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Phosphate Does A Body Good

Artist's representation of Cambrian marine life, based on fossil discoveries. Did increases in oxygen or phosphate play a role in their evolution?
by Leslie Mullen
for Astrobiology Magazine
Moffett Field CA (SPX) Jun 13, 2007
Life can be found nearly everywhere on our planet, but in a sense it has struggled to survive throughout Earth's history. One often-cited estimate says 99 percent of all life forms that ever existed have gone extinct. Individual species extinctions can be chalked up to the limitations of an organism, but mass extinctions involve the loss of many different species at the same time. Scientists think something catastrophic must occur during mass extinction events to make the environment widely unlivable.

For instance, a large meteorite is thought to have caused the K-T mass extinction of the dinosaurs and many other creatures 65 million years ago. Such massive meteorite strikes can upset chemical balances in the environment far from the site of impact by triggering forest fires, earthquakes, acid rain, and possibly even volcanic eruptions.

Jim Elser of Arizona State University studies ecological stoichiometry, or how multiple chemical elements interrelate in an environment. Such studies can help scientists better understand what causes life to thrive or die out.

Elser has been investigating what may have contributed to the Cambrian explosion that occurred approximately 543 million years ago. This event in the far past is called an "explosion" because of the wide array of animal forms that seem to suddenly appear in the fossil record.

Prior to the Cambrian explosion was a mass extinction of very different types of life forms known as "Ediacaran biota." Was the extinction of one kind of life related to the emergence of another? If so, what role did the environment play in tipping the scales?

Elser has been traveling to Cuatro Cienegas, Mexico to better understand how the chemistry of the Precambrian environment may have affected the evolution of life. Spring-fed pools in the Cuatro Cienegas valley contain different types of stromatolites - microbial mat communities that represent the earliest life known to exist on Earth.

The water the Cuatro Cienegas stromatolites live in is exceptionally low in phosphorus, an element that forms the backbone of DNA and is vital for many functions of life. But life in the phosphorus-poor pools has evolved to compensate for the lack of that element.

This scarcity of phosphorus makes Cuatro Cienegas a good analogue for the Precambrian Earth. The rock record shows that phosphorus, once scarce, became abundant around the same time as the Cambrian explosion. Could phosphorus be the key to unlocking the mystery behind the Ediacaran extinction and the sudden emergence of animal life on Earth?

Phosphorus is rarely found in its elemental form in nature - normally it is bound to other atoms such as oxygen and exists as phosphate molecules. Elser added different amounts of phosphate to the Cuatro Cienegas water. The stromatolites absorbed it, and then snails that fed on the stromatolites also took in the extra phosphate.

Moderate levels of phosphate addition seemed to benefit the snails, but adding higher levels of phosphate eventually caused them harm. Elser thought at first this finding was unique because the Cuatro Cienegas ecosystems are especially deficient in phosphorus, and the snails had adapted to live in this low phosphate environment.

Then he discovered studies of other animals which also showed that too much phosphate has a negative effect on their growth. This led Elser and his colleagues to suggest that animals exist on a phosphate "knife edge" where too little phosphate in the food supply provides poor nutrition, but too much is harmful.

"Phosphate is central in cellular metabolism in all kinds of ways - ATP turnover, nucleic acid synthesis, and other pathways," says Elser. "Cells maintain the phosphate concentrations in the cytoplasm extremely tightly; it's very strongly regulated. But if you have too much phosphate, then those equilibrium phosphate concentrations in metabolism get out of balance, and reactions in the cell are impaired."

Elser wonders if the Ediacaran biota, first stimulated by initial increases in the phosphorus supply in the biosphere 600 million years ago, may have been poisoned by further influxes of phosphate recorded in the early Cambrian rock record. The Cambrian animals that followed needed to find a way to accommodate the increased phosphate.

Elser suggests the solution, for at least some of the animals, was the ability to produce the mineral apatite. This calcium-phosphate mineral was deposited as hard body parts, and apatite is still the main component of our bones today.

