[meteorite-list] Seeking Deep Space Salt Lovers

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jul 21 14:49:36 2005
Message-ID: <200507211811.j6LIBja19136_at_zagami.jpl.nasa.gov>

http://www.space.com/searchforlife/seti_saltlovers_050721.html

Seeking Deep Space Salt Lovers
By Rocco Mancinelli
Principal Investigator, SETI Institute
21 July 2005

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.
Received on Thu 21 Jul 2005 02:11:45 PM PDT