Life Beyond Boiling

At 212 degrees water turns to steam, and proteins--the very stuff of life--turns to glop. But in some canisters in Georgia, a select group of creatures are just warming up.

By Will Hively|Saturday, May 01, 1993
Mike Adams is probably the only researcher in North America growing microbes that can flatten cinder-block walls. He doesn’t like you to confuse them with ordinary life. Let me call them organisms, he says. They’re not really bacteria. But they do have some nasty habits, a product no doubt of their bad environment. Most of them originally came from hot springs at the bottom of the ocean--spectacular smoking, sulfur- rich caldrons where pressurized water shoots from volcanic vents at temperatures as high as 700 degrees.

As you step inside the room where these tough little creatures grow, a strong metal door slams impressively shut. Cinder-block walls and a plain cement floor add bunkerlike touches. The incubating tank--a brand-new 120-gallon fermenter, freshly painted blue--looks sturdy enough for deep- sea diving. No one works near it while the microbes grow. A bundle of wires leads from the incubator to a sealed-off, computerized control room next door. The wall opposite this control room is designed to blow out under pressure. Everything is explosion-proof, Adams says.

Most of the organisms we work with are decomposers that make hydrogen, he explains. They feed on sulfur and give off hydrogen sulfide, which gives the fermenter the pleasant aroma of crate upon crate of rotten eggs. As a bonus, they also give off a little of the same explosive gas that filled the Hindenburg. But Adams says this is a minor consideration.

His microbes’ craving for heat, though, is a major consideration. These organisms enjoy being simmered in boiling-hot water. That sets them apart from all other life. At 212 degrees, the molecules that we’re made of--that all life as we know it is made of--fall apart. DNA comes unglued, and proteins collapse in a tangled heap, usually within seconds. We depend on the murderous efficiency of boiling water to purify food and sterilize medical instruments. Yet Adams could boil his organisms forever. They don’t merely survive the heat, they thrive in it. It makes them grow and multiply. Many types, in fact, find 212 a bit tepid. A hotter bath, say around 220 in a pressure cooker, massages their molecules, perks up their peptides, and makes them even more eager to reproduce.

Adams, a biochemist at the University of Georgia, wants to find out how their impossible life is possible. These are completely new organisms, he maintains. They promise to revolutionize ideas on the origin and chemistry of life. It’s obvious that to grow at these extreme temperatures, they can’t be doing exactly the same things as other organisms, so they must be doing something different. They’re also doing a lot the same, and we don’t quite understand how they do it.

When Adams calls the organisms new, he means only that they are new to science. And what he really means is that they could be very old. He believes, in fact, that they are the most ancient form of life known to us, closely related to the first single-celled organisms that emerged on a young, hot planet. If this theory, originally proposed in the early 1980s by microbiologist Carl Woese of the University of Illinois, is correct, the chemistry inside these microbes points back toward the original chemistry of life--a chemistry that works best above 212 degrees.

Traditionally, Adams says, organisms that can grow at high temperatures have been thought to be conventional organisms that somehow adapted. The new theory turns the original idea on its head, which is kind of dramatic if you think about it. We’re the adaptation.

While adapting to cooler environments, our kind of life may well have shifted toward the lower end of biology’s temperature scale. If a bit above 98 degrees, which is normal for us, is actually on the chilly side, and if 212 is closer to comfy, what’s the upper limit? The chemistry inside these organisms could give us a better idea. It could also suggest where life might have evolved elsewhere in the universe: not necessarily on Earth-like planets.

Right now Adams doesn’t speculate much about other planets. The earthly organisms he’s growing are alien enough. His pursuit of these exotic creatures began in 1981 at Exxon Research in New Jersey. Adams landed there after getting his Ph.D. in biochemistry at the University of London in 1979 and spending a brief period at Purdue. He was investigating a bacterial enzyme, hydrogenase, that can rip hydrogen away from water molecules. Exxon apparently hoped that this powerful enzyme might be useful for refining petroleum; Adams just wanted to see how it worked. In 1986, however, Exxon cut back on basic research in biology, and Adams decided to leave.

He went on to the University of Georgia, where he began searching for microbes that metabolized hydrogen. If they did, they would have an enzyme like hydrogenase. Looking through the literature, he found several reports by a German microbiologist named Karl Stetter. In 1982 Stetter had discovered the first organisms that thrive above 212 degrees, in shallow hot springs off the coast of Sicily. Later he and other researchers began finding them in vents up to three miles deep at the bottom of the ocean. Biologists had known that some bacteria could survive in hot springs such as those at Yellowstone, in water up to 190 degrees. These hot-springs dwellers were called thermophiles, meaning heat-lovers. The new ones, however, were off the heat-loving chart--hyperthermophiles.

I saw these organisms out there, says Adams, but there was no biochemistry. The timing was perfect. No one had purified any enzymes from them; no one knew anything about them. And they metabolized hydrogen. So I thought, ‘Well, hell, they can’t be that difficult to grow, even if they do grow at these extreme temperatures.’ He ordered two types from a culture collection in Germany, and they both grew. Well, in retrospect, they didn’t grow very well, Adams recalls, but we could harvest the cells. Even though he had read reports on them, Adams was still shocked by their seemingly impossible chemistry. These creatures indeed had hydrogenase--a superhot version. We could break them open, and we could measure the hydrogenase activity at 212 degrees. As far as I’m aware, that was the first enzyme purified from a hyperthermophilic organism.

