Astronomy and space exploration excel at planning ahead. Decadal surveys and years of engineering effort for missions give the field a much longer time horizon than many others. In the near future, scientists know there will be plenty of opportunities to search for biosignatures everywhere, from near oceanic worlds (i.e. Titan) to potentially habitable exoplanets far away. But we don’t know what these biosignatures would look like. After all, there is currently only Earth’s biosphere to study, and it would be unfortunate to miss any clues about another just because it didn’t look like those found on Earth. Now, a team led by researchers at the Santa Fe Institute (SFI) has come up with a framework that could help scientists search for biosignatures that may be completely different from those found on Earth.
This framework is based on stoichiometry. A common feature of high school chemistry classes, stoichiometry is the study of chemical ratios. There are obvious stoichiometric relationships on Earth that are clearly formed by life as we know it. Generalizing these ratios to be applicable anywhere was the goal of the SFI article. Three main principles collectively make up the new framework.
The first principle is that stoichiometric values change with the size of individual cells. For example, as bacteria grow larger, the concentration of RNA increases while the concentration of individual proteins decreases. When these cells die, their size would help determine the concentration of molecules released into the environment.
Environmental distribution is also affected by the second principle – that the number of cells in an environment follows a law of power distribution as a function of their size. For example, there are probably many more small cells than large ones, depending on the simplest power law distribution curve. This size ratio, as well as the stoichiometries associated with these different sizes, then led to the third principle.
Applying this stoichiometric principle an additional step leads to a result which can be applied to biospheres more generally. In this case, the size of a given particle is a determining factor in its relation to the fluid that surrounds it.
Let’s continue to use RNA and proteins as an example. RNA is an order of magnitude larger than a protein. It is also more prevalent in larger cells, according to the first principle. Larger cells, however, are less prevalent in the environment, according to the second principle. Therefore, in a biologically active system, proteins, which are smaller, are more likely to have a higher concentration in a surrounding fluid than RNA, which is larger, would. Hence the third principle according to which its size determines the concentration of a particle in a surrounding liquid.
The immediate application of this framework is the study of oceanic worlds, like Titan or Enceladus, where there are likely liquid bodies that could contain concentrations of biological molecules within. Unfortunately, as of yet, there is no system capable of accurately measuring the size of particles that can be launched during missions in these worlds. But that doesn’t mean there won’t be some in the future. Thus, the potential to use this framework now requires a little more engineering expertise to develop such a system. And it’s already clear how good the astronomy and space exploration community is in this regard.
SFI – Origins of life researchers develop new ecological biosignature
Journal of Mathematical Biology – Generalized Stoichiometry and Biogeochemistry for Astrobiological Applications
Astrobiology – Biosignatures of exoplanets: a framework for their evaluation
UT – What will it take to find life? In search of biosignatures in the universe
Artistic conception of life on another planet