Rock soup

Matt Clark uses an air-powered chisel to shave away weathered patches on a rock.  Once cleaned, the rock will be crushed to the size of sand and used for soil-generating experiments.


photo courtesy of Jason Torrey

Can life arise from simple rock?  

It seemed doubtful. At the edge of a makeshift science experiment, scientists poured powdered rock into a white plastic bucket filled with water to see if anything would grow.

Months later, this brown soup was teaming with life: clots of bacteria and waving algae clung to the sides of the bucket. The water’s surface was laced with sticky bubbles of slime-like growth.

Could this be what the primordial soup looked like?

This rocky concoction was a far cry from the churning oceans of microbes that scientists believe life evolved from billions of years ago.  But, Dragos Zaharescu, a postdoc at the University of Arizona, saw an important similarity.

“All these organisms live off of fresh rock soup,” he said.  “It may seem hard to believe, but this ecosystem-in-a-bucket shows how life can grow and thrive off the most bare-essential minerals from an unlikely source—rock.

“It’s just a test,” said Zaharescu, as he replaced the lid on the briny mixture. For him, this bacteria-filled bucket is only a side dish compared to his new project. Using a space-aged planting enclosure, Zaharescu hopes to see how plants, with the aid of some helpful microbes, can survive off nothing more than pure rock by transforming it into soil.

Zaharescu’s thick Spanish accent tells of years studying microbes in Europe.  The Romanian-born scientist came stateside to continue his work at Biosphere 2, the super-sized, three acre greenhouse rising from the Sonoran Desert just outside Oracle.  

The Biosphere, packed with plants and animals from all parts of the world, served as a live-in sustainability experiment for a group of seven scientists in the early 1990s. These Biospherians attempted to coexist with the sealed ecosystems—living off food and oxygen produced by the plants inside to show how life might survive in a similar closed habitat in space and other harsh environments.

Now, under the umbrella of the University of Arizona College of Science, Biosphere scientists like Zaharescu are aiming for a different type of sustainability based on a number of new earth-science experiments.

From Rock to Life

The fact that life can survive by extracting minerals from rocks is nothing new. Look no further than rock-eating lichens, small green and orange patches that grow on almost any rock in the wild.  Lichens collect their food from dust floating in the air and by digesting the rocks they grow on with acids and enzymes. This is fine for simple organisms, but for the plants Zaharescu is planning to grow— ponderosa pine and buffalo grass— he needs a nutrient-packed material to support their lifestyle—soil.

What is soil, and why is it so important?

Soil is a combination of crushed, weathered rock, clay, silt and organic matter from dead and decaying plants and animals. Soil forms the base of the food web, giving nutrients to the plants that provide oxygen and food to all forms of life, including humans.

Even the Earth’s climate is affected by soil. Carbon dioxide, a greenhouse gas, is trapped in soil by a process called carbon sequestration. Dying plants that have spent their lives breathing and absorbing CO2 decompose into carbon-rich organic material that becomes part of the soil.

“There is 5 times as much carbon stored in the soil as in life biomass globally,” said Katerina Dontsova, a research professor at Biosphere 2.  She believes soil has huge potential for helping with the world’s CO2 troubles.  “Depending on what we’re doing to the soil and how we’re growing crops, we can substantially increase how much [carbon] is being left in the soil.”

So how can fertile soil come from a lifeless rock?

The complex process of making soil, called soil genesis, involves both the chemical and physical breakdown of rocks.  Zaharescu thinks one additional piece to the puzzle is the interaction of life itself with rocks. “On Earth we have bio-geochemical cycling of elements, instead of just geochemical, as you have on other planets.” Looking at the white powder that covers the moon or the rusty reds of soil on Mars, anyone can see the difference from our own brown-tilled Earth. “On this planet, life has a very important role in modifying and shaping the landscape,” said Zaharescu.

What needs to be researched further is exactly how life modifies rocks to become soil. To do this, Zaharescu is building a special device.

It’s Not Your Mother’s Pressure Cooker

It stands roughly 4 feet tall and looks like something you’d find aboard a spaceship—a long, transparent, coffin-shaped box containing rows of foot-long tube planters. This is Zaharescu’s creation, a kind of mini-biosphere made out of Plexiglas and sealed completely from the outside environment.  

