The Methanomonas bacteria are the only ones that can utilize methane as a source of energy. The risk of contamination is minimal. The bacteria are cultivated as a submerged culture in a water solution of mineral salts and a source of nitrogen (ammonia or urea). Air and methane are bubbled through the liquid and dispersed with an impeller. A batch culture is harvested after three days and yields about 12 g of wet bacteria per litre. The dried biomass of bacteria is pinkish white, odourless, tasteless and non-toxic and consists of about 70-80% protein of balanced amino acid composition.
For practical application it has been suggested that natural gas together with air (certain proportions of air and methane are explosive) can be bubbled through a lake or a pond. As a source of mineral and nitrogen, manure will do. As the bacteria produce acid it will probably be necessary to add limestone to maintain a constant pH. The bacterial biomass may either be collected and used for feed or allowed to remain in the water as a food for fish. Bacteria are very versatile organisms that can grow on almost everything; however, only one group of bacteria - those which can oxidize methane - has been thoroughly investigated for the production of single-cell protein. These bacteria have not been isolated and clearly defined; usually called Methanomonas methanica Sohngen, they are probably not a single species. Methane-oxidizing bacteria need more oxygen for growth than yeast and algae, and this increases the cost of production. There are also problems in obtaining a concentration of bacteria in the medium that is high enough for profitable production. Methane is among the most inexpensive and abundant sources of energy to be found. It is the main component of the gas produced in the anaerobic treatment of sewage and of natural gas, which in some parts of the world is burned off for lack of demand.
