By Katherine Brown
Extremophiles are defined as organisms that may survive comfortably in environments that would be impossible for a human to survive in – whether this be extremes of temperature, pressure, pH, salinity or radiation. The word “extremophile” is a very loose definition, and is better treated as an umbrella term, from where you must specify in which extreme environment the organism survives. To best understand life at extreme conditions, it is perhaps best to discuss the limiting factors on life.
It’s a commonly known fact that the human body operates at 37°C, and in fact all mammals have a body temperature of between 36 and 40°C. Reptiles and fish, being cold blooded, have lower body temperatures than mammals, whereas for birds it is slightly higher. All told, most animals are found to have body temperatures in the range of 8 to 48°C. So what is it that sets this limit?
DNA is famed as being the basis of life: the molecule that stores the genetic code for many organisms. It has an iconic double helix structure, but like any molecule, there comes a point where the molecule denatures; this means that its structure gets changed such that the molecule can no longer fulfil its purpose in the organism. In DNA, this occurs in an interesting way: when you begin to heat the molecule, the double helix structure slowly begins to untwist to a ladder-like structure before the bonds between the two halves of the ladder break. This transition begins at 76.7°C, and the molecule is completely destroyed by 77.0°C, from where it cannot be recovered. Similarly, chlorophyll denatures at 75°C, meaning that plant life cannot photosynthesise above this temperature. In fact, most proteins and cellular structures denature by 100°C, with the cell walls becoming fluid in this region and the cells bursting.
This is clearly one limit for the existence of life but does not explain the body temperature upper limit at around 48°C for mammals and birds. Rather, this limit is explained by the low solubility of oxygen at high temperatures, meaning that any organism that depends on the absorption of oxygen will struggle to survive long before cellular breakdown.
At the other end of the temperature scale, the clear limiter for life is water. Water freezes at 0°C, but in fact is at its most dense at 4°C. Humans are 68% water, and most other animals fall within the range of 60-70% water (with a few exceptions: jellyfish are around 95% water).
Primarily, the water acts as a solvent: the medium through which molecules in cells may diffuse, pass through membranes, and therefore react with one another. Thus, if the water is very dense, or indeed solid, then life cannot realistically be sustained because no molecular reactions will occur. This is also an important point to consider when discussing life at high pressures. Here we are making no mention of the energy required for the reaction of biological molecules, which also sets limits on the minimum temperatures for life.
Extremophiles that can survive at temperatures above 80°C are described as hyperthermophilic, and organisms that can survive below 15°C are called psychrophilic. The animal that can survive at the lowest temperature is the Himalayan midge, which can survive at -18°C, and interestingly that is also the lowest temperature at which microbial colonies are active; though they can survive being preserved by temperatures lower than this. So, it seems that the lower limit due to freezing of water is important for all cells, regardless of the type of organism they belong to.
On the other hand, at high temperatures it is found that one strain of archaea (a type of microorganism, like a bacterium with no cell nucleus) – Strain 121 – is capable of growth at 121°C, and one enzyme – amylopullulanase – can be active at 142°C. Clearly this is well above the boiling point of water and the temperature where proteins break down, so how do thermophiles manage this?
Many extremophilic organisms have evolved specific traits and methods to survive under extreme conditions, past the point where their individual components should have ceased to work. Archaea microbes, for example, have evolved such that their DNA is formed in to a two-dimensional crystal of proteins, which has higher melting point than the individual protein molecules and hence avoids the problem of the proteins denaturing at high temperatures.
Besides temperature, there are many extreme conditions in which extremophiles can be found. Environments under extreme pressures are found in the deepest places on the Earth, where the pressure is 101kPa at the surface and increases at a rate of 10.5kPa per metre depth for hydrostatic pressure (underwater), compared with 22.6kPa per metre for lithostatic pressure (underground). The place on Earth with the highest pressure is in the Marinas Trench, where pressures can reach 108MPa – 1000 times that at the surface.
At these depths the pressure is so great that normal microbial functions start to fail. As we now know, the ability for molecules to diffuse across cells is key. The low volumes due to extreme pressures reduce membrane fluidity and affect the interactions of proteins, meaning that diffusion of molecules across the cell is problematic. Yet life still exists at these high pressures, in fact there are some microorganisms found in the Marinas Trench that require pressures of at least 50MPa in order to survive – for instance the strain MT41. These organisms are found to have modifications to the structures of their transport proteins in order that they may live under such harsh conditions.
Little is known about the behaviour of microorganisms that have evolved for low pressures, likely due to the lack of low-pressure environments on Earth. The pressure on the peak of Everest is 34kPa, which is about a third of atmospheric pressure at sea level: not a great enough change to observe a lot of differences. Studies of samples from outer space should yield more information on this, but any organism that survives the vacuum of space must also survive the challenges posed by low temperatures and high levels of radiation.
Radiation is dangerous to life because of its ionising nature, which can break bonds in molecules and damage cells. The organism that is able to withstand the greatest amount of radiation is a bacterium – D. radiodurans – which can survive 20,000Gy of gamma radiation, where the amount considered deadly to a human is a whole-body acute dose of only 5Gy. This resistance to radioactivity is, in this case, considered to be a side effect of this bacterium also having the ability to survive desiccation – severe lack of water. D. radiodurans is a prime example of a polyextremophile, able to survive multiple different extremes including also acidity, vacuum and low temperatures. It’s a well-studied extremophile and shows us how adaptations to one extreme may be useful in a variety of environments.
This is all very interesting but why is the study of extremophiles of use to us – and what relation does it have to condensed matter?
Extremophiles represent a great opportunity to better understand the origins of life of Earth and, furthermore, life in space. For instance, investigating the effects of space travel upon organisms may give us clues about the origin of life and will become increasingly relevant as we send more and more material (including humans) into space. The field of condensed matter gives us the tools that are necessary to study this area: from using scanning tunneling microscope (STM) technology to study the structure of microorganisms; to using scattering techniques to understand the behaviour of individual proteins; and on to aiding our understanding of large molecular objects and their interactions.
SEM image of Milnesium tardigradum in active state: doi:10.1371/journal.pone.0045682.g001. Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. (2012).
The National Oceanic and Atmospheric Administration: Pacific Ring of Fire 2004 Expedition. NOAA Office of Ocean Exploration; Dr. Bob Embley, NOAA PMEL, Chief Scientist.
Life in extreme environments; Lynn J. Rothschild & Rocco L. Mancinelli; Nature Vol 409 (2001)
Extremophilic Bacteria and Microbial Diversity; Enhancement Chapter; Raven and Johnson’s Biology, Sixth edition
Thermal Denaturation of DNA Studied with Neutron Scattering; Andrew Wildes et al.; PRL 106, 048101 (2011)
Pyrolobus fumarii, gen. and sp. nov., represents a novel group of archaea, extending the upper temperature limit for life to 113°C; Elisabeth Blöchl et al.; 14 Extremophiles (1997) 1:14–21
High Pressure Effects on Proteins and other Biomolecules; Karel Heremans; Ann. Rev. Biophys. Bioeng. (1982) 11:1-21