DURHAM, N.C. -- Microscopic Mojave Desert plants growing on the underside of translucent quartz pebbles can endure both chilly and near-boiling temperatures, scavenge nitrogen from the air, and utilize the equivalent of nighttime moonlight levels for photosynthesis, a new study reports. The plants, which receive enough light through the pebbles to support photosynthesis, could offer a model for how plants first colonized land, as well as how they might have evolved on Mars, said the scientists who performed the study.
"Here you have a really bizarre habitat," said William Schlesinger, dean of Duke University's Nicholas School of the Environment and principal author of a paper on the study that appears in the December, 2003 issue of the research journal Ecology, which was just published. "When I first went to the site in 1978 I thought: 'That's weird, how do these plants photosynthesize?' Then it dawned on me that they photosynthesized on the light coming through the rocks."
Years after he first noticed the primitive plants -- mostly species of blue-green algae -- growing under every quartz pebble he turned over at the site in California's Joshua Tree National Park, Schlesinger assembled a scientific team to investigate the phenomenon. He said what the scientists learned suggests a possible way that land plants established their first toehold in the harsh conditions of the early Earth: by staying under cover.
Such habitats may also be "prime locations to search for extraterrestrial life" on other planets, wrote Schlesinger and his other team members in their paper. Other authors include Schlesinger's technician Jeffrey Pippen and Duke graduate students Matthew Wallenstein and Kirsten Hofmockel; also Bruce Mahall of the University of California at Santa Barbara and Debra Klepeis, Mahall's graduate student.
Under Schlesinger's direction, Pippen counted 295 whitish, light transmitting quartz pebbles commingled with a much larger number of opaque black pebbles within a 1-by-50 meter desert test plot. The scientists found all quartz pebbles that were about one inch or less thick supported active plant colonies on their undersides. Quartz pebbles thicker than one inch still had rings of plant life around those parts of their bottom edges where sunlight could penetrate through the stone at an oblique angle.
By placing heat sensors above and below some of the pebbles in all four seasons, the scientists documented that living under the quartz pebbles kept the plants warmer in winter and cooler in summer compared to conditions underneath black pebbles. In fact, their Ecology paper suggested that sunlight transmitted through the translucent quartz might "confer a modest greenhouse effect" during the cooler months, in essence trapping some of the sun's heat.
Comparatively moderate though they were, temperatures underneath the quartz pebbles still logged as low at 41 degrees Fahrenheit in January and almost 150 degrees Fahrenheit at midday in August under harsh desert conditions.
The researchers then brought some pebble samples back to their laboratory at Duke and heated them to 194 degrees for six hours. Despite that ordeal in the lab, when the baked rocks were then moistened, their resident plant colonies proved still able to photosynthesize. Photosynthesis is the process by which plants synthesize sugars using atmospheric carbon dioxide through the action of light on green chlorophyll molecules.
The algae's demonstration of high temperature resilience presented a paradox, because chlorophyll molecules themselves normally begin to degrade at about 167 degrees, according to Schlesinger, who is a biogeochemist and ecologist. "Either they have some special kind of chlorophyll, or they were in a resting phase which bacterial groups can go into to get through really extreme conditions," Schlesinger said. Blue-green algae are more properly called cyanobacteria. Wallenstein's DNA identification of the algae species in plant colony samples revealed 26 different kinds of cyanobacteria. Of those, the Ecology paper suggested that five species may be previously unknown to science.
Cyanobacteria are suspected of being "one of the first colonizers of land" on Earth, Schlesinger noted -- a time when there was no atmospheric ozone shield to block harmful solar ultraviolet radiation and no nitrogen-rich topsoil covering the ground. The lack of soil nitrogen provided no obstacle for the plant colonies living under the quartz rocks. Hofmockel, another of Schlesinger's graduate students, found those algae obtain the nitrogen they needed for growth directly from the air like some less primitive plants are also able to do.
The UC Santa Barbara researchers found that the pebbles did not filter out more ultraviolet rays than they did other wavelengths of sunlight, meaning that quartz did not provide an especially protective environment. On the other hand, analysis also showed that that only about .08 percent of the light of any wavelength that entered one-inch-thick pebbles could reach plants on the other end. "That's pretty shady," Schlesinger added. "That's like photosythesizing by moonlight on the bottom of the thickest rocks." "The growth of hypolithic (beneath rocks) algae under diaphanous quartz pebbles in the Mojave Desert is another illustration of the successful microbial exploitation of a novel habitat in an otherwise harsh environment," the authors concluded in their Ecology paper. "Similar environments might harbor life on other planets," the paper added.
While the paper did not specify which other planets, Schlesinger singled out Mars, whose surface is known to harbor quartz rock, be extremely dry and cold, and receive larger doses of ultraviolet radiation than Earth's surface does today. "Right now Mars doesn't look too good for life," Schlesinger said. "But if Mars had something alive two billion years ago, when it is believed to have been slightly wetter, this might have been where that something lived."