"Your bones have more phosphorus by mass in them than calcium," says Elser. "Everyone knows you've got to drink milk to get calcium, but everyone forgets about the phosphorus part of the mineral. Our argument is that the first function of apatite formation in animals was not for structural support the way it is used now. Instead, it was originally a detoxification mechanism as a way of preventing excess dietary phosphate from affecting physiology."

The emergence of hard parts in animals by phosphate deposition could account for the "explosion" of fossils in the Cambrian rock record, since hard parts are better preserved over long time scales than soft body parts.

Elser also says that the increased phosphate - after eons where there was very little phosphate available in the environment -- may have allowed multi-cellular life forms to proliferate.

"Our proposal is that during the very early Earth, the phosphate deficiency contributed to holding the animals back," says Elser. "Any multi-cellular consumers didn't have a viable lifestyle because the food was too nutrient-poor."

Bruce Runnegar, an astrobiologist at UCLA, doesn't think phosphate played such a central role in the extinction of Ediacaran biota and the Cambrian explosion which followed.

"I would put more money on oxygen rather than phosphate as an intrinsic trigger of the Cambrian explosion," says Runnegar. "Phosphate may have been a follower rather than a leader."

For most of Earth's history, life was single-celled. The more widely accepted answer for why is that oxygen limits the body size of animals. Early Earth had little atmospheric oxygen. While the main component of the air was mainly nitrogen as it still is today, the other major gases were probably carbon dioxide and methane.

The evolution of photosynthetic life led to the production of oxygen, but it would have taken a very long time for this gas to build up in appreciable amounts in the atmosphere. Eventually, perhaps an oxygen threshold was reached that allowed larger animals to exist.

Elser acknowledges that the oxygen hypothesis for the Cambrian explosion is compelling in many ways, "but if you see something happening quickly, you'd think the cause of that event also would be happening quickly." He says the Cambrian phosphate deposits are remarkable because throughout Earth's history there was little phosphate available, and then suddenly there's a huge spike.

What caused so much phosphate to be released into the environment at one time? It may be due to the shifting of continents. During this era in Earth's history, most land was locked up in a supercontinent called Rodinia. Than about 750 million years ago, the supercontinent began to break apart into smaller land masses. This breakup continued to occur up to the Cambrian explosion, and may have led to exposure and weathering of phosphate-bearing rocks that had once been buried.

Or life itself may have been the cause. Although scientists have long believed that Precambrian organisms only lived in the oceans, recent discoveries have indicated otherwise. There is some evidence that microbial mats and even early plants could have colonized land about a billion years ago.

If so, then they would have greatly accelerated the mineralization rates of the rocks they lived on, and the released phosphate would have cycled into the ocean and affected marine life.

The production of fecal pellets by animals with evolved guts might be another explanation, suggests Runnegar. The new digestive tracts would have allowed animals to take in phosphate and perhaps even process it -- with enough animals and enough time, the concentration of phosphate in fecal matter would have resulted in large phosphate deposits.

Runnegar discounts supercontinent break-up as a cause of the phosphate spike, saying that the major phosphate deposits being mined for fertilizer don't seem to have been generated by such an event.

Runnegar notes that other types of body mineralization also appear in the fossil record along with apatite. There is aragonite, or calcium carbonate, which most bivalve animal secrete to form shells or pearls. Calcite, another form of calcium carbonate that forms limestone, is secreted out of seawater by corals, algae and diatoms to form their shells.

And opal, a hydrated silicon dioxide, is produced by various organisms to form hard body parts. Runnegar says all four types of biomineralization appeared almost simultaneously, but it is unclear why.

"Why some organisms used phosphate whereas others used silica or carbonate is hard to determine," adds Runnegar. "An old idea of phosphate first because it was easier now seems incorrect."

The cause of the Cambrian explosion and the mass extinction that preceded it is made complicated by the seemingly infinite pieces that make up the whole puzzle. Figuring out how the different environmental elements may have interacted is difficult, in part, because the world back then was such a different place then the one we now experience.

Whether phosphate is a key piece in that puzzle, merely a contributing factor, or simply a side effect of other events remains a mystery. What is certain is that the development of such biomineralization made nature the bloody battlefield of tooth and claw versus protective body armor that continues to this day.

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