And How We Can Detect It Through Information Theory
Beneath our feet, stretching for miles into the Earth's crust, exists a vast, living world. Microbes—bacteria and archaea—comprise up to half of all living material on the planet, thriving in the pores and fractures of deep rocks7 .
For decades, scientists have known that these microorganisms are more than just survivors; they are "ecosystem engineers"1 . They consume minerals, exude sticky substances, and fundamentally change the chemistry and physical structure of their habitat.
This fascinating concept, rooted in Shannon's information theory, provides a powerful new lens for detecting life. It suggests that life leaves a distinct signature in the very order and structure of its environment, a signature that could one day help us find life on other planets.
| Reagent/Material | Function in Experiment |
|---|---|
| Mineral Sediment Suspensions | Serves as a model geophysical system to observe how microbes colonize and alter mineral surfaces1 . |
| Nutrient Solutions (e.g., Carbon & Nitrogen) | Provide essential nutrients to support microbial growth and activity, often washed down from topsoil in deep subsurface studies2 . |
| Extracellular Polymeric Substances (EPS) | Mucilaginous metabolites secreted by microbes that glue mineral particles together, altering the physical structure of the environment1 . |
| Iron & Sulfur-bearing Minerals | Used in biofilm colonization studies; microbes derive energy from these minerals, forming dense "hotspots" of life6 . |
In information theory, Shannon entropy measures uncertainty or randomness. When applied to a geophysical system, we can ask questions like, "What is the size of a random particle in this sediment?" or "How effectively does it fill space?"
A system with low entropy (high information) is ordered and predictable, like a collection of identical marbles. A system with high entropy (low information) is disordered and uncertain, like a box of marbles of many different sizes and shapes1 .
Abiotic processes like erosion can also create disorder. However, microbial activity introduces a unique and more profound level of complexity. As they colonize surfaces, form biofilms, and glue mineral particles together with their mucilaginous metabolites, they dramatically alter physical characteristics like particle size distribution and space-filling capacity1 .
This microbial "engineering" increases the Shannon entropy of the system, making it more unpredictable than its abiotic counterpart. This measurable change is the microbial information signature.
Ordered, predictable particle distribution
Abiotic or biomass-freeMicrobes alter particle size and distribution
Increasing disorderDisordered, unpredictable particle distribution
Biomass-affectedComplementing the laboratory work, field studies have revealed where these microbial engineers prefer to operate.
In a landmark study conducted 1.5 kilometers below ground in a former gold mine, scientists discovered that subsurface microbial life is not evenly distributed6 .
Instead, microbes form dense biofilms, or "hotspots," that correlate strongly with iron-rich minerals6 .
This mineral selectivity drives the distribution of a massive amount of biomass, estimated to be:
of all bacterial and archaeal life in the continental subsurface6
Estimated range of subsurface microbial life forming mineral-associated hotspots
To test the hypothesis that life changes the information content of its environment, researchers designed a sophisticated experiment using a model geophysical system: mineral sediment suspensions1 .
The researchers conducted 105 experiments with different types of suspensions: pure mineral (abiotic), nutrient-affected, and actively microbially-colonized1 .
These suspensions were subjected to various abiotic conditions, including different nutrient concentrations, mineral concentrations, and background entropy production rates, to test the method's robustness1 .
For each experiment, they measured key geophysical properties of the mineral particles, specifically their size (L) and their capacity dimension (d₀), which is a fractal measure of their space-filling capacity1 .
Using Shannon's formula, they calculated the entropy of the particle size and capacity dimension distributions for each system. A higher entropy indicated a more disordered, less predictable system1 .
The results were clear. Systems that were colonized by microorganisms showed a significantly higher Shannon entropy than their abiotic counterparts1 . The microbes' activity had made the geophysical system more disordered and information-rich.
| BI Value | Interpretation |
|---|---|
| BI ≥ 0 | The system is biomass-free (has similar or lower entropy than the abiotic reference). |
| BI < 0 | The system is biomass-affected (has higher entropy than the abiotic reference). |
| BI = -1 | The system contains no information about the geophysical property (maximum entropy). |
| System Type | Average Shannon Entropy | Detection Outcome |
|---|---|---|
| Pure Mineral (Abiotic) | Lower | Correctly identified as biomass-free |
| Microbially-Colonized | Significantly Higher | Correctly identified as biomass-affected with <10% error |
The power of this method is its robustness. The study demonstrated that this Biomass Index could detect microbial colonization with an error of less than 10%, particularly in systems with a low background entropy production rate1 .
The discovery that living microorganisms change the information content of their environment is more than a laboratory curiosity. It represents a paradigm shift in how we can detect and understand life.
This approach provides a theoretical foundation for a universal life-detection method1 . Unlike searching for a specific chemical, it looks for the fundamental signature of life's complexity-generating nature.
As researcher Yuran Zhang noted, "If geological activity is a driver for early life formation or diversification, then maybe we should look for extraterrestrial life on planets that are geologically active"7 .
By using information theory as a guide, scientists are developing new tools to listen to the subtle signals of life in the rocks beneath us and, potentially, beyond our world.
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