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Re: phototoxicity of malachite green

> Date: Tue, 30 Sep 1997 22:53:17 -0400
> From: Bjorn Straube <straube at digital_net>
> Subject: phototoxicity of malachite green
> least as good at blocking light as fish scales if not better.  If this
> is true then would it not be possible for fish to feel the effects, 
> especialy fry, weakened, or very sensitve spiecies?   Now I realize that
> malachite green is used everyday without real problems, and that
> although we as hobbyists try hard to mimic the sun in spectrum and
> output, most of us probably don't even come close ;).  So, while this is
> probably more academic than practical, I think it is still interesting. 
> Any takers out there ?

Well, after doing the handwaving below, I figured I should do a little
literature search...

> > I've wondered whether it works by the same mechanism in the aquarium,
> > and whether the fish scales are protective from light, [....]

I dug through Medline and found apparent agreement that the action of
malachite green is by the formation of free radicals, probably hydroxyl
or hydroxymethyl, which gum up the DNA as well as lipids in membranes.
It is also an inhibitor of some enzymes.

I found reference to an article in which malachite green was added to
feed, and was quite effective as an ich treatment in fish that are
normally sensitive to malachite green in water, such as neon tetras.
This is surprising, to say the least.  

Malachite green is toxic to mammalian liver and kidneys, and acts as a
tumor promoter for some experimental cancers.  It is banned as a food
color but is sometimes still used by the unscrupulous.  Tests have been
developed for residues in edible fish.  Treated fish clear the drug
fairly slowly.

One abstract descibed tests of different concentrations of malachite
green on carp fry with different degrees of parasitization and concluded
that the most heavily parasitized fish were the most susceptible to the
toxic effects of the drug.

Another abstract described the internal membranes (e.g. mitochondrial
membranes) of ich trophozooites disintegrating upon exposure to malachite
green, killing the cells.

Anyhow, for more handwaving, it appears that photoactivation may or may
not be strictly necessary for malachite green to be toxic (how much light
is there in a mouse kidney?).  Perhaps scaleless fish are more susceptible
because their skins are more permeable?  Maybe small/thin-bodied fish are
more susceptible because of the surface area to volume ratio?  Maybe fry
are also more susceptible because they are growing?

Sorry, I was unable to find anything on phytotoxicity of malachite green!