This might help answer your question...

http://toxics.usgs.gov/definitions/methylmercury.html

As yeast dies off it gives off a toxin called acetaldehyde and that is what methylates mercury.... It acts as a catalyst.

That also is why we get so sick when we drink alcohol. The acetaldehyde formed from consuming alcohol methylates the stored merucury into the most lethal form...methyl mercury.

Hope this helps.

Sandi
xoxo