I´m glad that you use your own head
You asked me a question about nitrite toxicity in another place - I prefer to answer in your own thread
On request – I will write down my thinking about nitrite (NO2) and its toxicity in different waters.
First – nitrite in the bloodstream is toxic for nearly all organisms including most fish and even us. When nitrite reach the blood stream it will create another form of haemoglobin – methaemoglobin. The NO2 ion will oxidize the iron part of the haemoglobin from Fe 2+ to Fe3+. This form of haemoglobin can´t take up oxygen and the organism will suffocate till it dies. Among humans nitrite can enter the bloodstream through the digestive system from nitrite rich food (like spinach) or from nitrate not fully denitriated by the digestive bacteria. Therefore – in Sweden it is advised not to give a child below 2 years old spinach and there is a norm value of NO3 concentrations in drinking water (50 ppm in Sweden).
In water with high NO2 levels has it been shown that the NO2 ion can enter a fish body and hence coming into the bloodstream and causing methemoglobin formation and hence cause suffocation of the fish. It has been shown that the uptake take place in the gills and the uptake process use certain channels (or mechanisms) for this.
NO2 toxification in newly started FW aquaria is the most common reason for death of freshwater species IMO. When the forming of methaemoglobin take place – the blood become brownish – Brown Blood Disease.
However – it is rather easy to hinder NO2 toxicity in fresh water even if there is NO2 ions in the water – its only need enough of free chloride ions in the water –
as described here:
Further – FW fish do not drink normally – the digestive entrance is blocked for uptake
This means that we in saltwater with a chloride concentration of more than 19 000 ppm do not need to have any concern about high NO3 levels in the water and uptake through the gills.
However – most SW fish do drink water. Should we be worried about uptake through the digestive track. IMO not because I have measured NO2 levels of up to 2 ppm in the water with no problems with SW fish. Further - could high NO3 levels and a not fully completed denitrification in the digestive track cause NO2 toxicity among SW species – IMO – not normal likely. One reason for this is that most organisms using haemoglobin as oxygen transporter normally produce low amount of methaemoglobin too and therefore also have developed an enzyme in order to change it back to haemoglobin again. With FW species I´m pretty sure that this enzyme exist among them because the brown blood disease (NO2 poising) is reversable among the fishes I have had experiences with.
One interesting thing is how organisms that not use haemoglobin as oxygen transporter reacts to elevated NO2 levels in SW. Horseshoe crab is one example – anyone that have experiences with this organism and elevated NO2 levels in the water? They and some other arthropods and molluscs use hemocyanin instead of haemoglobin as an oxygen transporter. The active metal is here not Fe – instead it is Cu. The answer I can see is that it looks like a similar pattern between FW and SW with these organisms.
Sincerely Lasse