Water bearCreature from the underworld: Scanning electron micrograph of
an adult water bear (tardigrade).Photo: Goldstein labCross-posted from
Cool Green Science.
Water bears? Fungi that strangle worms? Roots that send off
reconnaissance soldiers (that somehow report back)? There's a world of
bizarre organisms under our feet -- millions of species that are also
critical for life on Earth. Nature Conservancy scientist Sophie Parker
specializes in soil ecology, and has just published a paper called
"Buried Treasure: Soil Biodiversity and Conservation." I recently asked
her about her favorite soil critters and why we should care about what
lives in the dirt.
Q. What kind of organisms are we talking about here? Worms? Rats?
Microbes? Stuff that crawls away when you turn rocks over?
A. All those things and so much more. Soil organisms are very, very
diverse, in terms of their position on the tree of life and also in
terms of their body size. They can range in size from very large
burrowing organisms (such as ground squirrels and gophers) down to
actinomycete bacteria, which give soil its "earthy" smell.
Q. Is it peaceful down there, or is it Wild Kingdom underground?
A. It's pretty vicious down there. Life as a soil bacterium is not an
easy life. You are subject to drying out at any point in time. There's
patchiness in your food resources, so trying to get enough food is
difficult. You've got these protozoa that are scavenging, like big
grazers just waiting to suck you up and eat you. If you're a pathogenic
bacterium, you can try to attack a root, but there are all these things
that the roots can do to defend themselves from you. If all this were
blown up in size a million times, it would be very dramatic. It's just
that we humans can't easily experience it.
Q. Do you have favorites?
A. I love water bears. These are organisms that are about 200 microns
long, so about the size of a period at the end of a sentence in a book,
really tiny but not as tiny as bacteria. They can survive extreme
dessication, extreme heat, and extreme cold. They can even survive the
vacuum of outer space. They look like little eight-legged bears with
claws. They're an example of charismatic microfauna.
sophieSweating the small stuff: soil scientist Sophie Parker. Photo:
Andrea MartinQ. What else?
A. There are lots of examples. There are species of fungus that can trap
nematode worms, and the way they do it is by creating a nooselike
structure. The nematode worm digs its way through the soil, and when it
intersects, when it goes through the loop, the fungus can feel it. The
fungus then tightens the noose, strangles the worm, and eats it.
Q. Wow!
A. Here's another one, a kind of reconnaissance troop for plant roots.
As a root grows, it produces a layer of mucilage. It's a slippery
material that helps the root to glide through the soil, and as part of
that layer, there are these little cells that slough off called "border
cells." The cells have the same genetic material as the rest of the
plant, but they actually separate from the plant and move off into the
soil and are still alive. They have been found to survive in the soil
for long periods of time -- on the order of weeks. Here's the cool part:
As they move off into the soil, they send information back to the root
about the conditions in the soil.
Q. What are they saying?
A. Well, they could be telling the root which way to grow. We don't know
very well why roots grow the way they do. We know about the molecular
triggers that make a root turn to the left or turn to the right, but we
don't know how well roots can sense their environment. Can they sense
water from a distance away and grow towards it? These border cells are
part of the answer to that question -- they are kind of like foot
soldiers that go out and investigate the soil and send information back
about the condition to the root. But how those messages get back and
forth is a mystery.
Q. Why is there all this diversity in soil?
A. Because the niches underground are both very, very small and very,
very complex. You have huge gradients under the ground in temperature,
in moisture, and in the availability of air, carbon, and nutrients. In
everything that the organisms need to survive, there are huge gradients,
and it is a constantly changing environment as rock is eroding and dead
materials fall to earth and the microorganisms are decomposing it and
creating soil. It's the complexity that drives biodiversity -- there
are lots of different roles because there are so many things to do.
There are so many ways to survive.
Q. It sounds like an alien war movie. But here's the "So what?"
question: Why should we care?
A. Because soils and the organisms they contain can have a large effect
on biodiversity above ground. Let me give you an example. Mycorrhizal
fungi -- they are ubiquitous soil organisms found all over the world, on
the roots of most plants. We're just beginning to understand the
relationships between these fungi and their plants. One mycorrhizal
fungus species may cause one plant species to grow more, but it may
cause another plant species to grow less, and so depending on the mix
and match between fungi and plants, you get different outcomes for the
plant community above ground.
This can have big, very important outcomes for conservation. There are
some species of weedy plants, for example, that can take advantage of
relationships with mycorrhizal fungi and become more successful than
they would be otherwise.
nematodeMadonna and child: a soybean cyst nematode with egg, photo taken
with electron microscopy.Photo: Ethan Hein
Q. How did you get interested in this stuff?
A. I studied nitrogen cycling in soils in California grasslands for my
doctoral work. The first time I dug a soil pit was actually on Cape Cod
in a forest ecosystem there. The amazing thing about digging a soil pit
is each time you dig one, you don't really know what you're going to
see. Each time is a different layering of colors and textures, and soils
can be very beautiful, kind of like a layer cake as you're digging down.
You have no idea what it is going to look like until you begin to
recognize patterns between plants and soils, because there are strong
associations there. It's always kind of a mystery.
Plus, it's something that, in order to do it well, you have to be
willing to get dirty, and that's really appealing. It's a lot of fun.
It's like making mud pies when you were a kid, you know?
Robert Lalasz is the director of science communications at The Nature
Conservancy. A long-time editor and writer, he was previously the
Conservancy’s associate director of digital marketing and the editor of
Cool Green Science. He now blogs on Cool Green Science about the
Conservancy’s scientific research and on-the-ground work as well as
larger conservation science issues.
[permaculture] A scientist dishes on the wild kingdom beneath our feet | Grist,
Lawrence F. London, Jr., 03/08/2011