Wednesday, 11 January 2012

New Research Provides Insight Into The Biochemical Mechanism Which Caused The Oxygen Catastrophe.

Banded iron formations composed of layers of iron oxide provide direct
geochemical evidence of the appearance of free oxygen in the atmosphere
Between 2.4 and 2.3 billion years ago, free oxygen appeared in the atmosphere. Yet the entire planet was transformed in just 100 million years. Free oxygen allowed organisms to manufacture collagen proteins and become multicellular. It allowed for more efficient and faster metabolic processes, thereby speeding up the rate of evolution. We have evidence of how it happened. We know its effects upon early life and when the change began. However, as with most evolutionary events, there was a mystery.

Oxygen, in its free form, is toxic, and in a world where all organisms respired anaerobically, it would have had destructive capabilities, comparable to those which caused the Permian Great Dying. This raises a question: what organism could produce a waste product on a global scale, a waste product which would cause its demise? A quick and easy answer is that life processes are numerous and there are many which release waste oxygen. 

There would have been organisms which used such processes and hence, oxygen appeared in the atmosphere. Now scientists believe that they may have found geochemical evidence for the precise chemical pathway that led from anaerobically respiring bacteria and archaea to a world dominated by an oxygenated atmosphere and the aerobically respiring organisms. A catalyst is a substance which speeds up the rate of a chemical reaction. 

The name given to biological catalysts are enzymes, a group of proteins present in all organisms. These are used by all life processes, such as digestion or growth and repair. By looking at traces of these enzymes, a team of scientists, led by Dr Gustavo Caetano-Anolles from the University of Illinois Crop Sciences  and Institute for Genomic Biology, believe that they have found the so called 'molecular culprit.' They studied a region of the proteins called folds as they are the most stable parts of the enzyme, being more resistant to changes of function or amino acid sequence; the building blocks of proteins.

These act like molecular fossils and by calibrating their origins with the emergence of different microbial fossils, Caetano and his team were able to identify the most ancient reaction of aerobic metabolism: a reaction involving the synthesis of pyridoxal, a form of vitamin B6, using the oxygen-generating enzyme manganese catalyse. They found that this metabolic pathway appeared around 2.9 billion years ago.

At this point one might say that the Oxygen Catastrophe only began 2.4 billion years ago. Yet this can be countered by the fact that, for oxygen to appear in the geochemical record, there has to be sufficient quantities of it in the atmosphere. Therefore an oxygen spike would only become detectable after the event, in this case around 400 million years; enough time to allow micro-organisms to generate sufficient levels. The specific enzyme type, catalase, converts hydrogen peroxide into water and oxygen as part of the metabolic pathway.

The team hypothesise that bacteria or archaea evolved to be able to use manganese catalase in order to deal with hydrogen peroxide rich environments. Some geochemists believe that the oceans of the early Earth were saturated with hydrogen peroxide, as the lack of an ozone layer would have allowed intense solar radiation to bombard ancient glaciers, converting many tonnes of ice to hydrogen peroxide, resulting in environments flooded with the compound.

Early life forms would have simply found a way to metabolise this abundant source of potential energy, using manganese catalase to decompose the hydrogen peroxide. This would provide the organism with energy to live whilst simultaneously creating the waste products of water and free molecular oxygen which, over 400 million years, would have accumulated in the atmosphere until a sudden spike 2.4 billion years ago. The result is the appearance of an oxygenated atmosphere and aerobically respiring organisms.