September 25, 2012
Land-based Microbes Were Producing Oxygen 2.7 Billion Years Ago
Brett Smith for redOrbit.com - Your Universe Online
Life on earth began in the oceans, but it would eventually spread to land and a new study suggests that land-dwelling bacteria could have covered large swaths of territory about 2.7 billion years ago, despite a thin ozone layer that would have offered little protection against the sun´s ultraviolet radiation.
According to astrobiologists at the University of Washington, these earliest microbes produced oxygen and broke down pyrite, an iron sulfide mineral, releasing sulfur and molybdenum into the primordial seas.
"This shows that life didn't just exist in a few little places on land. It was important on a global scale because it was enhancing the flow of sulfate from land into the ocean," said Eva StÃ¼eken, a UW doctoral candidate and the lead author of the report that appeared this week in Nature Geoscience.
The influx of sulfur into the ocean is extremely important in the development of life on Earth, because it would have enabled marine microbes to thrive on methane, paving the way for oxygen to slowly dominate the atmosphere. Until this time, most of the oxygen created by processes like photosynthesis was absorbed by reactions with methane that rose from the ocean into the air.
By analyzing existing data and models of the primordial sulfur cycle, StÃ¼eken and her colleagues were able to determine the terrestrial oxidation of pyrite by microbial action. The comprehensive review culled data from almost 1,200 marine sediment samples.
"The data has been out there for a long time, but people have ignored it because it is hard to interpret when it is not part of a large database," she said.
According to StÃ¼eken, volcanic activity could have also released massive amounts of sulfur, but her research showed that molybdenum was being released at the same time and this suggests that both substances were being liberated by bacterial action on continental rocks.
The study indicates that if land-based microbes were producing oxygen 2.7 billion years ago, it predated the "Great Oxidation Event" by about 300 million years. That event essentially produced the atmosphere that we know today.
"It supports the theory that oxygen was being produced for several hundred million years before the Great Oxidation Event. It just took time for it to reach higher concentrations in the atmosphere," StÃ¼eken said.
Many experts believe that the Great Oxidation Event began when oxygen absorbing compounds became saturated. This allowed the resulting excess oxygen to build up in the atmosphere.
One of the consequences of this oxygenating event was probably the wiping out of a large amount of the Earth´s anaerobic microbes that once dominated the planet. To Cyanobacteria, oxygen is toxic and the Great Oxidation Event represents one of Earth´s largest extinction events.
Another side effect of the GOE was the massive amount of glaciations that occurred as a result of the interactions between methane and oxygen. As the methane was oxidized into carbon dioxide and water, glaciers stretched out for several hundred million years. This time period is dubbed the Huron glaciations period because of the evidence it left behind around Lake Huron that was later collected and analyzed by researchers.