Enzymes are the movers and shakers inside an organism, powerful proteins that bring other molecules together to produce the chemical reactions essential to life. Adams decided to let enzymes guide him through the strange terrain of life above 212 degrees, a landscape teeming with exotic species, beyond the known world on conventional maps. By following the enzymes, Adams thought, he’d find out how these creatures really worked.

The first step in isolating an enzyme from a microbe is to break open a gob of cells. You can squash them in a press, crack their cell walls with another enzyme, or shake them to pieces with sound waves. Then you centrifuge. After an hour at 19,000 g’s, cell wall debris packs down as leaden glop at the bottom of a tube. Enzymes from inside the cell remain in solution at the top. To sort them out, you pour the solution into a column packed with material that enzyme molecules, which are relatively large, have trouble penetrating. The larger or stickier molecules--those with a higher electric charge, which tend to stick to the filtering medium--take longer to reach the bottom. The work is often tedious, requiring several runs of several hours each to purify similar enzymes that refuse to separate. Someone has to be there when each enzyme comes out, and the column doesn’t care about day or night.

By sheer coincidence, says Adams, during the purification of one enzyme on one of these columns, we saw this bright red-colored molecule. So I said to one of my students, ‘Why don’t you purify that to see what it is?’ Since it was red, my idea was it might be a metal- containing compound, like heme. Heme is a molecule containing an iron atom, and it’s an essential part of the protein hemoglobin in red blood cells. The iron snaps up dissolved oxygen, and the combination appears red. Adams was very interested in metals. Metals, he says, make the world go round. As iron does with hemoglobin, metals often give an enzyme its power to accelerate a reaction. Adams and his student analyzed the enzyme for metals, and they did indeed find some iron. They also found something else. Lo and behold, says Adams, there was tungsten.

For Adams, the exciting thing about tungsten was its scarceness in ordinary life--only one other tungsten-containing enzyme had ever been isolated from a living organism. Now Adams and his co-workers on this molecular disassembly line began finding tungsten right and left. Each of five organisms they could grow had tungsten-containing enzymes, and those enzymes did not resemble anything the researchers had seen before. They seemed to belong to an unknown series of chemical reactions that the organisms used to break down sugars and proteins for energy, making hydrogen in the process. Adams’s team discovered that when they fed tungsten to these creatures, it stimulated their growth.

Although the tungsten-based metabolism set these organisms apart from ordinary life, additional enzyme purification revealed that the creatures also had conventional enzymes containing metals like iron, and they seemed able to use both metabolic pathways at the same time. It wasn’t clear where tungsten fit in the scheme of things. Was it part of an older chemistry that worked better at higher temperatures? Did the conventional enzymes evolve later to cope with cooler temperatures? If so, why didn’t they fall apart at high temperatures, as ours do? It was time to dig deeper, to descend from the level of chemistry--the reactions between molecules--to the level of the molecules themselves. To me, says Adams, it seemed we needed to answer a very fundamental question: What makes the molecules stable?

When a protein boils, it becomes unstable. It falls apart, and then it just falls--or, technically, it precipitates. The essence of a protein, says Adams, is that it has to be in solution. You might even say that water is part of its structure. Tiny water molecules, jiggling with heat energy, suspend the long chains of amino acids that make up proteins. In water at a congenial temperature, these stranded molecules twist and fold like balls of yarn. Water that’s too hot jiggles too hard and hammers at the folded strands until the protein unravels, tangles with other unraveling proteins, and precipitates out of solution. Egg white at room temperature is a rich solution of yarn balls. Boiled egg white is the same yarn in a jumbled, gelled heap.

Why didn’t the hyperhot proteins cook like egg white? The best guess Adams could make was that their strands have more cross-links than ours--weak chemical bonds at points where strands are coiled close together. Those interactions might keep them from uncoiling in hot water. The number of possible cross-links depends on a protein’s sequence of amino acids, which determines how the strands fold over on themselves.

To get a better grip on the problem, Adams turned to a protein called rubredoxin. What’s nice about rubredoxin, he says, is that it’s a very small protein. It contains 53 amino acids. This protein has been isolated from about a dozen conventional organisms, and its structure in them has been well determined. So we isolated rubredoxin from one of these high-temperature organisms, and we determined its structure by three different techniques--X-ray crystallography, magnetic resonance imaging, and computer modeling. Last year they all came up with essentially the same structure.

Conventional rubredoxin consists of one long strand, knotted in a ball. The strand contains three regions, called beta sheets, that line up next to one another when the enzyme is folded and keep the whole structure stable. The middle beta sheet has an end sort of sticking out, Adams says, flapping in the breeze.

