"Oceans In Our Solar System"
Dr. Rita R. Colwell
Director
National Science Foundation
American Association for the Advancement of Science Annual Meeting,
Seattle, Washington
February 14, 2004
See also slide
presentation.
If you're interested in reproducing any of the slides,
please contact
The Office of Legislative and Public Affairs: (703)
292-8070.
It's useful to remind ourselves how very far we've come in a very short while in establishing the parameters of life in extreme environments ... and the likelihood of its occurrence on other worlds. Let's look at some of what we know about life on this planet.
[title slide]
(Use "back" to return to the text.)
Ten years ago, Columbia University Press published the proceedings of Nobel Symposium #84, titled "Early Life on Earth." The preface began with this bold and provocative statement: "Alone in our solar system, Earth harbors life."
Only a decade later, that notion seems almost as antiquated as the "warm little pond" that Charles Darwin imagined as the likely birthplace of the common original ancestor we share with bacteria, pine trees and platypuses.
[start animation] animation not available
(Use "back" to return to the text.)
We now know that the Earth's forbidding prebiotic landscape was utterly unlike the placid pool of Darwin's musings. And based on what we have learned in only the past few years, the prospect that life exists elsewhere in the solar system has become substantially more plausible.
Much of that new insight comes from two sources: study of extremophile life forms, and deep marine research. Combine those two, and life becomes imaginable in previously unsuspected venues, such as this fanciful depiction of a vent community on Europa's presumptive subsurface ocean.
Fanciful - but not entirely science fiction.
[slide not available]
(Use "back" to return to the text.)
Europan sea is Lake Vostok, a lightless body of water roughly the size of Lake Ontario trapped under the pressure of 4,000 meters of Antarctic ice.
[Vostok microbes]
(Use "back" to return to the text.)
Imbedded in a 100-meter-deep margin between the glacier and the lake, researchers recently uncovered a host of unexpected microorganisms. They may be indicative of a unique ecosystem that has been isolated from Earth's atmosphere for a million years or more ... with what evolutionary and ecological results, we can only speculate.
A flood of such findings obliges us continuously to reconsider the boundary conditions within which life is possible, especially the range of adaptability for microorganisms. We have characterized only a very tiny fraction of the microbial life on this planet - probably less than 1 percent. But even in that small sample, specimens keep turning up in places previously assumed to be uninhabitable.
[start faulty video] video not available
(Use "back" to return to the text.)
Many are found on the sea floor in locations such as the so-called Faulty Towers complex on the Juan de Fuca ridge, where exciting new proposed projects such as the Neptune observatory network are poised to expand our understanding dramatically.
In other locations, above and below the surface, we have found microorganisms that can live at near zero pH, and others that positively thrive at a pH of 12.
[Dry Valley slide]
(Use "back" to return to the text.)
In the formidably arid Dry Valleys of Antarctica, we have seen that bacteria persist beneath ice sheets and inside rocks in a region where annual mean temperature is about -20 C and water exists in liquid form for as little as a few hours every year.
[Lake Vida slide]
(Use "back" to return to the text.)
Investigators taking samples from ice-covered Lake Vida in Antarctica were astonished to find that it was not frozen solid, as might be expected at -10° C, but remained liquid - owing, in part, to the fact that it is seven times as salty as seawater. And it contains ample evidence of microbial life. In this CT scan of a core sample, organic material appears bluish-purple and sediment appears orange-red.
[deinococcus slide]
(Use "back" to return to the text.)
At South Pole station, researchers identified cyanobacteria in the snow. And they discovered a bacterium genetically similar to Deinococcus - an extremely ancient and hardy organism - that had an active metabolism and was synthesizing DNA and protein despite an ambient temperature of -17° C and intense 24-hour ultraviolet irradiation.
In addition, we now know that certain organisms can survive thousands of atmospheres of pressure, whether deep in the ocean or imbedded in rock thousands of meters deep. Some appear able to endure the stupendous temperatures associated with volcanism.
[slide not available]
(Use "back" to return to the text.)
A few weeks ago, scientists working in the Hawaii Scientific Drilling Program reported indications of bacterial activity in volcanic rock 1,350 meters below sea level. Here you see a micrograph of the vesicles that formed in the rock, and an enlargement of the edge of one vesicle showing apparent signs of biotic activity.
[slide not available]
(Use "back" to return to the text.)
Here both fluorescence studies and electron microscope imaging of the samples strengthen the case. Analysis of these and other samples may eventually help provide astrobiologists with a new set of diagnostic criteria for use on- or off-world.
We are well on our way to characterizing many of these unfamiliar organisms.
