Earth shifted from an anaerobic atmosphere to an aerobic one early in its life. However, for a long time, the question as to how it got there was still unresolved. A recent paper in Nature Geoscience proposes to explore the link between the length of the earth's day and the amount of oxygen produced by cyanobacteria to offer a possible solution. Earth scientists have tried to figure out for some time how the earth's atmosphere shifted gears. This proposal gives evidence that could change the way geoscientists view that transformation and may hold the key to future attempts at terraforming.
The Earliest Microbes and the Anaerobic Environment
When the earth was yet young, before the formation of an oxygen-rich atmosphere, the single-celled microbes that populated the globe breathed carbon dioxide. The first organisms that started to photosynthesize were cyanobacteria. The modern descendants of those bacteria do much of the same as their predecessors. In studying the transformation of the earth's atmosphere from something that couldn't support oxygen-breathing life to something that could, these cyanobacteria proved a key consideration. Unfortunately, modeling studies didn't jive with the hypothesis. Scientists were stumped as to how the output of these bacteria could have led to world-changing occurrences such as the Great Oxygenation Event (GOE) around 2.4 billion years ago. The math simply didn't work out. One proposal was that photosynthetic organisms helped contribute to the GOE, but fossils for those types of organisms weren't found until well after the GOE occurred. What was then responsible for the spike in early earth's oxygen content?
The answer lies in something that most studies of cyanobacteria neglected to factor in - the earth's spin. We know that the earth's speed of spin has diminished over the years. Today's rate of spin is much less than it was when the earth was still in its early days of formation. The University of Sao Paolo's Institute of Advanced Studies mentions that, about four and a half billion years ago, the length of a day on earth was a mere six hours. The GOE, which occurred significantly after the earth's rapid spin had decelerated, saw days extend to around 21 hours. The correlation between cyanobacteria's photosynthesis rates and the length of the day might have been responsible for the bump in atmospheric oxygen levels to approximately 20% of today's value.
Testing the Hypothesis
Seeing the correlation between oxygenation increase and the length of a day is one thing, but testing it to get realistic results is something entirely different. A team from the University of Michigan led by oceanographer Brian Arabic sought an environment that would mimic the low-oxygen settings of the early earth. They found it in one of the deepest areas of Lake Huron and extracted a mat of cyanobacteria from the locale. To test their theory, the researchers set up the mats and varied the amount of light they got to simulate the conditions of a longer or shorter day. Naturally, the amount of oxygen produced was more when there was more insolation.
The rates of photosynthesis for these organisms were calculated and plugged into equations to test the hypothesis for longer days being responsible for the bump in oxygen production. Both variables moved alongside each other. Even the GOE corresponded with a 21-hour day, which scientists expected would happen, given the results. As days lengthened, cyanobacteria managed to produce far more oxygen than they consumed, suggesting that the GOE was probably due to those more extended periods of insolation. Other oxygenation events such as the Paleozoic Oxidation Event and the Neoproterozoic Oxygenation Event were also due to more extended daytime periods. These periods were due to friction in the earth's spin or interactions with the moon's gravity.
A Shorter Day Means Less Oxygen Produced
Cyanobacteria consume oxygen and, when there is a balance, they will produce just about as much as they consume. However, when there is a slower spin to the earth's surface, these bacteria produce excess amounts of oxygen, which make its way out of the oceans and start oxygenating the atmosphere. This massive oxygenation and subsequent events were crucial in helping the earth develop an atmosphere that could support larger and more complex organisms. Had the earth remained the barren place it was before, it's unlikely that organisms would have advanced to such a degree that they could leave the oceans for the land. The results explain quite accurately the lack of photosynthetic fossils for such a long time in the record. Cyanobacteria may have been the most significant contributor to the change in atmospheric composition, but they were by no means the only cause. Changes in levels of oxygen-binding materials from volcanoes that pockmarked the earth in its early days may have also contributed significantly. More research needs to be done to confirm these findings. So far, the hypothesis looks so bright that you may need prescription sunglasses to see it.