The concentration of elements
In nature the quantity of elements is extremely important: the substances that are necessary to living organisms are often more present in their tissues more than in the nearby air, soil or water. This concentration is due to the capacity of cells to absorb only some substances and to the various passages that occur along food chains. (If you want to have more details, read the main text “Alteration of the food chain”).
In some cases element accumulation is extremely relevant. The coral reefs are an example. They are made of calcium compounds (carbonates), deposited by generations of microscopic polyps that extract these substances from the seawater.
If you want to have more details about this phenomenon go the section “Coral reef”.