Salty Old Cellulose: Tiny fibers found in ancient halite deposits

Researchers have unearthed the planet’s oldest-known intact biological macromolecules, microscopic bits of cellulose from 253-million-year-old salt deposits in the southwestern United States.

The remarkable preservation of the material suggests that under the right conditions, cellulose could last more than 1 billion years. Such a long-lived molecule, a chain of simple sugars, might give scientists searching for past extraterrestrial life on other planets a new target.

Cellulose, best known as the tough material in trees, shrubs, and grasses, is one of the most abundant biological materials on Earth. Altogether, plants, algae, and some bacteria produce an estimated 100 billion metric tons of the stuff each year, says Jack D. Griffith, a biochemist at the University of North Carolina in Chapel Hill. He and his colleagues analyzed samples of salt mined about 650 meters below ground at a site some 50 kilometers east of Carlsbad, N.M. Previously, scientists have grown bacteria from ancient spores found in the same deposits (SN: 6/12/99, p. 373).

The newly discovered cellulose, like those bacterial spores, was recovered from brine-filled pockets, or inclusions, within large crystals of salt, says Griffith. Radioactive dating of the 600-meter–thick halite deposits, and of the sediments above and below them, indicates that the salty strata formed even before dinosaurs walked the Earth. The configuration of layers suggests that the salts crystallized on the floor of a shallow marine lagoon and haven’t been disturbed since.

The researchers analyzed only material taken from inclusions in salt crystals that weren’t cracked, thereby ensuring the contents of the inclusion hadn’t washed in after the salt had formed. The team also dissolved material from the outer surface of the crystals to remove any modern contaminants that might have been introduced during excavation, says Griffith.

Most of the fibers found in the inclusions were small and flexible, Griffith and his colleagues report in the April Astrobiology. The bits didn’t dissolve in a concentrated 65°C solution of sodium hydroxide and sodium borohydride, as all biological materials except cellulose would. Also, the material quickly dissolved when placed in a 37°C solution of cellulase, a cellulose-digesting enzyme.

“This is a beautiful discovery,” says R. Malcolm Brown Jr., a molecular biologist at the University of Texas at Austin. “It’s incontrovertible that this is cellulose.” Further analyses of the microstructure of the fibers and mats found in the salt deposits may allow researchers to identify the organism that produced the cellulose, he adds.