Macronutrients are nutrients required in large amounts for cell growth. All cells require carbon. On a dry basis, a typical cell is about 50 % carbon, and this element is the major element in all classes of macromolecules. Some organisms are autotrophs and use CO2 as the source of carbon. The rest are heterotrophs and they use reduced organic compounds as a carbon source. Heterotrophs require autotrophs for their carbon source. The next abundant element is nitrogen. A typical nitrogen content in a prokaryotic cell is about 12 %. Nitrogen is found mainly in proteins and nucleic acids. Nitrogen is available in both organic or inorganic forms. However, the bulk of available nitrogen is in inorganic form, either as ammonia (NH4), nitrate (NO3−), or diatomic nitrogen (N2). Most organisms can use ammonia or nitrate as a nitrogen source but only a few are able to fix atmospheric N2because of the strong triple bond. Other macronutrients are...