Carbon is an essential element for life on Earth. During evolution, living organisms have learned to create and process large number of different types of carbon compounds. Carbon is the key element of most biologically developed organic compounds such as carbohydrates, protein, fats, and DNA.
In addition to carbon, these compounds contain many other elements which include nitrogen, hydrogen, and oxygen.
In fact, elemental carbon forms when organic carbon compounds on Earth expel all other elements such as nitrogen and hydrogen without biological influence in high temperature and pressure conditions. For example, under high temperatures, wood deep in the ground first becomes coal then with increasing pressure and temperature results in the formation of graphite and anthracite with very high carbon content. The carbon is in crystalline form in these compound.
When gas, wood or oil is burned this results in soot that mostly contains an amorphous form of carbon. Previously, it was not known that living organisms can themselves generate elemental carbon.
Meanwhile, researchers have been cultivating microorganisms for more than 15 years that use methane without oxygen to produce the energy that is required. These are archaea that live symbiotically with bacterial partners. This, however, does not generates much energy for either partner for the consortia to grow with double amount of time of several months to be very long for microorganisms.
Some time ago, researchers noticed microbial consortia to be unusually dark close to black. At an initial stage, a part of the black mass was already explained as metal sulfides. Metal sulfides are formed from iron that is mixed with the culture resulting in the formation of sulfide by the partner bacteria.