Nitrification

Nitrification is the dominant respiratory process on Pontret.

Chemical Process
The process of nitrification is divided into three parts: nitritation, nitratation and

Nitritation
Firstly, ammonia reacts with oxygen and oxidising agent hydrogen sulphide to produce hydroxyl-amine, water and sulphur dioxide (which is not involved any further in the process). The hydroxyl-amine then reacts with water to form nitrogen dioxide, releasing protons and electrons. These subatomic products react with oxygen to form water.

NH3 + 2O2 + H2S ==> NH2OH + H2O + SO2

NH2OH + H2O ==> NO2- + 5H+ + 4e-

5H+ + 4e- + ½O2 ==> H2O + H+

Overall nitritation reaction:

NH3 + 2½O2 + H2S ==> NO2- + H2O + H+ + SO2

Nitratation
Here, the nitrogen dioxide reacts with water to produce nitrate ions, protons and electrons. These react with oxygen to form water.

NO2- + H2O ==> NO3- + 2H+ + 2e-

2H+ + 2e- + ½O2 ==> H2O

Overall nitratation reaction:

NO2- + ½O2 ==> NO3-

Overall Equation
NH3 + 3O2 + H2S ==> NO3- + H2O + H+ + SO2

Photosynthesis and Nitric Acid Recycling
Photosynthesis on Pontret involves the conversion of nitric acid back into ammonia. This requires a greater energy input than carbon dioxide-based photosynthesis on Earth but the greater ultra-violet output of Pontret's star Ro-a and the planet's closer proximity to it help to provide this energy.

In exoanimals, nitric acid recycling occurs. The process begins with the reduction of nitrates produced in respiration to from nitric acid. This reacts with sodium acetate taken is as part of the organism's diet, along with an iron catalyst, to produce ammonia. This reaction requires some energy release from an ATP-equivalent energy currency molecule but ultimately allows for net production of energy currency as the ammonia produced can then be respired again. Energy is lost each cycle and this recycling process is not enough to sustain the organism so regular respiration must continually occur.

In exoplants, the nitrate ions are taken in through the roots and a similar reaction occurs, once again with sodium acetate. The energy here is provided by photoexcitation due to ultra-violet radiation.

Use of Sulphur Dioxide in Digestion
The sulphur dioxide produced as a waste product in respiration by some organisms reacts with water to produce dilute sulphuric acid. This is removed and released into the exoanimals' stomach for digestion, before being diluted with sodium hydroxide, acquired elsewhere, to produce sodium sulphate which is then excreted. This process requires an efficient lymph system to stop a harmful build up of the acid.