The hyperhot version of rubredoxin was different. The end of the middle beta sheet was missing an amino acid; it didn’t stick out of the molecule. Our hypothesis, says Adams, is that if you take the conventional protein and heat it, one end starts to flap around by thermal motion, and eventually it pulls out and unzips the structure. The loose end pulls up the middle beta sheet, and the molecule falls apart. In the hyperhot protein, the vulnerable end is shorter and closer to the whole structure. So it doesn’t flap around, and the ball is much more difficult to unravel.

One thing that surprised us a little, says Adams, was how close the structures in these rubredoxins were. There weren’t any major, global changes. The conventional and hyperhot versions were similar enough to carry out the same chemistry. On the gross level they’re more or less the same three-dimensional structure, but there are these few key differences. Equally small changes in structure, Adams believes, could stabilize the other seemingly conventional molecules in heat-loving microbes. I think it’s going to become apparent that the mechanism involved is unique to each protein, and that these rather minor changes are at critical points on the protein. This hypothesis seems to puncture the organisms’ mystique. Their remarkable resistance to heat boils down to a trivial adaptation. But it’s not trivial, says Adams, from the point of view that minor changes in protein structure can give you dramatic changes in stability.

Of course, he adds, I tend to look at it the other way. The heat-resistant proteins, he’s convinced, never became more stable by adapting to higher temperatures. Instead, proteins like ours became less stable as they adapted to colder environments. As Earth cooled, they had no choice. At 60 degrees, the average temperature now prevailing on Earth’s surface, a tightly cross-linked protein becomes rigid, essentially frozen. A rigid enzyme can’t participate in chemical reactions because it’s not limber enough to get close to other molecules and affect them. By shedding cross-links, enzymes became flexible enough to react at colder temperatures. These same adaptations, however, made them too flimsy to survive at higher temperatures.

This, Adams says, tells us something about the evolutionary history of these proteins. If you accept this simple scheme, tightly knit proteins tend to be older, and disheveled ones younger. Sooner or later an evolving, unwinding protein might split into separate molecules. Each one could develop a specialized role in a more complicated chain of chemical reactions. The idea makes sense of a pattern Adams has noticed in some enzymes that function at high temperatures: Some of these enzymes are much simpler than, or appear to be precursors of, enzymes that you’d find in higher organisms. One heat-resistant enzyme called pyruvate oxidoreductase, for example, combines several functions that our own colder chemistry delegates to separate molecules. It’s as if metabolic pathways need to be compact at high temperatures, like the heat-resistant proteins themselves. On the molecular level, this is biology’s version of the Big Bang: the further back in time you look, the hotter and more compact the chemistry of life.

Adams fits the puzzle pieces he’s deciphered so far into a hand waving history of life. It started with heat-tolerant organisms, just as Woese claims. As the environment cooled, proteins shed cross-links to stay loose and functional. Meanwhile, most organisms traded tungsten for another element, molybdenum, which is now common in all other organisms and which might have worked better at colder temperatures or with different nutrients. Tungsten might have been better for metabolizing chemical reactions at higher temperatures, such as reactions involving hydrogen and sulfur. Organisms that retained it became the hyperthermophiles. The others evolved into the precursors of today’s plants and most other forms of life, from earthworms to us.

It’s plausible chemistry--but it would take a long time to prove. I keep reminding myself, Adams says, not to take any of this too seriously. Ideas in this field are constantly changing because the chemistry in hyperhot life often turns out to be more complicated than first imagined--it’s been evolving, after all, for billions of years. These are not simple organisms, Adams points out. They carry out a chemistry as complicated as bacteria’s. But no matter how the details shift, the theory that knits everything together--Woese’s big picture of life evolving from hot to cold--hasn’t failed Adams yet. I use that as a framework, when the need arises, to give us somewhere to hang our structural data, or tungsten dependency. It’s still being challenged in the field, but there’s more and more evidence.

The evidence from rubredoxin is especially intriguing. If a small structural change in the hyperhot version--a change that makes one end a little looser--really does let the molecule function at 60 degrees rather than 212, what would happen with that end a little tighter? Would equally small changes to tighten the molecule further let it function near 300 degrees--the highest setting on Adams’s new incubator? How about more extensive changes? Some researchers believe, in fact, that organisms are still growing on Earth at temperatures considerably higher than 300 degrees.

It’s easy to see now, in retrospect, that 212 degrees was an arbitrary barrier: the boiling point of water at sea level, under one atmosphere pressure. Three miles under the ocean, the pressure is higher than 300 atmospheres. There are microbes living in hyperhot vents down there we just can’t get close to. We haven’t developed the tools yet to isolate them, Adams says. The ones that do make it to labs may well be the ones with chemistry closest to ours. Are older, more heat-resistant creatures still out there? Adams sees no reason why not. We haven’t looked for them hard enough, he says. Why did it take until 1982 for somebody to find a bug that can grow in boiling water? Nobody looked before. I think there are going to be quite dramatic breakthroughs very soon, he adds. It really does boggle the mind, because 20 years ago, if you asked what’s the upper temperature limit, I’m sure we would have said it’s got to be less than 212 degrees. Who knows what it’s going to be 20 years from now?
ADVERTISEMENT
Comment on this article
ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT
DSC-CV0917
+