Carbon Limitation Leads to Thermodynamic Regulation of Aerobic Metabolism

Organic matter (OM) metabolism in
freshwater ecosystems is a critical
source of uncertainty in global biogeochemical cycles, yet the processes
regulating aerobic respiration in aquatic environments remain poorly
understood. Aerobic respiration is typically predicted through kinetic
controls such as organic carbon and oxygen concentrations, while assuming
thermodynamic regulation has no influence. The role of OM thermodynamics
has been mainly explored under anaerobic conditions; however, recent
observations have suggested that thermodynamic properties of OM may
influence aerobic respiration rates. Here, we explicitly test OM thermodynamics
as a key regulator of aerobic respiration. We pair controlled microcosm
experiments with ultrahigh-resolution OM characterization to show
that aerobic respiration increases with OM thermodynamic favorability
under carbon limitation. We also demonstrate a shift in the regulation
of aerobic respiration from OM thermodynamics to nitrogen content
when carbon is in excess, highlighting a central role for OM thermodynamics
particularly in carbon-limited ecosystems. This illuminates a structural
gap in aquatic biogeochemical models by demonstrating that both kinetic
and thermodynamic constraints can modulate aerobic respiration. Our
work therefore proposes a new conceptual model in which thermodynamic
and nutrient limitations dually control aerobic respiration.