The Charms of Cold

by Elio
 

Figure 1. Scanning electron microscope image of Chry­seo­bacterium greenlandensis found in a Greenland glacier. (Credit: Jennifer Loveland-Curtze, Penn State). Source. Frontpage: Scan­ning electron microscope of Planococcus halo­cryophilus cells grown at −15°C in 18% NaCl and 7% glycerol, encrusted in dense nodular mate­rial. Source

The upcoming "protein spotlight" #209 by Vivienne Baillie Gerritsen on ice-binding proteins brings up the near ma­gic things that happen to microbes in the cold. At a time when our worry is global warming, it may be good to con­sider that some bacteria do not shun cold habitat and thrive in such uninhabitable-sounding places as the per­ma­frost, polar ice, glaciers, snowfields, and cold deep ocean waters. Of course, these places are being severely affected by the change in climate. But for now, even a partial list of what microbes do in the cold is impressive.
 

• Fish, insects, plants, and above all microbes can thrive in extremely cold climates and make use of a variety of strategies to overcome such harsh condi­tions. Certain fish resist sub-freezing temperatures by making proteins that serve as antifreeze. This lowers fishes' freezing point by about 1°C. Using such antifreeze proteins, insects and plants can do even better than that, but it is microbes that win the prize.
   
• Bacteria and archaea are said to survive in ice for 50 million years, that is, are able to grow after thawing. Not unexpectedly, the data become more credible the more recent the time of their frozen permeance. A reliable datum is that they survived frozen for 750,000 years in ice samples from Western China. Microbial activity has been measured in soils frozen below −39°C.
   
• Bacteria in ice can repair their DNA. One would expect that bacteria living under such harsh conditions should have active mechanisms to repair damage to their DNA, and this has in fact been reported for viable bacteria frozen in ice for 500,000 years. The authors propose that the active metabolism need to sustain such activity is also present. For a discussion of this issue in these pages, see here.
   
• Bacteria survive temperatures near the absolute zero as long as ice crystal formation is pre­vented, as when they are plunged into liquid ethane in preparation for electron cryo­to­mo­graphy (CryoET). It is said that bacteria frozen this way swim away happily when thawed.
   

Figure 2. Phase contrast micrograph of Psychro­monas ingrahamii. Bar 1 µm. Source

• The word champion bacterium for growth at a low temperature appears to be Psychromonas ingra­ha­mii, which can grow at −12°C with a generation time in the lab of 240 h (Figure 2). It is named after my buddy from way back, John Ingraham, for his contri­butions to 'cold microbiology'. These include the first charac­te­ri­za­tion of cold-sensitive mutants in essential func­tions. ('buddy' refers, among other things to having jointly authored four books, to­gether with the late Fred Neidhardt.) Challenging the long-time champion is now a Firmicutes bac­terium, Planococcus halocryophilus, which grows at −18°C and is still metabolically active at −25°C in its natural habitat, Arctic permafrost (see frontpage).
   
• Some bacteria from Antarctic lakes cling to the underside of the ice. They do this via a se­cre­ted long protein thread reminiscent of a pilus, but one that is much thinner than a regular pilus. Yet, like many pili, it has an adhesive end that does the actual binding. It is made by the Antarctic bacterium, Marinomonas primoryen­sis. Now, you ask, why would bacteria want to cling to ice? The reason proffered is that the underneath of ice floes is a good a place for bac­teria to grow in seemingly inhospitable places. Just underneath the ice is a region with higher concentration of oxygen and nutrients, and more light. In the Antarctic lakes that are perma­nently covered with ice (see a recent post), these regions are an ideal habitat for strictly aero­bic organism such as M. primoryensis.
   

Related: listen to this episode of Meet the Microbiologist (an ASM podcast) where Christine Foreman, Asso­ci­ate Professor, Montana State University, explains how microbes can survive and grow on glaciers, and what we can learn from microbes in glacier ice cores.