Each recent report of liquid water existing elsewhere in the solar system - be it ice on comets, oceans on Europa, or more recently water on Mars - has reverberated through the international press and excited the imagination of humankind.
Why? Because in the last few decades we have come to realize that where there is liquid water on Earth, virtually no matter what the physical conditions, no matter where, there is life. What we previously imagined were insurmountable physical and chemical barriers to life, such as extremes in temperature, pH, and radiation, are now seen as yet another niche harboring so-called "extremophiles." This realization, coupled with new data on the survival of microbes in the space environment, as well as modeling of the potential for transfer of life between planets, suggests that life could be more common than previously thought.
This raises several profound questions, one of which is: If life were to be found beyond Earth, would it be the result of an independent origin, or merely a distant relative?
There are several potential niches for life elsewhere in the universe, as well as terrestrial niches that we consider extreme, that may not be at all extreme from either an evolutionary, or even a physiological point of view. UV radiation tolerance, acidophily (acid lovers), alkilophily (base lovers), thermophily (heat lovers), halophily (salt lovers), and anaerobiosis (oxygen haters) may all be cases in point. Here I concentrate on the geochemical extremes of salinity and desiccation. Although not identical, they are related.
Love of Salt
Organisms live within a range of salinities, from essentially distilled water to saturated salt solutions. Halophily refers to the ionic requirements for life at high salt concentrations. Osmophily refers to the osmotic aspects of life at high salt concentrations, especially turgor pressure, cellular dehydration, and desiccation. Although these phenomena are physiologically distinct, they are environmentally linked. Thus, a halophile must cope with osmotic stress.
On Earth, halophiles are everywhere there is salt. They represent a phylogenetically, physiologically, evolutionarily, and ecologically diverse group of organisms. Halophiles occur in all three domains of life, archaea, bacteria and eukarya. Most halophiles are found interspersed among non-halophiles in the phylogenetic tree. They can be heterotrophs or autotrophs, and some have light harvesting pigments either for photosynthesis or for energy production via rhodopsin. They live in cold or hot environments, wet environments (e.g. lakes and ponds), dry environments (e.g. soils and salt crusts), and alkaline as well as neutral environments. They can be aerobes, anaerobes, or facultative anaerobes. Some possess true cell walls (bacteria and most eukarya) and some do not, such as most of the archaea. They even differ with respect to their modes of osmotic adaptation. The one characteristic halophiles have in common is their ability to live in hypersaline environments.
Adaptation to life at high salt concentrations can be achieved in different ways. The most commonly occurring strategy involves the accumulation of organic osmotic solutes without the need for specialized adaptation of intracellular proteins to high salt. This mechanism occurs in all three domains of life.
The second strategy is the intracellular accumulation of high concentrations of K+. This strategy, unlike the use of organic solutes, requires extensive adaptation of the intracellular enzymatic machinery to be functional in the presence of high ionic concentrations. The great diversity in strategies used by the halophiles to cope with the high salinity in their environment, coupled with the fact that halophily occurs throughout the tree of life in all three domains, suggests that adaptation to life at high salt concentrations is easy to evolve, and probably arose many times during life's evolution.
Cells respond to desiccation the same way they respond to osmotic stress from increasing salt concentration. This is not surprising since as a cell desiccates, the salts in and around the cell become more concentrated. As desiccation continues, organic solutes, or K+, are produced. These solutes accumulate away from proteins pushing the scarce water molecules next to the proteins and stabilizing them.
Life in Space
Space is a "new" category of extreme environment in the search for life in the universe. Space flight technology has enabled biological studies to be conducted in the space environment. This has allowed us to understand the potential for life to survive interplanetary space travel aboard spacecraft, meteors and comets.
From an organism's perspective, the space environment is not only inhospitable, but downright nasty. An organism in space faces extreme cold, is exposed to unfiltered solar radiation, solar wind, galactic radiation, space vacuum, and negligible gravity. Terrestrial organisms most likely to survive these conditions are microbes, with comets or meteorites as conveyance.
Microgravity is not lethal. Cold tolerance and anhydrobiosis (vacuum desiccation) are survivable. Because of the extreme cold and anhydrobiosis, the organisms are not metabolizing, so nutritional needs would not exist. Thus, we are left with one potential "show-stopper": radiation.
The two types of radiation most likely to cause cellular damage are heavy ions and UV radiation. Most damage to microbes exposed to space is due to UV radiation, especially during the short term, unless it is protected by being buried in a meteor or inside a spacecraft. During the long term, however, heavy ionizing radiation has a greater probability of being lethal no matter if the organism is inside a meteor or spacecraft.
Remarkably, some terrestrial organisms can survive this very extreme environment. Microbes tested in the space environment to date include Bacillus subtilis spores, bacteriophage T-1, Tobacco Mosaic Virus, and most recently halophilic microbes. Bacillus subtilis spores will survive for at least six years in space if either in a bi-layer, or mixed with glucose to protect them against high solar UV-radiation flux. But if they are exposed in a monolayer, they are killed within minutes. For comparison, viruses lose viability on the order of days. Halophiles can survive for at least two weeks in space and probably much longer. The halophiles are the first demonstration of a vegetative cell surviving exposure to the space environment.
Panspermia, as proposed by Richter, Lord Kelvin, and Arrhenius during the late 1800s and early 1900s posits that reproductive bodies of living organisms exist throughout the universe, wandering through intergalactic space, and living and evolving wherever the environment is favorable. This implies that conditions favorable to the development of life prevailed at different locations in the universe and at different times.
Major criticism of this idea includes the fact that unaided living organisms will not survive radiation exposure for the long period of time required to travel from solar system to solar system. Additionally, this original proposal avoids the issue of where and how life began. However, results from the Long Duration Exposure Facility and BioPan space experiments showing that microbes can survive in space has led to a reexamination of the feasibility of the notion of interplanetary transfer of living material, particularly microbes within a solar system.
With that, the proposed definition of panspermia may be re-defined to state that life may originate on one planet and be transported to other planets in the solar system. If the environment is favorable then life may evolve on that planet. Therefore, life, as we know it could travel between other bodies within the solar system and the Earth, and indeed the life we find on Earth may have been, or may be, present elsewhere.