2.1.1 Living in extreme environments
All exploration missions – their objectives, landing sites, the instruments they carry and the analyses of the data they generate – are informed by investigations here on Earth where life is ubiquitous.
Studying the microbes that inhabit extreme environments on Earth – those with extremes of temperature or pressure, or where there is no liquid water for long periods of time – helps us to recognise which environments beyond Earth may be capable of supporting life. But it can also help us to understand which microbes might be capable of surviving a journey through space and into an unknown extraterrestrial environment.
The range of conditions over which an organism is adapted to live is called the tolerance range. This can be represented graphically (Figure 7) by plotting a biological factor associated with the organism (e.g., growth rate on the y axis) against variable environmental factors (e.g., temperature on the x axis). In the case of Figure 7, this shows the response of growth rate to changes in temperature for five different species of microbes (represented as curves labelled A to E). For this example, the temperature at which each species grows and reproduces most rapidly is the ‘optimum temperature’, shown as the peak of each curve. At temperatures above and below this optimum, but relatively close to it, the microbe will survive, but its growth rate will be reduced.
Similar responses can also be shown for other environmental conditions (both physical and chemical, e.g., amount of radiation, salinity, pH, etc.) against any biological processes (e.g., reproductive success, respiration rates), for any type of organism. In each case, the organism will have its own set of response functions that define its limits of survival.
Microbes that can tolerate exposure to these extremes are extremotolerant. Microbes that require extreme conditions to survive and grow are known as extremophiles, with this survival requiring specialised biological adaptations. Indeed, since Earth hosts an incredible diversity of extreme environments, with each type of physical or chemical extreme (temperature, pH, salinity, pressure) requiring a separate collection of biological adaptations to survive. Table 4 represents the current state of knowledge for all categories of extremophile.
| Table 4 Our current understanding of extremophiles | |||
|---|---|---|---|
| Environmental factor | Limiting conditions | Type of extremophile | Comment |
| temperature | <15 °C | psychrophiles | Consist mainly of bacteria, algae and fungi. Psychrophiles have adaptations that enable them to survive low temperatures, e.g. cell membranes and enzymes that function optimally at relatively low temperatures. |
| 15–50 °C | mesophiles | The vast majority of organisms on Earth are mesophiles, inhabiting all the major temperate and tropical regions. | |
| 50–80 °C | thermophiles | The majority are single-celled organisms found in hot springs and undersea hydrothermal vents. Thermophiles have adaptations that enable their cell machinery to function at high temperatures, including structural modifications of their proteins, nucleic acids and cell membranes to give them greater heat stability. | |
| 80–121 °C | hyperthermophiles | Similar to thermophiles but able to tolerate even higher temperatures. | |
| radiation | The bacterium Deinococcus radiodurans is the most radiation-tolerant organism known, having been recovered from irradiated materials and also from rocks from regions of Antarctica that are thought to most closely resemble martian surface conditions. | ||
| salinity | 15–37.5% salt | halophiles | Bacteria that are able to grow in high concentrations of salt. |
| pH | 0.7–4 | acidophiles | Organisms able to tolerate highly acidic environments. |
| 8–12.5 | alkaliphiles | Organisms able to tolerate highly alkaline environments. | |
| desiccation | dry conditions | desiccation-tolerant organisms | A wide range of organisms are able to survive very dry conditions, including plants, fungi and bacteria. |
| pressure | high pressure | piezophiles | Organisms able to tolerate pressures hundreds of times that of atmospheric pressure at the Earth’s surface. Some organisms may be able to tolerate the pressures produced by shock waves in meteorite impacts. |
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Which one of the following names would least accurately describe the kind of extremophile you might expect to find in an ice-covered Antarctic lake?
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Mesophile is the correct answer.
Antarctic ice-covered lakes contain multiple potential extremes including low temperature and high pressure. Some may have high salinity or extreme pH, depending on the chemistry of the water. Therefore, an array of different types of extremophiles can be expected to be found within an ice-covered Antarctic Lake.
a.
Hyperthermophile
b.
Psychrophile
c.
Acidophile
d.
Piezophile
e.
Mesophile
The correct answer is e.
Extreme environments are the dominant environments identified on other planetary bodies today. Only where there have been geological investigations by space missions have clement environments been identified, but these have existed only in the past.
Now watch Video 3, which introduces some of the ideas around whether life might have once existed on Mars. Once you’ve watched the video, answer the questions that follow.
Transcript: Video 3 Could life have ever existed on Mars?
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Give two examples of extreme environments on Earth.
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The video mentioned high temperature hot springs in New Zealand, freezing temperatures of the High Arctic and high pressures at the bottom of the ocean, and desiccated salt flats of northern India.
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Why is present day Mars inhospitable to life?
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The surface of Mars receives extremely high levels of radiation from the Sun, and has low temperatures.
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What type of organism from Table 4 might survive on the surface or near sub-surface of Mars today?
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A psychrophile (one that can survive extremely low temperatures) or one that is radiation tolerant might survive. A desiccation resistant could also survive.
To determine what life could survive in these environmental extremes, and how, requires a study of life on Earth in environments that are similar – these are known as analogue sites.