Water bears go where no animal has gone before

A water bear (OMG, they're so cool!)

Timeline, 2008: They say that in space, no one can hear you scream (or at least that Alien movie said it). The reason for that is that sound waves require matter to be propagated, and in the vacuum of space, there is no matter. Hard as your vocal cords may push, they can’t make the sound travel through nothing.

That vacuum carries other complications for people beyond an inability to chat unprotected. Space is cold—temperatures run close to absolute zero, or -272 Celsius. With the vacuum, there is no oxygen and no pressure. And let’s not forget the bombardment with deadly cosmic rays and, if you’re hanging around just above Earth, UV rays 1000 times more powerful than those we experience on terra firma.

Boiling saliva and bubbling blood. Ewww.

Thus, if you send person or a dog or an ape unprotected into space, within minutes, the lack of pressure would lead to an uncomfortable death involving boiling saliva and bubbling blood. Yet, there are organisms known to survive the environment of space, primarily some bacteria and lichen, the symbiotic combination of algae and fungus. Now, we’ve learned that a little critter that lives on lichen also can be quite the intrepid space traveler.

Water bears–not even remotely like bears

The space animals—the first animals, in fact, known to pull off unprotected space travel—are representatives of two species of tardigrades. There are up to 1000 species of these little animals, more familiarly known as “water bears” because of their rotund, bear-like appearance. Researchers had noted their hardiness under earthbound conditions, observing that although the animals thrive in a damp environment, when conditions go dry, they can shut down for as long as 10 years until the environment moistens up again. Some accounts have compared the water bears to “Sea Monkeys,” the brine shrimp of comics advertisements that come to you dried up in a little packet, only to “miraculously” come to life when you add water.

On a September 2007 European Space Agency mission, scientists decided to test the tardigrades in the most hostile environment available—in space just above planet Earth. They tested two species of water bear, Richtersius coronifer and Milnesium tardigradum, both entering space as dried out versions of their usual selves. The 120 animals from each species were divided into four groups, one that remained on Earth as a control, two that were exposed to the vacuum of space and different combinations of UV rays, and a fourth that experienced only the vacuum of space without the radiation.

Vacuum OK, UV bad

The animals spent 10 days traveling unprotected far above the earth before returning to Earth, where they were hydrated in the lab. Amazingly enough, all three groups of space-traveling tardigrades initially perked right up and lived for a few days. After that, however, only the vacuum-alone group maintained that rate of survival. In the UV-exposed groups, animals that had experienced the vacuum of space and exposure to UV-A and UV-B survived at rates between 10% and 15%. Animals that had been exposed to all three types of UV—A, B, and C—all died.

Nevertheless, the animals that did survive appeared to thrive, reproducing heartily and generally living the usual life of the unusual water bear. Researchers find their ability to withstand radiation particularly intriguing, given that the bombardment would normally shred the DNA of most organisms. Investigators hope to find out more about how the bears  resist the perils of space, seeking perhaps to co-opt some of the tardigrade’s techniques to use in protecting astronauts.

Space, schmace. How about 4000 m deep?

Tardigrades didn’t really need to travel into outer space—or really, inner space—to prove their toughness. They are known to live as high as 6000 meters up in the Himalayas and as deep as 4000 meters down in ocean trenches. Water bears have also been found living in apparent ease in hot springs above boiling temperature.

Is the tree of life really a ring?

A proposed ring of life

The tree of life is really a ring

When Darwin proposed his ideas about how new species arise, he produced a metaphor that we still adhere to today to explain the branching patterns of speciation: The Tree of Life. This metaphor for the way one species may branch from another through changes in allele frequencies over time is so powerful and of such long standing that many large studies of the speciation process and of life’s origins carry its name.

It may be time for a name change. In 2004, an astrobiologist and molecular biologist from UCLA found that a ring metaphor may better describe the advent of earliest eukaryotes. Astrobiologists study the origins of life on our planet because of the potential links between these earthly findings and life on other planets. Molecular biologists can be involved in studying the evolutionary patterns and relationships that our molecules—such as DNA or proteins—reveal. Molecular biologist James Lake and astrobiologist Mary Rivera of UCLA teamed up to examine how genomic studies might reveal some clues about the origins of eukaryotes on Earth.

Vertical transfer is so 20th century

We’ve heard of the tree of life, in which one organism begets another, passing on its genes in a vertical fashion, with this vertical transfer of genes producing a tree, with each new production becoming a new branch. The method of gene transfer that would produce a genuine circle, or ring, is horizontal transfer, in which two organisms fuse genomes to produce a new organism. The ends of the branches in this scenario fuse together via their genomes to close the circle. It is this fusion of two genomes that may have produced the eukaryotes.

Here, have some genes

Eukaryotes are cells with true nuclei, like the cells of our bodies. The simplest eukaryotes are the single-celled variety, like yeasts. Before eukaryotes arose, single-celled organisms without nuclei—called prokaryotes—ruled the Earth. We lumped them together in a single kingdom until comparatively recently, when taxonomists broke them into two separate domains, the Archaebacteria and the Eubacteria, with the eukaryotes making up a third. Archaebacteria are prokaryotes with a penchant for difficult living conditions, such as boiling-hot water. Eubacteria include today’s familiar representatives, Escherichia coli.

