New areas of research have begun examining the different mechanisms here on Earth to have a metabolism in the absence of gaseous oxygen. Since most planets that have been identified so far lack atmospheric oxygen, the need to study anaerobic metabolisms here on Earth becomes apparent. The lack of oxygen in many environments (e. g., undersea sulfur vents, compost piles, or intestinal lumens) here on Earth has created a selective pressure for numerous organisms to evolve new metabolic pathways to maintain the metabolic reactions required for energy production. In aerobic organisms, oxygen is required in cellular respiration and photosynthesis to act as a final electron acceptor at the end of the electron transport chain. A high-energy electron is passed through a series of proteins that use the electron's energy to transport a hydrogen atom across a membrane to set up an electrochemical gradient. After the excited electron has been sent through the transport chain, the electron is paired with a hydrogen atom and bound to an available oxygen molecule to form water. The lack of oxygen as a final electron acceptor in oxidative phosphorylation has forced some organisms to utilize other molecules, such as ones containing sulfur, to be reduced in the final step. Other alternate mechanisms include the use of fermentation as a means of oxidizing NADH so that more cycles of glycolysis can continue. If these steps are happening here on Earth, should we not be looking for similar compounds on other planets?The reduction of nitrate to nitrite and water by Mycobacterium tuberculosis is the anaerobic version of the use of the reduction of oxygen as a final electron acceptor under aerobic conditions. This metabolic characteristic may be useful in distinguishing this bacterial species and other nitrate reducers from nonreducers when trying to identify a new species or possible environmental niche. Choose TRUE or FALSE for the above statement.