29 July 1998
"We don't know all the possibilities for life," says Andrew Ellington of the University of Texas, Austin. "If we found life on another planet, what could it be like, where could it live, what might it eat?" The search for life on Mars and other planets might eventually answer some of these questions, he said last week at an astrobiology workshop at the NASA Ames Research Center near San Francisco.
Meanwhile, however, Ellington has decided to take a different approach. If he couldn't find an extraterrestrial, he could try to make one instead. The building blocks of life might be very different on another planet, so he and his team decided to see if a terrestrial microbe could adapt to a major change in its diet.
They grew a strain of the bacterium Escherichia coli that can't make the amino acid tryptophan, an essential part of proteins. As a result, it requires a constant supply of tryptophan to grow.
The researchers also gave it a related synthetic amino acid, fluorotryptophan. This might conceivably be an essential component of life on another planet, but to life as we know it, fluorotryptophan is poison. When substituted for tryptophan, the impostor is incorporated into E. coli proteins and cripples the cells, killing them after about three divisions.
When the researchers gave the bacterial cultures a mixture of 95 per cent fluorotryptophan and 5 per cent normal tryptophan, the organisms barely coped and grew only slowly. But after many generations, the microbes started to divide more rapidly, suggesting they had developed mutations that helped them cope with fluorotryptophan's toxic effects. In small steps, the researchers increased the amount of the synthetic amino acid. The cells were eventually able to survive on a diet with 100 per cent fluorotryptophan and have divided for seven generations.
"The bacteria grow incredibly slowly," says Ellington, "but they are continuing to divide." His team is eager to discover what genetic tricks the microorganism has used to develop a taste for this unusual dish.
Ellington thinks this "unnatural selection" could encourage terrestrial microbes to live on inhospitable planets with very different sources of food. "There's no reason to think we can't simulate a piece of Mars in the lab and then coax something to live on it," he says.
He even suggests that such bacteria could terraform other worlds, by digesting toxic compounds in the soil and giving off gases that would help to make a planet's atmosphere more suitable for people to live on.
Author: Philip Cohen
New Scientist issue 1st August, page 14