Genomic fusion

According to the findings of Lake and Rivera, the two prokaryotic domains may have fused genomes to produce the first representatives of the Eukarya domain. By analyzing complex algorithms of genomic relationships among 30 organisms—hailing from each of the three domains—Lake and Rivera produced various family “trees” of life on Earth, and found that the “trees” with the highest cumulative probabilities of having actually occurred really joined in a ring, or a fusion of two prokaryotic branches to form the eukaryotes. Recent research If we did that, the equivalent would be something like walking up to a grizzly bear and hand over some of your genes for it to incorporate. Being eukaryotes, that’s not something we do.

Our bacterial parentage: the union of Archaea and Eubacteria

Although not everyone buys into the “ring of life” concept, their findings help resolve some confusion over the origins of eukaryotes. When we first began analyzing the relationship of nucleated cells to prokaryotes, we identified a number of genes—that we call “informational” genes—that seemed to be directly inherited from the Archaea branch of the Tree of Life. Informational genes are involved in the processes like transcription and translation, and indeed, recent “ring of life” research suggests a greater role for Archaea. But we also found that many eukaryotic genes traced back to the Eubacteria domain, and that these genes were more organizational in nature, being involved in cell metabolism or lipid synthesis.

Applying the tree metaphor did not help resolve this confusion. If eukaryotes vertically inherited these genes from their prokaryotic ancestors, we would expect to see only genes representative of one domain or the other in eukaryotes. But we see both domains represented in the genes, and the best explanation is that organisms from each domain fused entire genomes—horizontally transferring genes—to produce a brand new organism, the progenitor of all eukaryotes: yeasts, trees, giraffes, killer whales, mice, … and us.

Did we find life on Mars–and kill it?

Did we kill life on Mars?

The movie War of the Worlds may have it wrong. The collision between life forms from different planets may not involve fire, mutual bloody conflict, and pathogen exchange. Instead, it may already have happened, ending simply, quietly, and unilaterally, in water and heat at the microscopic level.

In the 1970s, we sent two Viking space probes to Mars on a mission to test for life on the red planet. The prevailing dogma among scientists was that water was required for life, anywhere in the universe. This viewpoint has evolved since then as we’ve discovered some Earthbound organisms, including the microbe Acetobacter peroxidans, relying on very different, non-water-based mechanisms for survival.

Organisms living alternative lifestyles

These discoveries of organisms living alternative lifestyles here at home led to the formation of the “weird life” panel, a U.S. group that addresses concerns that we may be too Earth-centric as we search for life beyond our planet. This narrow view may have led to a terrible accident on our Viking visits: we may have encountered Martian life and then drowned or baked it to death.

The Viking probes performed a series of experiments on samples from the Martian surface. The labeled release experiment involved exposing Martian soil samples to water and a nutrient source with incorporated radiolabeled carbon. To everyone’s excitement, the exposure elicited a spike in radiolabeled carbon dioxide (CO2), meaning that the nutrient source could have been metabolized, the carbon incorporated with oxygen to make CO2. However, the experiment seemed to fizzle as the CO2 levels dropped and plateaued.

Evidence of life–and death?

In the pyrolytic release experiments, radiolabeled CO2 appeared to have been incorporated into organic molecules by something in the Martian soil. This type of experiment would reflect activity similar to photosynthesis, which grabs CO2 from the air and incorporates it into the organic macromolecule glucose. In this Martian test, four of the seven soil samples showed significant production of organic molecules that had incorporated the radiolabeled carbon from the CO2. Interestingly, the sample treated with liquid water produced even less organic product than the control. In addition, the Viking probes found evidence of chemical oxidation (oxygen breaking down macromolecules), but we found no oxidative activity in the Martian soil on subsequent analyses.

The results seemed disappointing—they at first held promise for a life form metabolizing and catabolizing molecules. But as each experiment fizzled, expectations diminished, and everyone assumed that there was simply no life on Mars.

Oops

A review of the data from a different, less Earth-centric perspective, however, results in a potential confirmation of life on Mars. The scientists who devised this story point out that Mars is conducive to life based on hydrogen peroxide (HP). For example, the higher the HP content, the lower the freezing point of an aqueous HP solution. Even at freezing, HP does not form cell-destroying crystals, as water does. An HP also pulls water vapor from the air, an efficient way to grab water in a dry place like Mars.

Exposing an HP-based cell to water would drown it, and applying heat to the soil would bake it. These two things are exactly what the Viking probes did to Martian soil samples in executing the experiments. If HP-based life forms were present, they would have produced results exactly like those the probes found. For example, they could have taken the radiolabeled macromolecules and metabolized them, releasing CO2, but eventually dying as the water added to the samples overwhelmed them. Every single outcome of the Viking experiments appears to be consistent with a hypothesis that there might have been HP-based life on Mars.

We’ve now sent a new probe, the Phoenix, to the red planet. Data from the probe don’t answer all questions but do indicate the presence of “organics” on Mars. The next step is the Mars Science Laboratory, expected to launch next year with a Mars ETA in 2012. Will it settle the Life on Mars question once and for all?

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