From useful ammonia to even more useful nitrate

I'm trying to answer the plant biology question: why does nitrogen make leaves? (Which is something my mum told me.) This led me to look at the nitrogen cycle and I started off investigating how nitrogen from the atmosphere is converted to ammonia (nitrogen fixation). This can be done industrially but in the biological world it's done by nitrogen fixing bacteria.

Nitrogen in the form of ammonia can be taken up by plant roots and as such is a bit useful to them, but not that useful. It gets even more useful by being converted into nitrate. This is a two step process and is called nitrification. 

• In the first step ammonia (NH3) is converted to nitrite (NO2-)
• In the second step nitrite is converted to nitrate (NO3-)

Both of these reactions are technically examples of oxidation reactions because oxygen is combined with the nitrogen. Nitrite and nitrate are charged molecules so are referred to as ions (specifically anions – molecules with a negative charge).

In the plant world nitrification, like fixation, involves specific micro-organisms.

Nitrosomonas europeae
Stan Watson,  Woods Hole Oceanographic Institute     http://genome.jgi-psf.org/niteu/niteu.home.html

In the first step ammonia is converted to nitrite by ammonia-oxidising archaea and also some ammonia-oxidising bacteria (mainly types of Nitrosomonas). Archaea look a lot like bacteria and until the 1970s were thought to be types of bacteria. Like bacteria, archaea are single-celled organisms and their genetic material is not contained within a nucleus. But more recently scientists have shown that genetically archaea are very different to bacteria so are now classified in their own group for the purposes of biological classification(1). Archaea are really interesting and I could go on a bit more about them but the details are not really relevant to my leaf question. It's thought that archaea are the most abundant ammonia-oxidising micro-organisms in many habitats(2), but I couldn't find a suitable image anywhere on the internet.
 

In the second step the main group of bacteria that are involved in converting nitrite to nitrate are called Nitrobacter.

 

Like in nitrogen fixation the micro-organisms involved in nitrification use specific enzymes to facilitate the conversions(3).

Unlike nitrogen fixation, in which the bacteria need energy (in the form of ATP) to convert atmospheric nitrogen to ammonia, nitrification actually releases energy that the micro-organisms use to make ATP (which then provides energy for other purposes). The term to describe this type of organism is a chemoautotroph – loosely translated meaning “obtaining own energy from a chemical reaction”.

The micro-organisms involved in both steps can be found in a wide range of environments, including, importantly for the purposes of my investigation, soil.

 

One of the important things about nitrate is that it is much more water soluble than ammonia, which means that it is easier for plants to take it up through their roots, and why it is most useful. Nitrite is also quite water soluble but is toxic to plants in large amounts so it is important that it is converted to nitrate.

Interestingly, nitrate can be formed directly when lightning strikes. The energy in a bolt of lightning enables atmospheric nitrogen to be directly combined with atmospheric oxygen. The resulting nitrate is soluble in moisture in the atmosphere and when it rains the nitrate is delivered to the soil, from where the plants take the nitrate up through their roots(4).
 



Diagram showing the stages in the conversion of atmospheric nitrogen to nitrate

 

So now we've found out how:

1. Atmospheric nitrogen is converted to ammonia

2. Ammonia is converted to nitrite and then to nitrate

 

Next I'm going to be looking at how the plants take nitrate up through their roots and what they then do with the nitrogen. This will involve a little break from the nitrogen cycle (don't worry we'll be coming back to it) and a diversion into cell biology.

 

 

References

(1) http://www.microbeworld.org/types-of-microbes/archaea accessed 17.8.13

(2) Bernhard, A. (2012) The Nitrogen Cycle: Processes, Players, and Human Impact. Nature Education Knowledge 3(10):25 Available http://www.nature.com/scitable/knowledge/library/the-nitrogen-cycle-processes-players-and-human-15644632 accessed 17.8.13

(3) http://nitrificationnetwork.org/Introduction.php accessed 17.8.13

(4) http://www.howstuffworks.com/nitrogen-fixation-info.htm accessed 17.8.13