Aimed primarily at beginners to the hobby, this series of articles will take you step by step through the process of understanding how a good koi pond works.
Part 14: Nitrite
The question I posed last month was; which of the two remaining important water parameters, nitrite or nitrate, is affected by pH? The answer is nitrite. Its toxicity to koi is affected by both pH and by temperature. Over the years, there has been a huge amount of research done on the subject of nitrite toxicity and its effects on fish. None of the studies, as far as I can tell, have actually researched the effects on koi but there are different species that have near identical physiology so we can derive a great deal of information from them. The various research projects have repeatedly shown that, in a similar manner to ammonia, nitrite toxicity is dependent on both pH and on temperature. As with ammonia, its toxicity increases with increasing temperature but, unlike ammonia, its toxicity actually decreases with increasing pH.
Nitrite toxicity and temperature
Increasing water temperature increases nitrite toxicity. In one study, groups of common carp were subjected to different nitrite levels at different temperatures and it was found that toxicity was 70% higher at 20°C than it was at 14°C. In view of the temperatures involved, and the species of fish in the test, it is safe to assume that this is exactly what would happen in our ponds when nitrite is present. In a koi pond, nitrite toxicity can be expected to increase by almost 12% per degree Centigrade rise in temperature. In this respect, the effect of temperature increase on nitrite toxicity is greater than that of ammonia which only increases its toxic effects by approximately 10% per degree Centigrade temperature increase.
Nitrite toxicity and pH
The toxicity of nitrite increases as pH falls. The pH of water is caused by the H+ ions described in part twelve of this series. As a quick reminder, the more of these that are floating freely in water, the lower is its pH. Nitrite has the chemical formula NO2 and some of these molecules combine with the little H ions to form nitrous acid (H + NO2 = HNO2). The lower the pH, the more of these H’s are available to combine with any nitrite molecules in the pond to form nitrous acid. This is a very weak acid, almost too weak to measure, because not all the nitrite will combine in this way but it has a seriously bad effect on fish if it should enter their bodies.
Research is continuing but, at the present time, it is suggesting that there are two ways in which nitrous acid can enter a fish’s body. Fish have chloride cells in their gills and these serve different functions. A prime function of these cells is to excrete waste ammonia and at the same time take up an equal amount of chloride from the surrounding water as a sort of molecular balancing act. The presence of nitrous acid in the water, even in minute concentrations, can cause these cells to enlarge and allow the acid to enter the gills through them. A second way nitrous acid can enter is via epithelial cells. These are cells which form the tissue covering internal organs and other internal surfaces of the body.
In tests, it was found that twice as much nitrite would enter fish in the form of nitrous acid at a pH of 6.5 than at 8.0. This fall is slightly outside the normal range for koi keeping but it shows a definite relationship between falling pH and rising nitrite toxicity.
Nitrite toxicity in fish
Whichever way nitrous acid gets into a fish, the effects can be severe. It should be made clear that, even though nitrous acid is obviously acidic, the major harmful effects are not due to acid burns, it is far too weak to be greatly corrosive. Instead, it undergoes a chemical conversion and becomes nitrite again. The most serious effect it then has is for it to enter the bloodstream and interfere with the way haemoglobin carries oxygen around the fish’s body.
Figure 1 shows a simplified diagram detailing the way waste carbon dioxide is exchanged at the gills for oxygen which is then carried around the body by haemoglobin. This was described in greater detail in part 11, but in essence relies on the ability of the haemoglobin to deliver oxygen to body tissues and, at the same time, pick up waste carbon dioxide. It then exchanges carbon dioxide for oxygen again the next time it passes through the gills.
Figure 1 Simplified representation of the koi respiration cycle
When nitrite enters the bloodstream, it combines with haemoglobin. Once haemoglobin has picked up nitrite it can no longer carry oxygen because the “spaces” where oxygen, shown in black in the diagram, would normally be carried are now occupied by nitrite so there is no room available for oxygen molecules. Since any haemoglobin that has been affected in this way can’t take oxygen to cells that need oxygen, it can’t take carbon dioxide, shown in yellow, back to the gills to exchange for new oxygen in order for the respiration cycle to be continuously repeated.
Haemoglobin that has picked up nitrite where it should be carrying either oxygen into a fish or carbon dioxide out of it is called methaemoglobin (pronounced met-haemoglobin) which is brown in colour. This causes the colour of the gills, which are normally ruby red due to the blood flowing through them, to look brown instead, and therefore the common name for this effect is “brown blood disease”.
How nitrite kills
To repeat the above, it isn’t the corrosive effects of nitrous acid inside their bodies that is the immediate threat to life as far as koi and other fish are concerned. It will have a mildly corrosive effect but the greater threat will come from it reconverting back to nitrite and entering the blood circulatory system where it combines with haemoglobin and prevents it from carrying oxygen. As more and more haemoglobin becomes unable to carry oxygen, the respiration cycle becomes less and less efficient. The lower the pH, the more nitrite converts to nitrous acid and the greater will be the effect that this has on the fish’s blood. As the percentage of haemoglobin that is unable to carry oxygen increases, a fish will have to breathe harder and faster than normal. If the nitrite situation isn’t quickly rectified, more and more haemoglobin will be affected until so much has been disabled that there isn’t enough left that is functioning well enough to sustain life and the fish eventually suffocates.
