Tide-Triggered Production of Reactive Oxygen Species in Coastal Soils.

We report an unrecognized, tidal source of reactive oxygen species (ROS). Using a newly developed ROS-trapping gel film, we observed hot spots for ROS generation within ∼2.5 mm of coastal surface soil. Kinetic analyses showed rapid production of hydroxyl radicals ( • OH), superoxide (O 2 •- ), and hydrogen peroxide (H 2 O 2 ) upon a shift from high tide to low tide. The ROS production exhibited a distinct rhythmic fluctuation. The oscillations of the redox potential and dissolved oxygen concentration followed the same pattern as the • OH production, suggesting the alternating oxic-anoxic conditions as the main geochemical drive for ROS production. Nationwide coastal field investigations confirmed the widespread and sustainable production of ROS via tidal processes (22.1-117.4 μmol/m 2 /day), which was 5- to 36-fold more efficient than those via classical photochemical routes (1.5-7.6 μmol/m 2 /day). Analyses of soil physicochemical properties demonstrated that soil redox-metastable components such as redox-active iron minerals and organic matter played a key role in storing electrons at high tide and shuttling electrons to infiltrated oxygen at low tide for ROS production. Our work sheds light on a ubiquitous but previously overlooked tidal source of ROS, which may accelerate carbon and metal cycles as well as pollutant degradation in coastal soils.