Weird place on Earth Mono Lake in eastern California is where researchers found a type of bacteria that appears to break the rules for how we think life should survive. Credit: NASA image gallery

You may not know phosphorus when you see it, but your body does. Phosphorus is a sturdy workhorse element. In DNA molecules, phosphorus helps support the whole double helix. Within cells, energy shows up as ATP — and the “P” stands for phosphorus (specifically, phosphate, a form of phosphorus).

All life as we know it, in other words, depends on phosphorus. For that reason, scientists around the world were shocked December 2 when a team of scientists announced finding life forms that didn’t necessarily depend on this all-important element. In laboratory tests, the scientists grew bacteria that were able to use arsenic — a different element with similar chemistry — in the place of phosphorus.

It’s a surprising discovery because living organisms have never been found without all six of the ingredients crucial to life: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (all together known as CHNOPS). Arsenic, though, is a potentially fatal poison.

Many scientists say they would like to see more evidence that the research team did in fact observe life forms using arsenic instead of phosphorus.

“This is an amazing result, a striking, very important and astonishing result — if true,” Alan Schwartz told Science News. Schwartz researches chemistry at Radboud University Nijmegen in the Netherlands. “I’m even more skeptical than usual, because of the implications. But it is fascinating work.”

The bacteria came from Mono Lake, a lake in eastern California that is well known for its unusual population of living organisms, including shrimp and algae. The lake doesn’t drain, so the only way for water to leave is through evaporation. As a result, the lake is much saltier than the ocean.

An up-close picture of the bacteria GFAJ-1 grown on arsenic. Credit: Jodi Switzer Blum, NASA

Several researchers had been studying a number of tiny organisms that lived in Mono Lake mud. Astrobiologists study life in the universe and want to know how it started, how it has changed, and what will happen to life in the future. They also want to know whether life exists on other planets and if so, what it might look like. Many astrobiologists study what lives in Earth’s strangest places, such as Mono Lake, as a way to understand the possibilities for life.

The study was led by Felisa Wolfe-Simon of NASA’s Astrobiology Institute and the U.S. Geological Survey in Menlo Park, Calif. She and her team removed organisms from the Mono samples and grew those bacteria in the lab. The scientists fed the microbes with sugar and vitamins — but left out phosphate. Then they changed the diet again, and gave the microbes arsenate, which is a form of arsenic.

In one type of bacteria, called GFAJ-1, the researchers observed that arsenic wasn’t fatal. The bacteria continued to grow, though not as fast as if they’d had phosphorus. After studying these bacteria, Wolfe-Simon and her team concluded that the organisms had begun to make use of the arsenic the way they usually used phosphorus. The researchers suggest that arsenic was being used as a building block in the bacteria’s DNA.

“This microbe, if we are correct, has solved the challenge of being alive in a different way,” Wolfe-Simon told Science News.

If the scientists are right, then “life as we know it” may not include all the life that actually is possible. For astrobiologists, that conclusion suggests that life on other planets may not necessarily look like life on Earth.

It’s possible that follow-up studies will show that the researchers were mistaken. Wolfe-Simon and her team could not get rid of all the phosphorus when they were growing the bacteria. Some scientists say minute amounts might be enough to keep the microbes alive. It’s possible that, in the experiment, the bacterium GFAJ-1 was still getting small amounts of phosphate.

Can life exist using poison instead of phosphorus? Life of a different type is an exciting prospect, so stay tuned to see how the scientific community reacts. Next up, scientists will want to know how, exactly, the arsenic substitution works.

POWER WORDS

arsenic A highly poisonous metallic element having three allotropic forms, yellow, black and gray, of which the brittle, crystalline gray is the most common. Used in insecticides.

phosphorus A highly reactive, nonmetallic element occurring naturally in phosphates.

DNA A nucleic acid that carries the genetic information in the cell. DNA consists of two long chains of nucleotides twisted into a double helix and joined by hydrogen bonds between the bases.

molecule A group of like or of different atoms held together by chemical forces.

microbe A minute life form; a microorganism, especially a bacterium that causes disease.

bacterium A life form that is a single cell and too small to see without using a microscope. Bacteria (plural of bacterium) live in almost every environment on Earth, including very cold places, very warm places, in all types of water, in the air, even on and in plants and animals. These microorganisms can also cause disease in plants and animals.

New England’s brilliant fall colors attract many visitors.

National Park Service

“Being in the Northeast during autumn is just about as good as it gets in this country,” says David Lee. He’s a botanist at Florida International University in Miami.

Lee studies leaf color, so he’s biased. But plenty of other people share his admiration. Areas of the United States with especially colorful fall displays attract thousands of leaf peepers.

Even as they “ooh” and “aah,” few people know what makes many plants blush in the autumn. Research has shown that leaves change color when their food-making processes shut off. The chemical chlorophyll, which gives leaves their green color, breaks down. This allows other leaf pigments—yellow and orange—to become visible.

No one knows exactly how global warming will alter forests and affect fall colors.

J. Miller

But “there’s still a lot we don’t know about this,” Lee says.

