Life in Extreme Environments: The Universe May Be More Habitable Than We Thought: Part 1

Artist's concept of an astronaut examining a rock sample on Mars. Life has been found living inside rocks in the various extreme environments on Earth. Clues derived from finding life in such terrestrial locations will serve as a guide to understanding where we might find life on other worlds.
Note: A shorter version of this article appeared in the May/June 2002 issue of the National Space Society's Magazine Ad Astra. Reprinted here with the author's permission

The crew of Starship Enterprise regularly boasts that they go where no man has gone before. Alas, scientists have discovered that life on Earth has already done that - and has done so for billions of years. Life flourishes in physical and chemical extremes that, until recently, were thought to preclude life - hence the term "extreme" that is often used to characterize these locales. These extreme environments - hot, cold, acidic, saturated by radiation are also similar to what we expect to find on other worlds. As such astrobiologists view these environments and the life that flourishes there as a preview of what we might find elsewhere in the universe.

The various unique physiologies that have evolved to meet the challenges poised by these "extreme" environments demonstrate that life could exist in some of the extreme environments found in space and beyond. Indeed, adaptations have been found in some terrestrial organisms that could allow travel between planetary bodies.

Where are these extreme places? What challenges do they present to life - and how has life adapted? And what does this say about life elsewhere?

Chemistry

Organic chemistry - chemistry based on reduced forms of carbon ("reduced" carbon has had hydrogen added) has been shown to operate not just upon on our planet, but across our solar system, and far, far beyond. Carbon can trump even silicon (a common component of rocky planets) in its ability to form an astonishing variety of long and complex compounds. It is these long complex molecules that make life capable of what it does. Indeed, there is quite a ubiquity of organic chemistry in the universe: many of the compounds associated with terrestrial life have been found to be floating in the vast spaces between stars.

Water - Life's Solvent

Water is an excellent solvent for organic molecules - it provides a context wherein increasingly complex chemical reactions can occur - and be sustained. Based on what we have seen of life, it appears that liquid water is the sine qua non of life. Based on this understanding, the official mantra of the current Mars program at NASA is to "follow the water". Admittedly organic carbon (in contrast to carbon dioxide and carbon monoxide) has yet to be detected on Mars - but we're looking for it!

If water is indeed essential for life, a variety of physical limits to life seem apparent. But "seems" is the operative word. What may seem to be true in a theoretical or experiential context may not be true once sufficient observations have been made.

Water is a liquid and remains so within certain physical criteria such as temperature and pressure. To much and too little of either can bring life's processes to a halt. As water becomes scarce, a struggle for survival ensues. For life to continue, temperature has to be within the range wherein water can exist in liquid form. We have yet to find any form of life that can directly utilize solid (i.e. frozen) water.

Temperature has another importance: organic molecules lose the structure necessary for them to function (i.e. they "denature") at certain temperatures. For both DNA and chlorophyll (the molecule at the core of photosynthesis) this temperature is around 70°C. On the other hand, as temperature drops, biochemical reactions slow. Ice crystals which begin to form within cells which can cause irreparable harm as they slash through cellular membranes. Membranes are the surfaces upon which life's myriad reactions occur. They also serve to contain a cell's contents. Their damage slows down an organism's biochemistry.

Other factors inhibit life's ability to operate: extremes in pressure can destroy molecular structures and inhibit enzymatic reactions. Then there are toxins in the environment, such as mercury, arsenic and cadmium which can poison metabolism. High levels of radiation can damage a variety of organic molecules, most notable among these being the very genetic material of the cell, DNA. Ditto for oxygen.

My Air is Your Poison

Wait! - oxygen as an extreme environment? Oxygen allows the production of ATP, the energy currency utilized by all cells. This process is 18 times more efficient than anaerobic metabolism i.e. metabolism that occurs in the absence of oxygen. However, this increased efficiency comes at a steep price. The reduced (hydrogenated) forms of oxygen, such as hydrogen peroxide and especially the hydroxyl radical, may be extremely dangerous. The resulting oxidative damage they can cause can damage DNA causing mutations, or even death. Anaerobes (organisms that do not use oxygen in their metabolic processes) do not have the ability to detoxify the various forms of oxygen, and accordingly find oxygen lethal.

As such, from the perspective of a substantial portion of the life on Earth, the ability to live in an aerobic (oxygen rich) world confers upon our own species the distinction of being an extremophile. But there are other things that organisms can "breathe". The bacterium Shewanella putrefaciens uses metal atoms in its metabolism in the same fashion as we use oxygen atoms. As such, it "breathes" metal - in this case, manganese.

Clearly there are physical and chemical extremes that should make life based on organic carbon difficult if not impossible. Yet, within the last few decades we have found organisms that have punctured these seemingly insurmountable limits and have come to called "extremophiles" from the Latin "extremus" (being on the outside) and the Greek "philos" for love. Organisms that can live in more than one extreme, for example Sulfalobus acidocaldarius (a member of the Archea - an ancient branch off the family tree of life) lives at pH 3 and 80°C, are called polyextremophiles.

