细菌如何在冰冻环境中生存

Most microbial researchers grow their cells in petri-dishes to study how they respond to stress and damaging conditions. But, with the support of funding from NASA, researchers in LSU's Department of Biological Sciences tried something almost unheard of: studying microbial survival in ice to understand how microorganisms could survive in ancient permafrost（永久冻土）, or perhaps even buried in ice on Mars. Brent Christner, associate professor of biological sciences, and colleagues at LSU including postdoctoral researcher Markus Dieser and Mary Lou Applewhite Professor John Battista, recently had results on DNA repair in ice-entrapped microbes accepted in the journal of Applied and Environmental Microbiology. To understand how microbes survive in frozen conditions, Christner and colleagues focused on analysis of DNA, the hereditary（遗传的） molecule that encodes the genetic instructions used in the development and function of all organisms.

"Microbes are made up of macromolecules that, even if frozen, are subject to decay," Christner said. "We know of a range of spontaneous reactions that result in damage to DNA."

The worst kind of damage is known as a double-stranded break, where the microbe's DNA is cleaved into two separate pieces that need to be put back together to make the chromosome functional.

"This kind of damage is inevitable if cells exist frozen in permafrost for thousands of years and cannot make repairs," Christner said. "Imagine that a microbe is in ice for extended periods of time and its DNA is progressively getting cut into pieces. There will eventually be a point when the microbe's DNA becomes so damaged that it's no longer a viable informational storage molecule. What is left is a corpse."

The situation would seem dire for the longevity of microbes in ice. But curiously, researchers have been able to revive microbes buried in ice and permafrost for hundreds of thousands to millions of years. In fact, Christner managed to revive several different types of bacteria from near the bottom of the Guliya ice cap on the Qinghan-Tibetan plateau in Western China -- ice that is 750,000 years old, from long before the age of humans.

But how is it possible for microbes to counter expectations and survive for such long periods when frozen? The survival of microorganisms in ancient glacial ice and permafrost has typically been ascribed to their ability to persist in a dormant, metabolically inert state. But even this explanation does not account for the background levels of ionizing radiation that cause damage to these microbes' DNA, frozen at the bottom of a glacier or not.

"In order to survive that long, different studies for instance point towards dormancy, or 'slow motion metabolism,' but regardless of the physiological state, without active DNA repair an organism will accumulate DNA damage to an extent that will lead to cell death," Dieser said.