Miraculous things happen to the desert when it rains – everything
changes from brown to green and organisms that have not been seen
for months make a brief emergence from underground lairs.
In fact, even the desert’s soil turns visibly green following the
rare desert rain, as hidden filaments of photosynthesizing cyanobacteria
suddenly hydrate. Lying a few millimeters deep, these primitive
prokaryotes quickly glide upward, migrating en mass to the surface
for an hour or so of light exposure until the dirt begins to dry.
Then, just as suddenly, they return again to the subsurface, where
they begin the long wait for the next rain.
The existence of such "cryptic" communities of microbes
has long been known, and it has long been assumed that the organisms’
behavior can be explained by common light-responsive behavior. Now,
a new finding by Arizona State University microbial ecologist Ferran
Garcia-Pichel and Olivier Pringault of the Biological Oceanography
Laboratory at the University of Bordeaux shows that phenomenon is
actually more complicated, with significant implications for the
behavior and ecology of other underground microbes. The research
is reported in the September 27 issue of the journal Nature.
Observing several different species of soil crust-inhabiting cynobacteria,
the team found that the bacteria’s movements were affected by the
presence or absence of water, not just light – the first time such
behavior has ever been observed in bacteria.
According to Garcia-Pichel, the team was first intrigued by a "serendipitous"
field observation. "What we discovered was that when one of
these wetting events took place, the cyanobacteria came up to the
surface of the soil. But once the soil started drying out, the cyanobacteria
returned to the subsurface though the light didn’t change. Essentially
nothing changed except the availability of water," he said.
Subsequently, the bacteria were moved to a laboratory setting and
were tested under controlled lighting conditions, using microprobes
to measure the relation of bacterial movement to water content in
the soil surface. Test results showed clearly that the bacteria
"tracked" the water.
"These migrations are really population migrations that occur
in millimeter scale — close to 100 percent of the population will
come up to the surface," Garcia-Pichel noted. "Their tendency
to track the water overwhelms their tendency to track the light.
We’ve never seen this before."
Water, Garcia-Pichel hypothesizes, is critical to the bacteria
not just for metabolism, but also for movement. "They go down
because by tracking the water, they protect themselves. They will
get dry eventually, and when they get dry they can’t move. At the
surface they would be more subject to hazardous conditions."
Garcia-Pichel points out that the finding may have large implications
for investigating the ecology of the still poorly understood bacterial
species that live deep beneath the earth’s surface.
"Once traits like this are found, they’re usually not restricted
to one organism. We’ve seen this in a variety of cyanobacteria.
If this really a widespread ability of bacteria, it also has implications
on how we understand the bacterial communities in the deep subsurface.
Bacterial communities may be following water in the subsurface over
large distances," he said.
Similarly, there are implications for locating life in another
extreme environment – Mars. Though cyanobacteria are among the most
primitive living things, they have developed sophisticated skills
for dealing with an environment where water is both scarce and transitory.
"Desert soils are one of the earthly ecosystems that may have
some significance on Mars. If Mars had some water in the past, then
these desiccation-resistant environments are probably going to be
the last to have existed there. This is one of the most likely ecosystems
to have left an imprint that we can find some evidence for,"
Garcia-Pichel said.
"’Follow the water’ has become a productive shorthand for
expressing the scientific directions of our exploration of Mars,
and beyond," said Rose Grymes, Associate Director of the NASA
Astrobiology Institute, of which Arizona State University is a member.
"This fascinating research contributes directly to our understanding
of how living systems adapt to and impact the planetary environment,
and how they leave their signature; even in places that appear highly
inhospitable."
The research was funded by a grant from the U.S. Department of
Agriculture.
James Hathaway,
(480) 965-6375
Photos: http://lsweb.la.asu.edu/fgarcia-pichel/moab.html
Source: Ferran Garcia-Pichel, 480-727-7534