[deep vents]
(Use "back" to return to the text.)
Two weeks ago, scientists working on a joint NSF/DOE program reported that they had recovered all or part of five separate genomes from a bacteria-archaea biofilm discovered deep underground in the toxic drainage from a mine in northern California. The microbial community was flourishing in runoff with a pH of 0.8 and high concentrations of iron, zinc, copper and arsenic.
The investigators were able to reveal the pathways used by each strain for carbon and nitrogen fixation, and to provide enough data so that the entire community metabolic network may soon be understood.
[strain 121 smoker]
(Use "back" to return to the text.)
But ocean research is a particularly rich source of results. Last summer, scientists from the University of Mass. at Amherst announced the discovery of a coccoid called "Strain 121" because it was found living at 121° C in a hydrothermal vent two kilometers deep on the Juan de Fuca Ridge.
[slide not available]
(Use "back" to return to the text.)
Laboratory experiments indicate that Strain 121 survived nicely for two hours at 130° C. There is no reason to believe that it represents the thermal upper limit to terrestrial life.
[slide not available]
(Use "back" to return to the text.)
The recent discovery of the "Lost City" -- a hydrothermal vent complex in the mid-Atlantic - showed that vents and associated biotic communities can form without volcanism, thanks to heat and pressure generated by the action of seawater on mantle rock. If analogous processes occur elsewhere in the solar system, there may be far more energy sources for biogenesis than we had presumed possible.
Much of what we have learned about extremophile and deep marine life is the result of progress on remote sensing equipment - progress which must continue.
[slide not available]
(Use "back" to return to the text.)
All of you, I'm sure, recall the excitement in 2001 when investigators from Woods Hole and the Monterey Bay Aquarium Research Institute uncovered archaea-bacteria aggregates in deep marine sediments that may consume about 300 million tons of methane per year - most of it generated by bacterial symbionts. The bacteria provide the archaea with methane, and the archaea provide the bacteria with carbon.
What got less attention was the fact that the evidence was made possible by employing a device called an ion microprobe - which had previously made its name in paleontology and geology - to examine the biochemical composition of the cell walls.
[slide not available]
(Use "back" to return to the text.)
Other research programs using the deep submersible Alvin, funded by NSF's Biocomplexity priority area and related programs, are developing and testing a new suite of instruments designed to provide data on deep marine life in their extreme environments.
[Alvin instrument]
(Use "back" to return to the text.)
Prototype instruments, chemical sampling tools and experimental systems will measure density of hydrothermal vent fluid and salinity of seawater surrounding vents, monitor the size distribution and relative abundance of mineral precipitates, detect trace organic molecules that may be forming biotically or abiotically in the vent fluid, survey the mineral and microbial distribution on the vent walls, determine how animal/microbial symbioses develop at the genetic level, and characterize larger biomolecules such as amino acids, proteins and DNA fragments. Also being developed are microbial incubators placed in the walls of active, high-temperature sulfide chimneys to examine colonization and survival.
[Vostok drill slide]
(Use "back" to return to the text.)
And, of course, the drill probe technologies under consideration for Lake Vostok will provide an excellent test-bed for sterile exploration techniques.
[slide not available]
(Use "back" to return to the text.)
In parallel with those efforts, we are also devising experiments to better understand puzzling data that may or may not be evidence of biological activity. For example, last summer researchers from Indiana University and the University of Texas at Arlington published results of a study designed to interpret the peculiar nanoscale structures in Martian meteorite ALH84001, shown in the center.
They took samples of bean, squid and beef, soaked them in pond water, and then buried them in clay to simulate the action of sedimentary rock formation. After only a short period of time, the deposits formed spherical features they call "nanoballs" around 40 to 120 nanometers wide - shown here across the top and bottom - ostensibly as the result of enzyme action.
It remains to be seen how this will influence our views of the Martian meteorite and related evidence. But at a minimum, it provides a cautionary reminder that we must maintain an open-minded definition of what might constitute evidence for "life," lest we overlook critical data because they don't fit our preconceived notions. If we have learned nothing else from recent studies in complexity, it is that nature has the capacity to generate a plethora of forms and processes from deceptively simple ingredients and rules.
It is not necessary to embrace the extreme hypothesis that life is "an obligatory manifestation of the combinatorial properties of matter," as Nobel laureate Christian de Duve put it, to have a profound regard for the possibilities inherent in self-organization.
[slide not available]
(Use "back" to return to the text.)
We should approach the search for novel life forms in our oceans and on other worlds with the same sense of respect for nature's abundance and variety that Darwin expressed when he marveled that: "from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."
|
