Thursday, February 28, 2013

Antarctic Life Finds a Way

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?’. 

Figure 1. Metabolism of carbon dioxide by methanogens.

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.

Figure 2. Splitting of molecular hydrogen by the hydrogenase enzyme.

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.