Bacteria encased in ice can be resuscitated after thousands, perhaps even millions of years. How these hardy bugs manage to survive deep freeze is something of a mystery. If nothing else, the low levels of radiation hitting Earth’s surface should cause any ice-bound bacterium’s DNA to break apart over time, eventually leading to irreparable damage. Some scientists think bacteria survive cryosleep by encasing their DNA in protective shells known as spores and entering a state of dormancy. Following spore formation, a bacterium can withstand harsh environmental conditions, including desiccation, strong acids, heat and UV radiation.
But other researchers think we aren’t giving enough credit to the ice dwellers. Recent studies have shown that some psycrhophiles– technical-speak for cold-loving bacteria – are able to maintain basic metabolic functions at subzero temperatures. Could psychrophiles trapped in ice be repairing their DNA faster than the UV radiation bombarding our planet pulls it apart? Microbiologist Markus Dieser at Lousiana State University was interested in finding out. In a study published in the journal Applied and Environmental Microbiology, Dieser and colleagues show for the first time that one bacteria- Psychrobacter arcticus– can repair it’s DNA at temperatures as low as -15ºC, or 5ºF. Moreover, it can do so 100,000 times faster than damage occurs.
P. articus is an innocuous little bacteria that is famous for one thing: it really likes the cold. It can grow and metabolize at -10 ºC, making it one of the most psychrophilic organisms on Earth. To investigate P. articus’’s ability to repair DNA in deep freeze, Dieser and colleagues isolated viable P. articus cells from Siberian permafrost that has been frozen for 20 to 30 thousand years. In the lab, the researchers dosed their cell cultures with a large pulse of ionizing radiation- roughly equal to what P. articus might experience over 225 thousand years of field exposure. By using such an intense burst of radiation, the team hoped to induce many “double-strand breaks”, or breaks that cause small DNA fragments to separate off from P. articus’s main chromosome.They incubated the irradiated cultures at -15ºC and monitored their survival over the course of 505 days.
Rather astoundingly, the scientists found no significant difference between the survival rates of irradiated and non-irradiated bacteria over the year and a half long study. While this finding alone suggests P. articus can repair its DNA at subzero temperatures, Dieser and colleagues wanted direct evidence. They used pulse-field electrophoresis, a technique which separates DNA fragments by size, to determine how may DNA double-strand breaks occurred after radiation exposure, and whether the DNA fragments reassembled themselves over time. Like Humpty Dumpty rebuilding himself, the scientists could literally watch P. articus reassemble its genome. On average, P. articus was able to patch thirteen double-strand DNA breaks over the course of the study- quite close to the roughly sixteen breaks inducted by radiation.
Not only can P. articus repair its DNA at subzero temperatures, it can do so really fast. Using annual radiation exposure data collected in the field, Dieser estimates that P. articus can repair double-strand breaks 100,000 times faster than they occur. The discovery has important implications for the survival of life in extreme environments, including cold extraterrestrial environments. For instance on the surface of Mars, where radiation levels are ~400 times greater than the Siberian permafrost, P. articus can still patch DNA breaks 280 times faster than they would accrue. As scientists continue exploring the “cold limit” to essential cellular functions such as DNA repair, they will continue to refine, and perhaps expand, our understanding of the fundamental boundaries for life.
Markus Dieser, John R. Battista, & Brent C. Christner (2013). DNA Double-Strand Break Repair at −15°C Applied and Environmental Microbiology DOI: 10.1128/AEM.02845-13