Methylmercury is the form of mercury that is the most toxic to organisms. It originates as mercury, which is found naturally in many rocks and also originates from forest fires and volcanoes. The element is present naturally in trace amounts in the air, water, and soil. More notably, mercury is present in high amounts in coal, which, once burned, is deposited into waterways easily. This process causes the majority of anthropogenic mercury emissions worldwide.
Once present in waterways, mercury comes into contact with organic compounds, and is methylated to form methylmercury. Methylmercury is a liquid and can enter the ecosystem at the lowest trophic levels. From here, the methylmercury both bioaccumulates in each organism and biomagnifies to the top of the food chain. This means that the top predators in aquatic ecosystems are subjected to much higher mercury levels than are present ambiently in the water of the environment.
Methylmercury’s presence in these organisms can cause a variety of irreparable neurological, reproductive, and genetic issues. The ingestion of contaminated fish is the most common method of mercury exposure in humans, and this can cause many of the same issues. The most serious issues linked with methylmercury ingestion in humans are birth defects.