Bioaccumulation - The most important pathway for mercury bioaccumulation is through the food chain, as illustrated in the figure below. In the water, plants and small organisms like plankton take up mercury through passive surface absorption or through food intake. For "autotrophic" organisms (which do not eat other organisms), passive absorption is the only route of exposure. The amount of mercury that results in these species from even a lifetime of passive absorption is not generally harmful to the organism. On the other hand, heterotrophic organisms (animals which eat other life forms) may be exposed to dangerous concentrations via a second route. Methylmercury biomagnifies through the food chain as predators eat other organisms and absorb the contaminants that their food sources contained. Over time, an individual who consumes plants or prey contaminated with methylmercury will acquire levels greater than in either its habitat or its food. As a result, top predators acquire greater body burdens of mercury than the fish they consume.
How does mercury become a toxicological problem? - Like many environmental contami-nants, mercury undergoes bioaccumulation. Bioaccumulation is the process by which organisms (including humans) can take up contaminants more rapidly than their bodies can eliminate them, thus the amount of mercury in their body accumulates over time. If for a period of time an organism does not ingest mercury, its body burden of mercury will decline. If, however, an organism continually ingests mercury, its body burden can reach toxic levels. The rate of increase or decline in body burden is specific to each organism. For humans, about half the body burden of mercury can be eliminated in 70 days if no mercury is ingested during that time. Biomagnification is the incremental increase in concentration of a contaminant at each level of a food chain.