Microbial processes regulating the transformation of carbon, nutrients, and contaminants in sediments are difficult to predict using geochemical measurements alone because of the complex interplay between biotic and abiotic reactions and the underlying existence of cryptic elemental cycles that are not detected geochemically. Similarly, the competition between microbial processes is poorly represented in conventional kinetic rate laws used to describe complex biogeochemical processes in subsurface environments. In this study, a combination of high throughput omics and geochemical signatures were obtained in batch reactor incubations of wetland sediments from the Savannah River National Laboratory to elucidate the multitude of abiotic and biotic pathways involved in microbially-driven geochemical reactions. Several treatments were considered to investigate the competition between terminal-electron accepting processes. A gene centric model was then developed, calibrated with one treatment, and tested against the other treatments to identify the underlying anaerobic microbial processes taking place in these incubations. This model considers the thermodynamic driving force, growth yield, and the competition between the dominant metabolic processes based on new approaches to more accurately quantify carbon transformation and microbial responses to changes in redox conditions. Results demonstrate that iron and sulfate reduction dominate anaerobic respiration processes in these sediments and that sulfate reduction is outcompeted by iron-reducing bacteria when poorly crystallized iron oxides are available as terminal electron acceptor. The gene centric model allows to account for cryptic cycles and demonstrates that microbial competition can be explicitly accounted for using the thermodynamic driving force without the need for inhibition kinetics.

Kinetic studies to solve the complex interplay between abiotic and microbial reactions in sediments

ACS Spring 2022

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