It isn’t clear, for example, why different species of plants turn different colors. Or why some trees become redder than others, even when they’re standing right next to each other. And no one knows exactly how global warming will alter forests and affect leaf-peeping season.

Food factory

In summer, when a plant is green, its leaves contain the pigment chlorophyll, which absorbs all colors of sunlight except green. We see the reflected green light.

The plant uses the energy it absorbs from the sun to turn carbon dioxide and water into sugars (food) and oxygen (waste). The process is called photosynthesis.

When chlorophyll breaks down, yellow pigments in leaves become visible.

I. Peterson

As days get shorter and colder in the autumn, chlorophyll molecules break down. Leaves quickly lose their green color. Some leaves begin to look yellow or orange because they still contain pigments called carotenoids. One such pigment, carotene, gives carrots their bright-orange color.

But red is special. This brilliant color appears only because the leaves of some plants, including maples, actually produce new pigments, called anthocyanins.

That’s a strange thing for a plant to do without a reason, says Bill Hoch of the University of Wisconsin in Madison. Why? Because it takes a lot of energy to make anthocyanins.

Why red?

To figure out the purpose of the red pigment, Hoch and his coworkers bred mutant plants that can’t make anthocyanins and compared them with plants that do make anthocyanins. They found that plants that can make red pigments continue to absorb nutrients from their leaves long after the mutant plants have stopped.

Red leaves get their color from a pigment called anthocyanin.

I. Peterson

This study and others suggest that anthocyanins work like a sunscreen. When chlorophyll breaks down, a plant’s leaves become vulnerable to the sun’s harsh rays. By turning red, plants protect themselves from sun damage. They can continue to take nutrients out of their dying leaves. These reserves help the plants stay healthy through the winter.

The more anthocyanins a plant produces, the redder its leaves become. This explains why colors vary from year to year, and even from tree to tree. Stressful conditions, such as drought and disease, often make a season redder.

Now, Hoch is breeding plants for a new set of experiments. He wants to find out whether turning red helps plants survive cold weather.

“There’s a clear correlation between environments that get colder in the fall and the amount of red produced,” he says. “Red maples turn bright red in Wisconsin. In Florida, they don’t turn nearly as bright.”

More protection

Elsewhere, scientists are looking at anthocyanins in other ways. A recent study in Greece, for instance, found that as leaves grow redder, insects eat them less. On the basis of this observation, some scientists argue that red pigments defend a plant against bugs.

Leaves may turn red in the autumn to protect themselves from the sun’s ultraviolet rays.

J. Miller

Hoch rejects that theory, but Lee thinks that it might make sense. He points out that red leaves contain less nitrogen than green ones do. “It may actually be that insects avoid red leaves because they’re less nutritious,” Lee says.

However, “it’s pretty confusing at this point,” Lee admits. “People debate back and forth.”

To settle the debate, scientists will need to look at more species under more conditions, Lee says. So, he’s now researching leafy plants rather than trees. He’s especially interested in tropical plants, whose leaves turn red when they’re young rather than old.

You can do your own leafy experiments. Observe the trees in your neighborhood and keep track of weather conditions. When autumn begins, write down when the leaves change, which species change first, and how rich the colors are. You can even see anthocyanins under a simple microscope. After several years, you might start to notice some patterns.

Going Deeper:

Additional Information

Questions about the Article

Word Find: Leaf Color

Now, a geologist from the Field Museum in Chicago says that she has found a new way to figure out how the shape of Earth’s surface has changed over time. Her strategy? Leaf peeping.

Pores that take in carbon dioxide appear in this microscopic view of a present-day California black oak. The sample was stained orange so that details would be more visible.

Jennifer C. McElwain, Field Museum

A tree’s leaves have tiny holes called stomata. These pores allow the leaves to take in a gas called carbon dioxide, which the tree needs in order to survive.

With this fact in mind, geologist Jennifer McElwain collected leaves from living California black oak. These trees grow in a wide range of altitudes, from sea level all the way up to 2,500 meters (8,200 feet).

McElwain used a microscope to count how many stomata were inside a given area of each leaf. She found that the leaves had more stomata at higher altitudes. Then, she came up with an equation that links stomata numbers and elevation.

The black oak has been around for at least 24 million years. So, scientists can now count stomata on fossilized leaves to figure out how high the trees were when they lived, McElwain says. By comparing this altitude with the altitude at which the fossils were collected, the researchers can measure any changes in elevation that had occurred.

The new method should be more accurate than previous methods, McElwain says. Next, she wants to come up with equations for other tree species.

Someday, she says, her research may help scientists answer a major question in geology: When did the Himalayas in Asia rise?—E. Sohn

Going Deeper:

Shiga, David. 2004. Ancient heights: Leaf fossils track elevation changes. Science News 166(Dec. 18&25):390. Available at http://www.sciencenews.org/articles/20041218/fob7.asp .

You can learn more about leaf pores (stomata) at www.microscopy-uk.org.uk/schools/images/stomata.html (Microscopy-UK) and www.accessexcellence.org/AE/AEC/AEF/1994/case_leaf.html (National Health Museum).

Information about the geology of the Himalayas can be found at jan.ucc.nau.edu/~wittke/Tibet/Himalaya.html (Northern Arizona University).