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