  Non-equilibrium thermodynamics has long been an area of substantial interest
to ecologists because most fundamental biological processes, such as protein
synthesis and respiration, are inherently energy-consuming. Microbial
communities are a natural system to decipher this mechanistic basis because
their interactions in the form of substrate consumption, metabolite production,
and cross-feeding can be described explicitly in thermodynamic terms. Previous
work has considered how thermodynamic constraints impact competition between
pairs of species, but restrained from analysing how this manifests in complex
dynamical systems. To address this gap, we develop a thermodynamic microbial
community model with fully reversible reaction kinetics, which allows direct
consideration of free-energy dissipation. This also allows species to interact
via products rather than just substrates, increasing the dynamical complexity,
and allowing a more nuanced classification of interaction types to emerge.
Using this model, we find that community diversity increases with substrate
lability, because greater free-energy availability allows for faster generation
of niches. Thus, more niches are generated in the time frame of community
establishment, leading to higher final species diversity. We also find that
allowing species to make use of near-to-equilibrium reactions increases
diversity in a low free-energy regime. In such a regime, two new thermodynamic
interaction types that we identify here reach comparable strengths to the
conventional (competition and facilitation) types, emphasising the key role
that thermodynamics plays in community dynamics. Our results suggest that
accounting for realistic thermodynamic constraints is vital for understanding
the dynamics of real-world microbial communities.
