Background on the Discovery of Methyl Mercury in Fish

The discovery of methyl mercury toxicity in fish has an interesting background.  
Mercury was used as an antifungal agent for seed grains.  
In Sweden predatory birds were developing neurological disorders. It was found that these birds were getting the neurological disorders because of mercury poisoning. They were at the top of the food chain that began with small mammals eating the grain that had mercury used as an antifungal agent.  As a control in finding mercury toxicity in the birds, Swedish investigators checked mercury levels in fish-eating birds, which their mercury levels were thought to be low. Their mercury levels were surprisingly found to be high, though they had no connection with eating things connected with the treated grain. 
This led to the discovery that microorganisms in the aquatic environment are capable of converting mercury to methyl mercury. The methyl mercury bioaccumulates in different underwater creatures.  Here is a link to the journal article that discusses this,