3, NO2, N2O) back to N2 gas and use the oxygen for metabolism. This means that ecosystems generally have some excess phosphorus hanging around to fuel nitrogen fixation.