Thursday, 20 October 2011

New Research Sheds Light Upon The Great Oxygen Catastrophe

Ancient Banded Iron Formation
Around 2.5 billion years ago, free oxygen appeared in the atmosphere (free simply means not bonded into a compound). This gas is vital to many lifeforms on Earth. We use it to oxidise glucose molecules in the process of aerobic respiration to produce energy in order to live. The same process is carried out by most creatures on the planet, from giraffes to ants. Without free oxygen, we could not survive. However when this life-giving gas first appeared, it was poisonous to all.

When life first appeared, there was no free oxygen in the atmosphere and therefore early organisms had to find a different way of breaking down glucose to generate energy. They used a process called anaerobic respiration. This form of respiration can be expressed using a number of different chemicals and means. The introduction of oxygen into the atmosphere meant that many of the compounds and elements that the bacteria used in anaerobic respiration combined with the new gas, changing their chemical nature.

The bacteria would have either have been poisoned or unable to respire. Eventually, they evolved in order to live with this new gas and use it to generate energy from glucose. This was a big step in evolution as aerobic respiration produces more energy than anaerobic respiration, therefore accelerating the pace of evolution and increasing the levels of complexity which organisms could achieve. However two questions arise from this. The first is long standing: when did free oxygen first appear in the atmosphere? The second: when did bacteria first begin to utilise this new and, at first, unwelcome gas?

A recent study of ancient rocks called banded iron formations has shed light upon these two mysteries. A team from the University of Alberta led by Professor Kurt Konhauser and Professor Mark Barley from the University of Western Australia's Centre for Exploration Targeting made a study of the chemical traces within these formations.

Banded iron formations (BIFs) are very useful to scientists studying the conditions of the early Earth as they preserve the chemical traces found around them. BIFs are simply layers of iron oxide that form as the chemical is deposited by running water. As well as recording the levels of oxygen in the atmosphere, they preserve any chemicals that were in the water which created them. The team found, that as the BIF formations got younger, the contained levels of chromium increased from around 2.48 billion years ago.

They believe that this was connected to the respiratory activities of bacteria living near the formations. By using the data, they came to the conclusion that cyanobacteria were excreting oxygen as a waste product. This waste gas was then absorbed by aerobically respiring bacteria called chemolithautorotrophs (the name given to organisms which use the chemical compounds that make up rocks ) in order to oxidise pyrite rocks in a process which creates glucose.

When the pyrite was oxidised, it would have been broken down by various acids within the rock (pyrites can react with many chemicals to form acids). These acids would have destroyed the rock, carrying with it chromium compounds and sulphates which would have dissolved into the surrounding water and been laid down in BIFs. Yet what does this tell us about the Great Oxygen Catastrophe and its effect upon early life?

By studying the date of the BIFs, the team found that the chromium increase occured between 2.48 and 2.32 billion years ago. If their complex chain of organic chemistry is correct, then the two dates mark the period in which organisms evolved to be able to use oxygen in respiration. Fossil evidence at both ends of the time corridor support the theory. We have many potential fossils of aerobically respiring that are 2.5 billion years old.

The Francevillian Group Fossils, some of the oldest macroscopic and possible complex organisms on Earth, lie close to the other end of the time-scale at 2.1 billion years old. 'We think we've resolved a major debate about when the bacteria that produced oxygen existed and how long it took for oxygen levels to rise enough to support growth of life on earth,' said Professor Barley. Such events are so far back in time that fossil evidence becomes of little use. So studies like these shed light on such mysteries as they rely upon traces which are much more easily preserved and can be interpreted with greater ease, due to unchanging chemical laws.