When scientists hunt for life, they often look for biosignatures, chemicals, or phenomena that indicate the existence of present or past life. Yet it is not necessarily true that signs of life on Earth are signs of life in other planetary environments. How do we find life in systems that are not like ours?
As part of groundbreaking new work, a team * led by Professor Chris Kempes of the Santa Fe Institute has developed a new ecological biosignature that could help scientists detect life in very different environments. Their work appears in a special issue ofMathematical Biology Bulletin collected in honor of renowned mathematician biologist James D. Murray.
The new research starts from the idea that stoichiometry, or chemical ratios, can serve as biosignatures. Since “living systems display surprisingly consistent ratios in their chemical makeup,” Kempes explains, “we can use stoichiometry to help us detect life.” Yet, as Simon Levin, member and contributor of the SFI Science Board, explains, “the particular elementary relationships that we see on Earth are the result of the particular conditions here, and of a particular set of macromolecules like proteins and ribosomes, which have their own stoichiometry. “How can these elementary relationships be generalized beyond the life we observe on our own planet?
The group solved this problem by relying on two models of law, two laws of scale, which are entangled in the elementary relationships that we have observed on Earth. The first of these is that in individual cells the stoichiometry varies with the size of the cells. In bacteria, for example, as cells increase in size, protein concentrations decrease and RNA concentrations increase. The second is that the abundance of cells in a given environment follows a power law distribution. The third, which arises from the integration of the first and the second into a simple ecological model, is that the elemental abundance of particles relative to the elemental abundance in the environmental fluid is a function of the size of the particles.
While the first of these (that elemental ratios change with particle size) leads to a chemical biosignature, it is the third finding that leads to the new ecological biosignature. If we do not think of biosignatures simply in terms of chemicals or single particles, but rather consider the fluids in which the particles appear, we see that the chemical abundances of living systems manifest as mathematical relationships between the particle and the environment. These general mathematical models can appear in coupled systems that differ considerably from Earth.
Ultimately, the theoretical framework is designed to be applied in future planetary missions. “If we go into an ocean world and look at particles in the context of their fluid, we can begin to wonder if these particles exhibit a power law that tells us that there is an intentional process, like life, that makes them, ”says Heather Graham, Associate Principal Investigator at NASA’s Laboratory for Agnostic Biosignatures, of which she and Kempes are a part. However, to get through this applied step, we need a technology to sort particles by size, which we don’t have for spaceflight right now. Still, the theory is set, and when the technology lands on Earth, we can send it to icy oceans beyond our solar system with a promising new biosignature in hand.
Read the article * Christopher Kempes (Santa Fe Institute), Michael Follows (MIT), Hillary Smith (Pennsylvania State University), Heather Graham (NASA Goddard Spaceflight Center), Christopher House (Pennsylvania State University) and Simon Levin (Princeton University) , Santa Fe Institute) are co-authors of the article “Generalized stoichiometry and biogeochemistry for astrobiological applications”, in the Mathematical Biology Bulletin.
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