301. The Anaerobic Way of Life

Molecular oxygen in appreciable amounts is found only in those areas on earth that are in direct contact with air or are inhabited by organisms carrying out oxygenic photosynthesis. The solubility of oxygen in water is low. In equilibrium with air at 1.013 bar and at 20°C, pure water will contain approximately 9 mg/liter of dissolved oxygen. In aqueous systems, aerobic organisms rapidly consume dissolved oxygen, so that deeper layers of many waters and soils (especially if they are rich in organic compounds), as well as mud and sludge, are practically anaerobic. Nevertheless, these areas are inhabited by numerous organisms that fulfill the important ecological role of converting insoluble organic material to soluble compounds and gases that can circulate back into aerobic regions. Other important anaerobic habitats are the rumen, the intestinal tract, and man-made anaerobic digestors of sewage treatment plants.