Windows on the desert floor

Natural History, May, 1998 by Peter J. Marchand

Once established, the bacterial colony grows outwardly from scattered points, often starting in tiny depressions that capture the wind-transported minerals and retain moisture from dew or runoff (the bacteria themselves may have been similarly "trapped"). The colony may eventually cover entire cliff faces in and regions. As Ronald Dorn, of Arizona State University, and his colleagues elsewhere are finding, these colonies may persist for hundreds of thousands of years, adding paper-thin layers, one upon another, to the rock varnish, providing the longest record yet discovered of climate history on our planet (see "The Once and Future Climate," Natural History, September 1996).

Remarkable as these bacteria are, they are matched by an equally impressive algal community that flourishes where neither lichen cover nor varnish bacteria have been able to take hold, completely hidden within the matrix of the rock itself. Robert Bell, of the University of Wisconsin, has been working with different variants of Coconino sandstone from the Sonoran Desert and is finding, in the lighter-colored rocks, a zone of algal growth lying between 1.5 and 3.7 millimeters below the surface of the rock. Here, cyanobacteria and green algae develop and reproduce, obtaining energy through photosynthesis and even showing adaptive response to light of different intensity Cells exposed to greater light in upper portions of the band often possess protective orange and red pigments, helping to shield them from damaging ultraviolet radiation, while those receiving less light at the bottom of the band usually are vivid green. Bell and E. Imre Friedmann, of Florida State University, along with others, have identified twenty-one different species of cyanobacteria and green algae that live entirely within the interstices of crystalline sandstone and limestone of hot deserts. Like their rock-surface counterparts, these hidden microorganisms persist by the same stop-and-go pattern of growth, enduring extended periods of extreme desiccation between short bouts of metabolic activity. The secret to the good life on--or in--a hard rock, it appears, is to live intermittently.