One of the signposts of life - and something that will likely be exploited in the study of potential Martian critters - is isotopic fractionation. The general wisdom is that life prefers light isotopes. Given the choice between hydrogen and deuterium (always present due to its construction 13.7 billion years ago during primordial nucleosynthesis) living things will tend to choose the lighter hydrogen. The same is true for carbon-12 and carbon-13, the light guy wins and is preferentially incorporated into the chemical doings of cells. When the extraordinary plumes of atmospheric methane were confirmed on Mars a couple years ago it was clear that the next step would be to try to pin down the isotopic composition of the gas. Here on Earth one can quite readily distinguish methane produced by bacteria or archaea and that produced by abiotic processes (most is biological), by tasting the isotopic composition.
So why is this so, why does life like particular isotopes. Ask a biologist and they will typically say that it's all about enzymes. Processes inside cells that work with enzymes - fabulous catalysts - happen faster with light isotopes, and bingo, you naturally sieve out the heavy guys and leave them in the dust. At least that's the line I've heard again and again. I was fascinated then when I stumbled across a very interesting work by Calsciotti. In this and subsequent papers the discovery of cellular processes involving nitrogen are described that actually prefer the heavy isotope nitrogen-15 rather than the lighter nitrogen-14. It appears that in some cases the heavier atoms provide a more energetically lucrative reaction route, et voila, like the fickle thing it is, life spits out the lighter variants in favor of something meatier.
The light isotope rule is not even that. Biochemistry again demonstrates that after a few billion years of fine-tuning it can exploit just about any trick in the book to get ahead. What I find amazing is that only about 0.4% of nitrogen on the Earth is the weightier 15, yet here we have organisms that will make use of the better chemical throughput of that isotope. One's first reaction is 'why bother?'. Obviously it may just be an unintended consequence of the chemical network, but I think the odds are good that somewhere along the line that tiny advantage will have been important. Here's the big question though, would a different biosphere - a martian one perhaps - with a presumably different evolutionary history, make all the same biochemical choices? To throw a final spin on this; Mars has a higher nitrogen-15 abundance than the Earth, a relative enrichment of about 60%. How much would this environment have swayed things?