Closed ecosystems extract energy through self-organized nutrient cycles

TL;DR

This paper presents a conceptual model explaining how self-organized nutrient cycles enable closed ecosystems to sustainably extract energy, revealing convergent thermodynamic features across diverse communities and the impact of energy input levels.

Contribution

The study introduces a new model for self-organization in closed ecosystems, highlighting thermodynamic constraints and emergent energy extraction features regardless of species diversity.

Findings

Large, diverse communities extract about 10% of maximum energy

Communities show convergent nutrient fluxes in energy cycles

Energy input increases variability in energy extraction features

Abstract

Our planet is roughly closed to matter, but open to energy input from the sun. However, to harness this energy, organisms must transform matter from one chemical (redox) state to another. For example, photosynthetic organisms can capture light energy by carrying out a pair of electron donor and acceptor transformations (e.g., water to oxygen, CO$_2$ to organic carbon). Closure of ecosystems to matter requires that all such transformations are ultimately balanced, i.e., other organisms must carry out corresponding reverse transformations, resulting in cycles that are coupled to each other. A sustainable closed ecosystem thus requires self-organized cycles of matter, in which every transformation has sufficient thermodynamic favorability to maintain an adequate number of organisms carrying out that process. Here, we propose a new conceptual model that explains the self-organization and emergent features of closed ecosystems. We study this model with varying levels of metabolic diversity and energy input, finding that several thermodynamic features converge across ecosystems. Specifically, irrespective of their species composition, large and metabolically diverse communities self-organize to extract roughly 10% of the maximum extractable energy, or 100 fold more than randomized communities. Moreover, distinct communities implement energy extraction in convergent ways, as indicated by strongly correlated fluxes through nutrient cycles. As the driving force from light increases, however, these features -- fluxes and total energy extraction -- become more variable across communities, indicating that energy limitation imposes tight thermodynamic constraints on collective metabolism.