Oxygen Loss

• In the environment, oxygen is often lost from soils that are submerged or inundated with water. Air does not pass through water very quickly, so the oxygen that was present in the soil is consumed during respiration.
  
  

Alternate Electron Acceptors

• Oxygen ultimately accepts the electron that is removed from organic matter during the respiration process. When oxygen is depleted from the soil, some microbes have the potential to utilize other electron acceptors, which are less efficient than oxygen.
  
  

Preferential Order

• When anaerobic respiration begins in soils, there is a particular order in which alternate electron acceptors are utilized; however, there is some overlap. Nitrate or manganese are typically used first, followed by iron, sulfate and organic compounds. Reduction of organic compounds results in the production of methane gas.
  
  

Facultative vs. Obligate Anaerobes

• Some soil microbes are able to utilize alternate electron acceptors in the absence of oxygen, and these organisms are known as facultative anaerobes. Other soil microbes only survive in oxygen-free environments, and these are known as obligate anaerobes. If oxygen is introduced, obligate anaerobes either shut down their metabolic activities or they die.
  
  

Adaptations

• Many plants have adapted morphological advantages to overcoming surviving in soils with oxygen deficiencies, such as wetlands and ponds. For example, some plants have hollow shoots and elongated root systems that allow them to more effectively scavenge for, and transport, oxygen to meet their metabolic requirements.