Energy-ordered resource stratification as an agnostic signature of life

Abstract

The search for extraterrestrial life hinges on identifying biosignatures, often focusing on gaseous metabolic byproducts as indicators. However, most such biosignatures require assuming specific metabolic processes. It is widely recognized that life on other planets may not resemble that of Earth, but identifying biosignatures "agnostic" to such assumptions has remained a challenge. Here, we propose a novel approach by considering the generic outcome of life: the formation of competing ecosystems. We use a minimal model to argue that the presence of ecosystem-level dynamics, characterized by ecological interactions and resource competition, may yield biosignatures independent of specific metabolic activities. Specifically, we propose the emergent stratification of chemical resources in order of decreasing energy content as a candidate new biosignature. While likely inaccessible to remote sensing, this signature could be relevant for sample return missions, or for detection of ancient signatures of life on Earth itself.

Fig. 1. Energy-ordered resource stratification as an observable signature of life.

a Stratified profiles of chemical resources layered by energy content are commonly observed on Earth, e.g., microbial mats, early Earth fossils (stromatolites), Winogradsky columns, and in marine environments. b Such profiles are generally understood to be shaped by biotic species (typically microbes) that metabolize these resources for energy. Here, we propose that energy-ordered resource stratification is a robust signature of biotic action.

Fig. 2. Self-replication and ecology lead to energy-ordered resource stratification.

a Two universal features of life are self-replication and ecological interactions between different biological species—the simplest being antagonism. b Simulating a minimal model incorporating these two ingredients (for details see text) shows that these two ingredients lead to spatially stratified profiles of (b) species and (c) resources. Shown here is an example from a simulation for 5 species and 5 resources. Antagonistic interactions segregate species spatially, with species displacement order determined by the energy content of the resource they consume. In each segregated zone, species deplete resources proportional to their abundance.

Fig. 3. Both self-replication and interspecies antagonism are necessary for robust spatial stratification.

a Quantification of resource penetration depth: for each simulated resource profile (blue and red), we find the width of the rectangle with area equal to that of the resource profile and the same initial height. b Quantification of stratification order parameter: for all resource profiles obtained from one simulation, we compute the negative of the correlation between their penetration depths and energy content Y_α (shown is an example from a simulation with 5 profiles). c Heatmap of the stratification order parameter over multiple simulations, where we systematically varied the self-replication parameter γ and the density of ecological interactions ρ. Spatial stratification does not emerge in the absence of either self-replication or ecology. As γ and ρ both increase, stratification emerges robustly.