Our Earth is unique from the other planets in our Solar System for many reasons. Of the many differences, one of the most conspicuous is the presence of abundant oxygen in Earth's atmosphere. No other planet or moon in the solar system exemplifies an excess of oxygen like Earth. Earth is also the only planetary body known to harbor life. These two facts might lead some to believe that oxygen is therefore necessary for life to exist. However, a mere glimpse at the natural environment surrounding us quickly refutes this conclusion. We wouldn't be able to fully digest a meal if it weren't for the help of microorganisms living in the oxygen-free environment of our intestines. In fact, it is difficult to determine which is more abundant on Earth, those organisms that need oxygen and those that do not.
Life that can survive in the absence of oxygen is described as anaerobic. There are two types of anaerobic organisms, facultative and obligatory. Facilitatively anaerobic organisms are those that can exist both in the presence and absence of oxygen. Obligatory anaerobes, on the other hand, cannot exist in environments where oxygen is present; it is poisonous to them.
The metabolic reactions necessary for life can occur in both aerobic and anaerobic conditions. However, life reaps a greater energy yield from metabolic reactions if they occur aerobically. Why, then, wouldn't all life have evolved to strictly use aerobic metabolic pathways? To answer this question, it is important to recognize that the first life on Earth was most likely anaerobic. Although life subsequently evolved into an aerobic lifestyle, there were selective advantages for some organisms to maintain their anaerobic habits. In addition, there is a price to pay for the high efficiency of aerobic metabolism. A common byproduct of oxygen-related mechanisms is the manufacturing of oxygen reactive species. These are molecules, derived from oxygen, that are highly reactive and can easily cause cell damage. Perhaps the most well known is the hydroxyl radical. This small molecule is extremely reactive and can rip apart bio molecules with ease. Luckily, aerobic organisms have evolved effective mechanisms to combat oxygen reactive species. These strategies include the production of antioxidants within the cell, sophisticated cellular repair mechanisms, and enzymes that work to detoxify the cell.
The majority of environments beyond Earth that are targeted for future exploration are strictly anaerobic. Our search for life elsewhere, therefore, will require a robust understanding of how life on Earth functions in anoxic environments. What molecule other than molecular oxygen can life utilize for its metabolic reactions? What are the possible byproducts of photosynthesis other than oxygen? How can we recognize these molecules as signatures for life? Answers to these questions and more will enable us to more effectively scour our cosmic environment for the signatures of life.