Organic Matter Thermodynamics Mediate Microbe-Ecosystem Integration

Abstract:

Microbial communities fundamentally influence and are influenced by ecosystem dynamics. Understanding these integrated feedbacks is a major challenge for microbial ecology and ecosystem science. Towards this goal, we test hypotheses from a conceptual framework linking microbial community membership, properties, and processes to environmental features and biogeochemical function. To test hypotheses we exposed hyporheic zone sediments to different wetting/drying cycles in laboratory incubations spanning 14 days, and examined responses associated with the whole community, the putatively active community, organic matter thermodynamics, and aerobic respiration. To link community membership to the emergent property of community assembly, we used ecological null modeling. This revealed a threshold response to disturbance whereby deterministic homogeneous selection governed assembly when communities experienced more than 8 cumulative days of drying conditions. This response was only observed for the putatively active community, however, indicating a shift in relative activity but not relative abundances. The shift to deterministic assembly was paralleled by a threshold-based response in community processes realized as a decrease in respiration rates. We further observed clear relationships between thermodynamic properties of organic matter and both community assembly and respiration rates. Interpreting the results in context of the conceptual model points to organic matter thermodynamic properties as a key mediator of the iterative links among environmental disturbance and community membership, properties, and processes. Collectively, we infer that disturbance provides an external forcing that instigates dynamic feedback between system structure and function that is mediated, in part, through the thermodynamics of organic matter. Future studies that also interpret results via the conceptual framework used here will enable cross-system comparisons at the concept level to progressively advance theoretical principles linking microbes to ecosystem dynamics.