Re: Low pH nitrification THE ANSWER?

["A. Inniss" <andrewi@u.washington.edu>]

>      As a further note, someone was wondering how a low pH tank could function
> and not "crash" if no Nitrogen Cycle activity was occurring, I will repeat
> what was said earlier. The ionized ammonium ion (NH4+) does not pass through
> cell membranes (read "gills") and therefore is not take up by organisms, and
> is not toxic to our fish. Therefore, it does not really matter if nitrate or
> ammonium accumulates in our low pH tanks, as long as correct tank maintenance
> procedures are followed.

	I've read this very informative thread with great interest.  I'd
like to add a couple of comments and questions:  I'm not sure I'd go so
far as to say that NH4+ "does not" pass through the gill epithelium.  I
had thought that the reason NH3 had an easier time passing through is
because of its electrostatic neutrality.  Thus, unlike the cationic
NH4+, it does not require ion transport channels, and so presumably its
diffusion is not inhibited by other ions by competition for passage sites.
>From this understanding, I hadn't inferred that ammonium _could not_ or
_does not_ diffuse through, but that it did so more slowly and in
competition with similar ions.  Is my understanding inaccurate?  

	I'd also like to mention that while ammonia (as ammonium) is
relatively safe at an acidic pH, something like the opposite is true for
nitrite.  Nitrite (NO2-) has a pH dependent equlibrium with nitrous acid 
(HNO2).  However, while both are toxic because they oxidize hemoglobin to
methoglobin and thus inhibit oxygen transport in the blood, nitrous acid
is the more toxic of the two.  It's my understanding that HNO2's
electrostatic neutrality makes its passage through the gills easier.  (See
above explanation). Unfortunately, nitrous acid predominates at an acidic
pH, so having an acidic pH helps protect against (or perhaps prevents
ammonia poisoning, but just the opposite is true for nitrite (nitrous
acid) poisoning.  Further, HCO3-, which I would guess is in low
concentrations in acidic conditions, helps inhibit nitrite poisoning by
competing for passage sites.  (Chloride does a better job, though).

	I'm not a biochemist, so if I've made errors, please correct and
comment.

			Andrew 
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