It is difficult to put exact figures on the point at which this effect would become fatal, clearly the more highly aerated the water, and the higher its pH, the better the chance of survival for any affected fish.
Research indicates that only the most oxygen sensitive fish would be at risk of suffocation with 50% of its haemoglobin ineffective. Most fish in the tests would survive this situation, they would obviously be in distress but they could, at least, survive although mere survival isn’t a good situation. That brings with it, stress related problems such as a reduced immune system which could allow a pathogen to infect the fish and cause death that way. Rapid death seems to be nearer to a methaemoglobin level of 70% to 80%.
Although, strictly, nitrite is less toxic than ammonia in that it is less corrosive or damaging to internal organs, gills or body tissue, the way in which its effects can rapidly cause asphyxiation means that deaths due to nitrite poisoning (methaemoglobinemia) can occur much more rapidly than those due to high levels of ammonia.
How nitrite prevents haemoglobin from carrying oxygen
The red blood cells in koi blood contain molecules of haemoglobin which, typically, make up 6.9 gm per 100 ml of their blood. Each of these molecules contains four sites where it can attract and hold oxygen molecules.
From left to right;
At the gills: Haemoglobin (Hgb) attracts oxygen molecules (O2) into its four sites.
In the blood stream: Oxygen is carried by the haemoglobin towards cells in the body tissue.
In the body tissue: Individual cells take the oxygen they need from the haemoglobin.
Nitrite combined with haemoglobin: No cells in the body need nitrite so, if haemoglobin picks up nitrite (NO2) instead of oxygen, it remains in those sites making it impossible for it to pick up any more oxygen during its subsequent passes through the gills.
You heard it here first!
Once haemoglobin has picked up nitrite and been converted to methaemoglobin it cannot carry oxygen. This much is true but it is commonly said that this change is irreversible and any blood cell carrying methaemoglobin will never again be able to carry oxygen until it dies and is recycled again as a new blood cell through the spleen, liver or bone marrow. This isn’t true and you read it in this magazine first.
Several researchers have discovered that methaemoglobin forms spontaneously in small quantities in fish blood even when nitrite isn’t present in their environment and that the amount of methaemoglobin that forms naturally could even get as high as 10%. They also discovered that there is an enzyme (called a reductase) that removes the nitrite. The process isn’t quick, it may take up to two days but, if the nitrite in the pond could quickly be reduced to negligible proportions then, within two days, any red blood cells containing methaemoglobin could be restored to functioning as normal.
Does salt make nitrite non toxic?
This is another commonly held belief but although it can help as a temporary first aid measure, it isn’t true to say that it can actually alter the toxicity of nitrite in any way, either to make it less toxic or more so. As described above, one route of entry for nitrous acid into the body of a fish is through the chloride cells in the gills and they are called chloride cells because part of their main function is to take up chloride from the surrounding water. This provides us with a strategy to reduce the amount of nitrite that can enter a fish’s body.
By increasing the amount of chloride in the water to a higher concentration than that of the nitrite, we can make it more likely that the cells will pick up a chloride molecule than a nitrous acid one. I don’t normally recommend adding salt to a pond except in extreme cases because koi are fresh water fish and salt interferes with their natural osmoregulatory system but in cases of imminent death due to nitrite poisoning, it is by far the lesser of two evils.
Salt is sodium chloride and therefore is a very convenient way to quickly add chloride to a pond. Salt doesn’t actually prevent nitrous acid from entering the gills but, by being in the water in greater quantities, it makes it far more likely that it will be picked up instead. It’s purely a numbers game. If two different types of molecules can enter a cell that has no particular “preference” to which one it lets in, the molecule that is present in the greatest numbers will be the most likely to be accepted. To simplify that using easy numbers as an example, we could say that if there are ten molecules of chloride crowding around a particular cell for every one molecule of nitrous acid, the chances of the cell accepting a chloride rather than a nitrous acid molecule will be ten times greater.
Taken overall this means that although the salt can’t prevent nitrous acid gaining entry it will have a success rate that is ten times greater. Or looking at the situation from the other direction, the success rate for nitrous acid gaining entry will be reduced to 10% which means that the damage caused inside the fish to its haemoglobin will also be reduced to the same amount.
In no other way does salt make nitrite less toxic, it merely competes against, and outnumbers, the nitrite for entry into the fish’s blood via the chloride cells. The cause of high nitrite levels in the pond will almost invariably be due to the nitrite bug colony in the biofilter not having fully matured. Adding salt at 0.5 ounces per gallon, along with regular water changes is a short term measure against nitrite poisoning until the biofilter does mature.
Next month, a discussion of the last of the vital pond parameters that should be regularly checked, nitrate. Why nitrate isn’t quite as harmless as previously has been believed and what it can tell us about the health of the biological filter when its value is compared to the values of the other parameters we test.
 Sources for research on spontaneously occurring methaemoglobin in fish: Cameron 1971; Brown & McLeay 1975; Smith & Russo 1975.