Chemistry question... happychem!

ChicoRaton

ChicoRaton

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Jun 5, 2004
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Ok I was talking to my boss tonight and she said the reason nitrates are more of a problem in saltwater than in freshwater is because there are already so many dissolved solids in the water there's very little room for oxygen, so when you add nitrates it displaces the oxygen. I'd never heard this before... To me, it seems this would mean that if you increase the salinity a tiny amount, like 100ppm, it would displace all the oxygen in the water. I know saltwater tanks don't have THAT small of a margin for error. I've seen firsthand a fish living in water with a SG of 1.038, and he'd been in it for weeks.

So...I'm confused. Why exactly are saltwater inverts more sensitive to nitrate buildup?
 
I know, but as I said... what's the chemistry behind it? To quote myself:
Why exactly are saltwater inverts more sensitive to nitrate buildup?
Click to expand...
 
Well, Nitrates do displace oxygen. In terms of organisms, Nitrate poisoning happens when too much Nitrate enters the blood steam of the animal. This causes the blood cells to not be able to hold enough oxygen needed for the animals tissues. As a result a type of internal suffocation happens, and it can be noticed by blood being brown instead of the normal color it should be, as well as gasping for air.


To be more exact, Nitrate oxidises the iron in their haemoglobin to produce methaemoglobin, which cant carry oxygen (haemoglobin is what actually carries the oxygen to the cell tissues, Nitrate turns it into methaemoglobin which cant carry oxygen )

Nitrite and Nitrate are less toxic in a Marine enviornment because high levels of salt (which contains chloride), prevents methaemoglobinemia from forming, and makes it less toxic in the blood stream, and calcium chloride reduces gill permeability which means less nitrite/nitrate is absorbed into the body.

Invertabrates are not as efficiant in oxygen production and delivery compared in Vertabrates, hence inverts will die from nitrate poisoning faster then a fish because they process less oxygen (so they have to absorb more oxygen out of the water, which also means more nitrate) then a Vertabrate animal does (its the advantage of having lungs and a well developed circulatory system)
 
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ok... so nitrate displaces O2 in the blood of the animal, right?
Thanks for the excellent response. One more question, do the dissolved solids in saltwater displace oxygen _in_the_water?_
 
The only thing that would dissolve into the water to displace oxygen is nitrite and nitrate, however oxygen being displaced in the water isnt what actually kills the fish or invert, and the oxygen displaced isnt that noticable. Another interesting thing is Nitrate poisoning can also happen to terrestrial animals like live stock, since plants use Nitrate to produce proteins, and if the plant stores nitrate for unfavorable conditions, like cloudy or stormy weather, that Nitrate can also possibly kill any livestock or other animals that eat too much of those plants.
 
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Gealcath--would you be interested in writing up a summary of this for an article?

(Yes, I am shameless.)
 
A bit of confusion here - neither nitrite nor nitrate displaces O2 from the water, that is incorrect if it is being implied. O2 is a dissolved gas, not a dissolved mineral.

Nitrite competes with O2 for haemoglobin binding sites, so even at low levels is toxic to fatal to fish in FW. That binding however is blocked by chloride ion - which is plentiful in SW obviously.

Nitrate toxicity in fish and inverts is species specific. Some are very sensitive, other less so, some hardly at all. The same situation occurs in FW, and there too inverts tend to be more sensitive than vertebrates - most FW snails are not very sensitve.

Plants may well store certain trace elements in excess sufficient to be toxic to some grazers. I do not belive that nitrogen is one of those elements. I would need to see documentation to accept that. Nitrate is taken up by plants, but not stored or used as such, it is reduced to nitrogen.
 
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