In an organism, concentration of materials which are not components critical for that organism's survival. Usually it refers to the accumulation of metals or other compounds (e.g., DDT). Many organisms - plants, fungi, protists, bacteria, etc. - accumulate metals when grown in a solution of them, either as part of their defence mechanism against the poisonous effect of those compounds, or as a side-effect of the chemistry of their cell walls. Bio-accumulation is important as part of the microbial mining cycle (q.v.), removing toxic metals from wastewater, as a purification (bioremediation) process, etc.