In the last 100 years, humans have synthesized thousands of new chemicals, some of which have been released in large amounts into the biosphere. Other, naturally occurring pollutants such as cyanides, phenols, polycyclic aromatic compounds, or long-chain aliphatic hydrocarbons originate from coal or crude oil. These may be hazardous if accidentally or deliberately released, because they exhibit acute or chronic toxicity. However, many of these organic compounds can be eliminated by microorganisms (natural or genetically modified), if the appropriate process conditions can be established.