As we ponder the possibility of life existing on other
planets, we have recently expanded our understanding of the parameters that
define how life can exist on our own planet.
After years of planning and preparation, an American team of scientists have now drilled through
800 meters of ice to tap into a confined lake
in Antarctica in search of life. On
January 28, they announced
that they had found it.
We don’t know yet exactly what ‘it’ is, but ‘it’ has
microbiologists and astrobiologists really excited right now. This is not as much because Antarctica is a
cold place as it is because this particular lake is an isolated place. Most of life thrives within some sort of
ecosystem where the existence of one organism depends on the existence of an
extensive network of others – the ‘great circle of life’, if you will. Usually this can be traced back to a plant or
other organism that gets its energy from sunlight. But a lake that has existed in darkness under
a sheet of ice, undisturbed for thousands of years (until we poked a hole in
it) is cut off from most of that. But
inevitably, ‘life finds a way’. Or to be more specific, metabolism finds a
way. Metabolism can be loosely defined
as the way that an organism makes energy from what it consumes. It usually involves a complex series of
chemical reactions that begins with some form of outside energy that is not
inherently useful. For instance plants
metabolize carbon dioxide and sunlight energy via photosynthesis to produce ATP
and carbohydrates – chemical forms of energy that all living things rely
on. So the big question here is, in a
cold, dark, confined, really salty lake, devoid of oxygen and limited in biodiversity, what sources
of energy are available to sustain any kind of life?
Carbon dioxide
We often think of oxygen as being necessary for life, but
this is because it is essential for our life as human beings and we are therefore
biased. Actually, quite a few organisms outside
of the animal kingdom do perfectly fine without oxygen. But carbon is essential – being as how all
life forms on earth are carbon-based life forms. For such, carbon-dioxide
is often the breath of life. As an
example, a group of microorganisms called Methanogens consume carbon-dioxide
and hydrogen gas, and expel methane (natural gas). Methanogens do this by the chemical reaction
in Figure 1. But don’t let the
simplicity of the reaction fool you. The
reaction
proceeds through six intermediate steps that are catalyzed by ten different
enzymes. Methanogens live deep in the
soil where decomposition takes place; or in the intestinal track of another organism
where digestion takes place. It is
unlikely that they would be in the Antarctic lake, as they tend to avoid salty
environments. But if they did, then the
next question would be: ‘Is anything consuming the methane?’.
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| Figure 1. Metabolism of carbon dioxide by methanogens. |
Hydrogen
Hydrogen is but a proton orbited by an electron, but even
the simplest element in the universe can be used for energy by microorganisms. There are a few bacteria that have a very
special enzyme called hydrogenase that enables them to essentially split
molecular hydrogen (H2) into its constituent electrons and protons, and use the
resulting separately charged particles for energetic purposes. The reaction is easier said (Figure 2) than done,
and great efforts
are underway to understand the chemistry well enough to reproduce it
artificially. Hydrogen would be
inherently present in any aquatic environment, and this could be an initial
energy producing step in the metabolism of something in an Antarctic lake.
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| Figure 2. Splitting of molecular hydrogen by the hydrogenase enzyme. |
Minerals
Beneath the icy surface of Antarctica and at the bottom
of the hidden lakes, is terrestrial land – just like every other
continent. And with that comes minerals,
which can be used to sustain life in spite of their obvious non-biological
nature. There are a handful of bacteria
that scientists call ‘chemolithotrophs’, which is a fancy word for
‘chemical-rock-eaters’. In other words,
these bacteria can derive the energy they need for metabolism from elements
such as sulfur and iron that are present in minerals. Again, specialized enzymes make this possible
by shuttling electrons and harnessing their energy. Because minerals are usually deficient in the
necessary carbon, these bacteria often subsist on carbon dioxide as well. These microorganisms can feed on a variety of
minerals and are found in diverse environments, from hydrothermal sea vents
to less isolated Antarctic lakes. It is very likely that these newly reported
Antarctic bugs are some type of specialized ‘chemical-rock-eater’ related to
those found elsewhere in Antarctica.
In time, the American researchers will perform a genetic
analysis so that they can classify these microorganisms and give them a place
on the tree of life. This will lead to
further understanding of their metabolic nature. Meanwhile, two other teams
of scientists are in the process of performing similar experiments on other
lakes that are buried deeper under the Antarctic ice sheet and incidentally
more isolated. If they find anything, it
will be interesting to compare, and then wonder about the possibility of
similar microbes existing in similar environments, within our own solar system.