Production and significance of Reactive Oxygen Species in the subsurface

Abstract

Reactive oxygen species (ROS) play a crucial role in greenhouse gas emissions, nutrient and contaminant transformations, microbial dynamics and a range of biogeochemical processes in surficial environments including the atmosphere, surface waters, and oceans. In recent years, research in ROS has extended to the subsurface, hence, moving from predominantly photic, oxic and homogeneous environments into aphotic, anoxic and heterogeneous environments. In this review, we discuss the production mechanisms and significance of ROS in the subsurface. Production hotspots of ROS occur where O₂ is brought into contact with reduced species like ferrous iron and natural organic matter, hence creating thermodynamically unstable conditions. The time and space window for ROS production is therefore co-regulated by the reaction kinetics between O₂ and reduced species, with the latter acting as both ROS generators and consumers of long-lived ROS such as hydrogen peroxide (H₂O₂) and short-lived ROS such as hydroxyl radicals (OH). The quantitative description of ROS cycling in subsurface environments is still in its early stages, however. Modeling of a pulsed groundwater O₂ intrusion yields the rates of OH and H₂O₂ production of 0.003–0.049 and 0.09–2.52 mmol/h/kg dry soil/sediment, respectively. Advances in ROS analysis, footprint mapping and reactive transport modeling, as well as new knowledge about the molecular mechanisms of ROS production and cycling, would enable a more comprehensive assessment of the significance of ROS in subsurface biogeochemistry that, in turn, could benefit their potential applications, for example, in contaminant remediation strategies.