Zaharescu is building four of these sci-fi-esque enclosures.  Each will be filled with different combinations of crushed rock, plant seedlings and beneficial bacteria and fungi.  One of these combinations will hopefully make the perfect blend for creating soil.

Zaharescu is going to painstaking lengths to control everything that goes in or out of these controlled environments. Each device will be pumped full of filtered air and each planter given a set amount of water per week, but no fertilizers or other nutrients will be added. The plants will have to “mine” all the food they need from the crushed rock.

The typical nutrients in potting soils, such as phosphorus, potassium and other trace metals, are locked away in crystals within rocks where they are unavailable for use as food.  The plants and microbes in each planter can work together to produce a cocktail of different enzymes and acids to break down the rocks into minerals they can use, much like the lichens that eat through rock.

Zaharescu can’t use just any rock for this experiment.  He must find rocks in the wild that are untainted by weathering or enzymes from the environment to ensure there is no interference with his experiments.

The Perfect Ingredients

To make the best rock soup for his plants, Zaharescu started by finding the perfect rocks. His research group collected rocks from areas around Arizona and Mexico, hiking into collapsed magma chambers and extinct volcanic fields to gather literal tons of rock.

These rocks come in four “flavors”: basalt, schist, rhyolite and granite. Each of these has a different balance of minerals contained within, providing a palette of dining choices for Zaharescu’s plants.

In nature, rocks have to be broken down physically for them to become soil. Stones must be weathered into pebbles, sand and silt over thousands of years by tumbling rivers, ocean surf and blowing winds.


Zaharescu doesn’t have that kind of time

Instead, he opts for manpower and machines to pulverize his rocks. Every day he descends to a small side room in the musty depths of the Biosphere’s basement. Sunlight streams through dirty skylights, revealing dust-covered Biosphere equipment in what is now Zaharescu’s rock-processing workshop. The whirring of a rock chisel and the ring of a falling sledgehammer can be heard above the din of hissing pumps and humming electrical equipment.

One of Zaharescu’s technicians, Guillermo Molano, raises a sledgehammer above his head before smashing it down onto a rock.   When all that remains are smaller stones, Molano bends down to inspect the fragments, one by one, for signs of weathering.  

“[We] are going to study the evolution of landscapes,” said Molano “So we need to get samples that have not been exposed to weather.” He pointed to patches of brown spots on the shattered rocks. Weathering is one of the processes that break down rocks in nature.  It can appear as a dullish hue or rust stains caused by water or cracking on the surface. “When [rocks] have been exposed to weather, we clean them,” Molano said.

In a corner of the workshop another rock-technician, Matt Clark, sits at a table with an air-powered chisel, shaving away any weathering he sees on the rock fragments. The cleaned rocks are crushed further by machine to fine sand, which is sifted to remove particles that are too large or too small. The sand is then sterilized, resulting in a pristine, lab-grade sand.

The months of preliminary work for this project have been backbreaking for Molano and Clark, but the research payoff is potentially huge.

The Future

As his experiment gets under way, Zaharescu will measure how his soils are developing by analyzing the water that leaks from the bottom of his futuristic planters.  He will measure subtle differences in the amounts of nutrients dissolved in the water to see if the plant/microbe combinations are changing the crushed rock into life-bearing soil.

Dontsova points out that such an experiment could take a very long time to show results. “We are talking about geological scale processes,” she said. “You don’t have soil forming in two years.”  In fact, it takes a century or longer to produce just one centimeter of new soil by natural processes.

Still, Dontsova is hopeful that Zaharescu’s experiment will show results in the coming years.  The end product won’t be what we think of as soil, but it will give a better understanding of how living plants and microbes can interact with rock to enhance its life-giving properties.

But the possibilities don’t stop there. If his experiment shows promise, Zaharescu would like to try an otherworldly approach: making fertile soil from lunar or Martian soil. “There, you don’t have life,” he said. “But we can make it.” The potential of such endeavors —from growing food crops on the moon to reshaping the desolate Martian landscape to more Earth-like scenery—is enough to make science fiction fans giddy.

For now, Zaharescu is content to decipher the role of lifeless rock in our own life-filled world. “We’re all made out of rock,” he said. “If you see rock, it’s a source of life.”

(Editor’s Note: Jason Torrey is a student with the University of Arizona.)

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