Who are the extremophiles?

The word "Extremophile" often invokes images of microbes, and so-called "simple" ones at that, yet the taxonomic range spans all three domains. (Note that life itself is so complex that the human creation of life has remained elusive. Thus, it is unjustifiably arrogant of us to call any form of life "simple".) While all organisms that live at extremely high temperatures are Archaea or Bacteria, eukaryotes (organisms whose cells have nuclei) are common among organisms that thrive at low temperature, extremes of pH (high acidity or alkalinity) pressure, water, and salt levels. Extremophiles include multicellular organisms, cold-lovers include vertebrates such as penguins and polar bears.

To qualify as an extremophile, does an organism have to be an extremophile during all life stages? Under all conditions? Not at all. Spores, seeds, and sometimes eggs or larval stages are all far more resistant to environmental extremes than adult forms. Yet some adult organisms - trees, frogs, insects, and fish - can endure remarkably low temperatures during the winter as a result of seasonal shifts in physiology such as hibernation.


A Scanning Electron Micrograph of a Tartigrade
One of the most resilient organisms known are tardigrades ("water bears"). Tardigrades can go into a hibernation mode - called the tun state - one that is more akin to "suspended animation" whereby it can survive temperatures from -253°C to 151°C, as well as exposure to x-rays, and vacuum conditions. When you place tardigrades in perfluorocarbon fluid (again while hibernating), at a pressure of 600 MPa, (that's almost 6,000 times atmospheric pressure at sea level) they emerge from the experience just fine . Even the bacterium Deinococcus radiodurans, the most radiation resistant organism known, only achieves this resistance under some conditions such as fast growth and in nutrient-rich medium.

Who? What? How?

Living in a Goldilocks world that is not too hot, not too cold and so on is the easiest environment for life to exist. An extremophile must either live within these parameters, or guard against the outside world in order the maintain these conditions intracellularly. With these rules in mind, we examine selected environmental parameters, summarized in Table 1.

Table I. Classification and examples of extremophiles

Environmental parameter

type

definition

examples

temperature

hyperthermophile

thermophile

mesophile

psychrophile

growth >80°C

growth 60-80°C

15-60°C

<15°C

Pyrolobus fumarii, 113°C

Synechococcus lividis

Homo sapiens

Psychrobacter, some insects

radiation

   

Deinococcus radiodurans

pressure

barophile

piezophile

Weight loving

Pressure loving

unknown

For microbe, 130 MPa

gravity

hypergravity

hypogravity

>1g

<1g

None known

None known

vacuum

 

tolerates vacuum (space devoid of matter)

tardigrades, insects, microbes, seeds.

desiccation

xerophiles

anhydrobiotic

Artemia salina; nematodes, microbes, fungi, lichens

salinity

halophile

Salt loving (2-5 M NaCl)

Halobacteriacea, Dunaliella salina

pH

alkaliphile

 

acidophile

pH >9

 

low pH loving

Natronobacterium, Bacillus firmus OF4, Spirulina spp. (all pH 10.5)

Cyanidium caldarium, Ferroplasma sp. (both pH 0)

oxygen tension

anaerobe

microaerophil

aerobe

cannot tolerate O2

tolerates some O2

requires O2

Methanococcus jannaschii

Clostridium

Homo sapiens

chemical extremes

gases

metals

Can tolerate high concentrations of metal (metalotolerant)

Cyanidium caldarium (pure CO2)

Ferroplasma acidarmanus(Cu, As, Cd, Zn); Ralstonia sp. CH34 (Zn, Co, Cd, Hg, Pb)

Dry environments

Imagine a desert and a feeling of dehydration follows. In the absence of water, lipids (fats) , proteins and nucleic acids (DNA, RNA) suffer structural damage. The Atacama desert located on the high northern Andean plains of Chile is one of the oldest, driest hot deserts on the Earth, while the Antarctic dry valleys are the coldest, driest places on Earth. In both cases, despite environmental extremes, life exists in the form of microbes: cyanobacteria, algae, lichens, and fungi.

Anhydrobiosis is a strategy organisms use to survive dry spells. During anhydrobiosis their cells come to contain only minimal amounts of water. No metabolic activity is performed. A variety of organisms can become anhydrobiotic, including bacteria, yeast, fungi, plants, insects, the aforementioned tardigrades, mycophagous (fungi-eating) nematodes, and the brine shrimp Artemia salina (also known as "Sea Monkeys" when marketed to school age children). During the drying out process (desiccation), less available water forces substances to increase in their concentration. Such increases lead to stressful responses within a cell that are similar to those of a cell experiences when exposed to high salt environments.

The ultimate dry environment is the "desert" of space. Adaptations to desiccation are critical for organisms to survive in interplanetary space. One organism in particular (described below) is a natural born space traveler.

Continued in PART 2


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