When a fluid-filled inclusion in a 250-million-year-old salt crystal was reported in the Oct. 19 Nature to harbor ancient life, many scientists raised eyebrows over the likelihood that only one out of 53 grains sampled just so happened to contain the oldest bacterium ever discovered.
 
The authors of the controversial paper — Russell Vreeland and William Rosenweig from West Chester University in Pennsylvania, and Dennis Powers, a consulting geologist from Texas — were steeled for an onslaught of criticism. They took great care to quantify their sterilization processes in order to quell any doubts and prove the ancient bacterium they stumbled upon was not the result of surface contamination by a present-day bacterium.
 
“To survive for 250 million years, all systems had to be correct,” says Vreeland. There could have been large amounts of bacteria living in the briny water before it evaporated, but the probability that vast numbers of them survived the evaporation of the water, subsequent burial and eons of time, is slim.
 
Vreeland and his colleagues collected samples of salt from the Permian Salado Formation, accessible via an air duct in the Waste Isolation Pilot Plant near Carlsbad, N.M. To search for bacteria in the tiny fluid inclusions in the salt, the team could not rely on a microscope because of the contamination that would have occurred when preparing the slide. Instead, they inoculated several petri dishes, using both sodium hydroxide and hydrochloric acid solutions to sterilize the salt crystals before drilling into the inclusions and retrieving the liquid.
 
In the Nature article, they describe making detailed contamination checks, including collecting the drill shavings from each sample to test for bacteria. They threw out any samples that showed signs of contamination, either from their procedures or through natural processes.
 
The detailed sterilization processes, they estimate, resulted in a probability of less than one in 10-11 that the bacterium they discovered was a surface contaminant. “Sterility is measured in probabilty,” Vreeland says.
 
Before Vreeland’s results can be fully accepted by the scientific community, “they will require replication by other groups and similar studies to be carried out on different salt deposits,” John Parkes said in his article, also in the Oct. 19 Nature. Sterilization alone will probably not convince the scientific community of Vreeland’s almost incredible findings. He and his colleagues hope to replicate their findings next year using samples they plan to collect in March from other layers in the Salado Formation.
 
Plans to replicate their work using Australian opal are already in the works. Most recently, Vreeland has begun talking about sampling 2.5 billion-year-old quartz in his search for ancient life, but has not yet made any definite plans.

Laura Wright
 

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