A brown, polymeric constituent of soils, lignite, and peat; it contains the brownish-black pigment melanin. It is soluble in bases, but insoluble in mineral acids and alcohols. It is not a well-defined compound, but a mixture of polymers containing aromatic and heterocyclic structures, carboxyl groups, and nitrogen. An excellent chelating agent, important in the exchange of cations in soils. It is a natural stream pollutant and is thought to be capable of triggering the “red tide” phenomenon due to microorganisms in seawater. Detectable to 0.1 ppm in water.