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Arizona State University
Tempe, Arizona

Contact:
James Hathaway, jim.hathaway{at}asu.edu, (480) 965-6375 

February 14, 2003

Astrobiologists find phosphorus' role in regulating
 ecosystems is unexpectedly complex in living lab
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For several years now, Arizona State Unversity astrobiologists have 
been doing experiments in a living lab for evolution -- a set of 
remarkable desert springs located near Cuatro Cienegas, Coahuila, 
Mexico which may give scientists a modern "analogue" for conditions 
during the transition from Earth's ancient bacteria-based biosphere to 
the present, more complex one. 

The area's numerous clear-blue, spring-fed pools and outlet streams 
each have their own unusual thermal and chemical conditions and their 
own distinctive biota that have evolved to cope with the specific 
environments. Like a Galapagos Archipelago in reverse, these islands 
of water set in a sea of desert give the research team a range of both 
benign and extreme environments and their accompanying food webs to 
study in relation to the Cambrian transition. 

A series of papers being presented at the 2003 Astrobiology Institute 
General Meeting describe experiments being done in this unique setting 
that may ultimately help explain how the earth's biosphere became what 
it is today.

One of the hypotheses that the researchers have been eager to test at 
Cuatro Cienegas is the concept that the relative availability or lack 
of availability of certain critical nutrient elements in the early 
earth could have had a profound effect on the evolution of 
multi-celled organisms. The critical elements for life include carbon,
oxygen, nitrogen (without which there can be no proteins), and, as 
these investigators are emphasizing, phosphorus. 

The last of these, phosphorus has intrigued researchers because it is 
relatively rare in the environment, yet it is a critical component in 
nuclic acids, including ribonucleic acid (RNA), which is, in turn, a 
critical part in the cell's growth process. RNA is DNA's messenger and 
the cell's factory worker in the process of creating proteins. 
Previous work has shown that organisms must produce large amounts of 
this phosphorus-rich RNA to sustain rapid growth and therefore need to 
have relatively high concentrations of phosphorus available to them to 
be successful. An intriguing fact from the fossil record is that there 
are significant phosphorus- rich deposits ("phosphorites") that date 
to approximately the period of the Cambrian Transition (540 million 
years ago), when eukaryotic life took charge.

>From this, a logical question has arisen: could the limited 
availability of phosphorus have been responsible for keeping 
eukaryotic cells from exploding into multi- cellular life for three 
out of the 3.5 billion years that life has existed on earth?

In addition to having a variety of different chemical and thermal 
environments nearby to compare, Cuatro Cienegas is also interesting 
because many of the springs contain microbe-based ecologies that may 
bear a certain amount of resemblance to the ecologies that may have
existed at the period of the Cambrian Transition from a strictly 
single-celled biosphere to the biosphere dominated by multi-cellular 
organisms that we live in today.

The ecologies of many of Cuatro Cienegas's pools and run-off streams 
depend for food on stromatolite (coral reef-like deposits) forming 
microbial mats (based on photosynthesizing cyanobacteria), that in 
turn support "grazing" snails, that then support fish, and so on. A
key question then is: does the amount of phosphorus available to the 
these microbes affect the success of the (eukaryotic) snails that 
graze on them? A positive answer to this question might be a strong 
early indication of the importance of available phosphorus to the 
evolutionary dominance of multicellular eukaryotic life.

If only life (and science) were so simple. In 2001 and 2002, ASU 
biologist and Cuatro Cienegas project co-principal investigator James 
Elser and colleagues ran three experiments to test the hypothesis with 
mixed results.

"We added phosphate to the water and the cyanobacteria that were in 
the stromatolite mat picked it up," said Elser. We had some questions 
we wanted to answer: How does the phosphate effect the microbial 
producer community, the carbon to phosphorus ratio of their biomass? 
How does phosphate rearrange the community structure of the bacterial 
mat? Second, given that you've added phosphate to the system and done 
something ecologically or physiologically to the microbial mat, what 
effect does that have on the consumers of that material, the snails?"

Elser's group did, in fact, discover that the microbes readily 
increased in phosphorus content with the addition of phosphate, but 
the effect on the snails eating the phosphorus-rich bacteria was not 
completely expected.

"We were surprised. The first year we thought we got evidence that 
supported our hypothesis that the snails had been limited by the very 
low phosphorus content of the microbes that they were consuming. The 
snails themselves had a higher phosphorus content and appeared to be 
performing better," he said. "In 2002, however, we did the experiment 
longer and more extensively with more sites and instead of the 
stimulation response we got the opposite -- there was apparently a 
poisoning effect of some kind. It was too much of a good thing."

In the second round of experiments, Elser found that the mortality 
rate of snails was significantly higher on phosphorus-enriched 
stromatolites than on the untreated ones. What's more, new snails 
didn't move in to replace the dead ones -- regular numbers of new
snails were observed in untreated colonies, but not in the treated 
ones.

Elser points out that the snails in question are probably adapted to a 
phosphorus poor environment, so, in the short-term, a significant, 
sustained phosphorus increase is harmful to the population.

"The snails are normally exposed to a low phosphorus diet. When we 
improve that diet moderately for a short while, they do better, But 
when, like in 2002, we go too far and give them too much phosphorus, 
they don't.  They are apparently so well adapted to acquiring
phosphorus, so used to being phosphorus limited, their metabolisms 
don't know what to do," he said. "We think that perhaps the snails 
have adapted to the extremely phosphorus-limited environment and are 
living on a stoichiometric knife-edge with regard to phosphorus."

Of course, an increase in phosphorus supply over an evolutionary time 
period could have a different effect, eventually changing the 
population by selecting for snails who could withstand higher 
phosphorus concentrations, and even take advantage of them.

"You could imagine exposing them to slowly increasing phosphorus 
content in their diet and allow some sort of natural selection process 
to take place," Elser said, "but that's a different experiment."

The NASA Astrobiology Institute is composed of over 700 researchers 
distributed at more that 130 research institutions across the United 
States. Its central offices are located at NASA Ames Research Center, 
in the heart of Silicon Valley, California. Additional information 
about the NAI can be found at its website:  http://nai.arc.nasa.gov

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