Monday, 10 June 2013

On The Origins Of Life's Phosphorus

During the early 19th century, one of the many products produced by British factories were matches.  One of the active, combustible ingredients was phosphorus. Its use, however, came at a price. The factory workers suffered from a condition known as 'phossy jaw.' The raw phosphorus caused the bone to decay in a painful and debilitating process.
The effects of 'phossy jaw. While incredibly dangerous in its pure form,
organic compounds of phosphorus are vital to living cells
Yet despite its toxic and combustible nature, life could not survive without it.

Phosphates, compounds of phosphorus and oxygen, make up the backbone of DNA molecules in conjunction with a form of sugar. Adenosine triphosphate (ATP), an organic, phosphorus-containing compound is the energy storing molecule used by every living cell on the planet. Therefore, a major challenge facing biochemists investigating the origins of life is to identify a source of phosphorus on the early Earth.

Due to its reactive nature, it is not found in nature in its pure form, and as most compounds of the element are toxic, only certain molecules are suitable. Yet certain meteorites contain a mineral called schreibersite, the chemical formula of which is (Fe, Ni)3P. It is almost non-existent on Earth, with one tiny deposit on Disko Island off the coast of Greenland.

Dr Terry Kee from the University of Leeds realised that the schreibersite in meteorites could act as the source that bichemists were looking for. ATP is used by cells not only as an energy source, but as a way of building organic molecules vital to their existence such as proteins. Kee found a way to generate a chemical precursor to ATP using schreibersite. His method is beautifully simple. First, he took samples of naturally occurring acidic solutions from thermal springs in the Hveradalur geothermal area in Iceland. To each he added a small piece of material from a body known as the Sikhote Alin meteorite which contained schreibersite, then left the samples for incubation in the hot spring for four days, and then a further thirty at room temperature.
A reaction between phosphorus-containing minerals and compounds
within geothermal springs on the early Earth may have created a
primitive energy source for the first cells

When the contents were analysed it was found that a molecule known as pyrophosphite had formed. Pyrophosphite could react with a number of oxygen-containing compounds to create pyrophosphate, a precursor to ATP.

The question was could the same reaction occur in nature? Thermal springs with the same chemical composition as those at Hveradalur almost certainly existed on the early Earth. Schreibersite has been recorded in many other meteorites besides the Sikhote Alin specimens, all of which came from a body of rock 4.5 billion years old, showing that the source of phosphorus was present in the early days of the planet.

Matthew Pasek from the University of South Florida has confirmed that such a process may have occurred in nature. Pasek and his team extracted rock cores from localities in Australia, Zimbabwe, West Virginia, Wyoming and Florida and analysed their mineral composition. The oldest samples, collected from the 3.5 billion year old Archaean age Coonterunah carbonates in Australia, contained a mineral called phosphite. 

In Kee's experiment, this mineral was produced when the meteoric schreibersite dissolved in the acidic water from the thermal springs, which eventually formed pyrophosphite. While the latter molecule was not found within the Coonterunah carbonates one of its components, the phosphite salt, was present, meaning that the Archaean Earth had the capacity to manufacture the components of Kee's experiment and produce both an ATP precursor and a usable source of phosphorus.

An artist's impression of the first barren continents of the Archaean Earth
shrouded in a thick chemical laden smog. Yet life may have first evolved
the pools around the base of those rocky towers.
Phosphite is an unstable mineral and so the only way it could have been deposited within a carbonate formation was if conditions on the Archaean Earth were different enough to allow it to exist without decaying or reacting with other compounds in the surrounding environment. 'The present research shows that this is indeed the case,' said Patek. 'Phosphorus chemistry on the early Earth was substantially different billions of years ago than it is today.'

Other natural sources of phosphite include lightning strikes, geothermal fluids and occasionally microbial activity under extremely anaerobic condition, but no other terrestrial sources of phosphite have been identified. None could have produced the quantities of phosphite needed to be dissolved in early Earth oceans that gave rise to life. The only other available source would have been from the dissolution of schreibersite contained in meteorites.

'Meteorite phosphorus may have been a fuel that provided the energy and phosphorus necessary for the onset of life,' concludes Pasek.Yet while a more conclusive link will be needed before we can definitely say that life's source of phosphorus was extraterrestrial in origin, what this study does give us is a distinct method of manufacturing an ATP precursor, which may have made the difference between the first cells or a lifeless planet.