The nitrogen cycle is the conversion of nitrogen between the different forms in the biosphere. Air consists of 80% dinitrogen gas and, with sediments and primary rock minerals, is one of the sinks of nitrogen on the earth. All organisms require some form of nitrogen to produce amino acids and thereby proteins, nucleic acids and, in the case of bacteria, cell wall materials. Access to an adequate supply of nitrogen in some form is a prerequisite for all forms of life. Nitrogen is the fourth most common element in micro-organisms after carbon, hydrogen and oxygen. The nitrogen atom can be in many different oxidation states and many of the "shifts" between these states are mediated by microorganisms. Nitrogen (as opposed to carbon, hydrogen and oxygen) exists most easily in the gaseous state (dintrogen gas). Biologically, only bacteria can "fix" atmospheric dinitrogen into forms available for metabolism. Nitrogen in the form of nitrate is easily leached because of its high solubility. Micro-organisms are primarily responsible for most of the cycling of nitrogen in the biosphere. The concept of a "cycle" of nitrogen was first put forward by Lohnis in 1913 and he recognised the protein, amide, NH3, NO2 and NO3 forms.