b. Mercury in water systems is transformed through biological processes into a substance that is toxic to animals.
Mercury is found in many different forms in nature, and we use if for a variety of different products. However, while mercury is useful in many different ways, it can also become an environmental hazard when it is released into the atmosphere. This can happen naturally, but it can also happen as a consequence of the burning of coal, oil and wood. This airborne mercury can then fall as rain, dust or simply due to gravity, contaminating water systems and becoming toxic to animals.