Life at the Extremes

Some living species are able to thrive in inhospitable environments. How do they do it?

Life has flourished here on Earth. Typically, living things are found in warm, wet, sunlit zones, at pressures similar to those at sea level and in conditions that are neither acidic nor alkaline. But living cells have also been found in areas that seem inhospitable to life: in hot springs, lying dormant buried in ice, in acid-filled caves and in the depths of the ocean. By exploiting new environments, these pioneering organisms gain a competitive advantage that allows them to proliferate. How are they able to tolerate these conditions?

Credit:www.carleton.ca/~kbstor ey

Wood frogs survive cold winters by freezing solid and thawing out in the spring. They have developed 'antifreeze proteins' that control the shape of ice crystals that form, and minimise the physical damage caused by its expansion.

Normal versus extreme

Life is inextricably linked to water and is usually found within the range of temperature and pressure where liquids can exist. Organisms that live on land tend to favour a temperature range of 10 degrees C to 48 degrees C, while life in the ocean exists at around 2 degrees C year round. At higher temperatures, vital long chain carbon molecules acquire too much energy from their surroundings and lose their important 3D shapes. At lower temperatures, chemical reactions slow down, making it difficult to sustain metabolism. At subzero temperatures, water inside living organisms can form ice crystals which damage the delicate internal architecture of cells.

Low pressure environments are rare, even at the top of the tallest mountain ranges. High pressure is a much more common stress factor, particularly at the bottom of oceans where the weight of the water generates a crushing force up to 1,100 times the pressure at the surface. There is also a lot of pressure inside rocks and sediments beneath the surface of the Earth where some bacterial species have been found.

In addition to these physical constraints, chemical stress factors like salt concentration and pH, a measure of how acid or alkali an environment is, also affect what can survive in an environment. All living things separate themselves from the outside world with a cell membrane. This allows them to control the concentrations of important or dangerous chemicals within their cells. In very salty conditions, cells struggle to retain their water as it floods out to dilute the salt outside the cell. Many of the problems associated with salty environments are due to lack of water.

The pH of an environment also has many consequences for cells. Since most biological processes occur in the neutral range of the pH scale, biological molecules lose their function in both acid and alkali environments (low and high pH respectively).