Re: Survival of bacteria in Antarctica

Hi Jo,
Nice to see a question about the very thing that I work on. Life at low temperature is a very interesting problem-over 80% of the biosphere is cold (below 5 degrees C) and low in nutrients, but our understanding of the organisms that live in these places is very limited. We know far more about the microorganisms that live around hot springs and ocean vents at temperatures over 100 degrees C.

There are 2 classes of organisms that live in the cold, described as psychrotolerant and psychrophilic (psychro- is Greek for cold). Psychrotolerant species can tolerate low temperature, but grow much more happily at higher temperatures. Psychrophilic species actually prefer the cold and grow best at temperatures between 0 and 20 degrees C.

There are actually many habitats in Antarctica where things can live. Not all of the continent is solid ice; there are patches of rock and soil, open lakes, the surrounding ocean and even underground lakes. The most famous of these is Lake Vostok, which scientists think is a good model for Europa, a moon of Jupiter which may have an ocean under the surface. As you suggested, many of the organisms that live there are bacteria, but there are also single-celled protozoans, fungi such as yeasts, algae and simple plants such as lichens and mosses. There are also several kinds of fish and insects that live happily at 1-5 degrees C. My current research is concerned with the psychrophilic Archaea, which are similar in some ways to bacteria, but form a different domain of life. Many of the Archaea live in extreme environments-very hot, cold, salty, acidic and so on and they are thought to resemble the earliest life forms on Earth.

OK, now to answer your question in more detail. It's true that below freezing, life for microorganisms cannot really continue, in the sense that they can't grow or metabolise compounds. At these kinds of temperatures, life becomes dormant. As you suggested, one way to cope below freezing is to have solutes in the cytoplasm. Some organisms have various "antifreeze" substances, such as a sugar called trehalose. There are also special antifreeze proteins. These function by preventing the growth of ice crystals which can damage the cell membrane. Another strategy for bacteria is to become freeze-dried and form spores. These appear completely lifeless, but add water and increase the temperature and they will come back to life. There is a very radiation- resistant bacterium called Deinococcus radiodurans, whose radiation- resistance is thought to have evolved as a response to dehydration and many of the genes and proteins involved can be detected in other microbes.

So below freezing, organisms can survive but they don't do very much. Perhaps more interesting are the organisms that can live, grow and metabolise at low temperatures, between 1 and 15 degrees C. We know something of the adaptations used by these organisms. For instance, the composition of the lipids in their cell membranes is different, allowing them to remain fluid and transport solutes. A little work has been done on the structures of some important proteins from the psychrophilic bacteria and Archaea, including some involved with protein synthesis and with metabolism. Cold-adapted proteins are often more flexible or folded up less tightly than similar proteins from warm-growing species, which may allow them to work more efficiently. Cold-adapted enzymes can also bind to their substrates more efficiently or have other changes that allow them to do metabolic reactions at low temperature. These are just a few of the adaptations we know of, but there is still a lot to learn.

If you'd like to investigate this more, you could look at http://www.micro.unsw.edu.au/ rick.html, one of very few labs working on cold-adapted microorganisms. I also have a small website at http://www.crosswi nds.net/~nfws/archaea/index.htm which has some links to research, in particular to Lake Vostok which is really fascinating.
hope this helps you,

Neil Saunders