Impact of Ocean Acidification on Aphotic Biogeochemical Production of Reactive Oxygen Species

Reactive oxygen species (ROS) are critical to global maintenance of the global organic carbon cycle, sulfur cycle, oxygen cycle, and transition metal cycles. The primary source of ROS is commonly considered to be photolysis or photochemically driven reactions, however ROS also exist in aphotic zones. A geochemical mechanism for the same in dark environments based on the tidally driven, episodic movement of anoxic groundwaters through oxidized, Fe(III) rich sediments is shown. Predictive models were developed based on in vitro experiments and tested using sediment samples collected from a saline tidal creek in the estuary at Murrell’s Inlet, South Carolina. These sediments were air dried, resuspended in aerated solution, then exposed to aqueous sulfide at a range of concentrations chosen to replicate the conditions characteristic of a tidal cycle, beginning with low tide. No detectable ROS production occurred from this process in the dark until sulfide was added. Sulfide addition resulted in the rapid production of hydrogen peroxide. The mechanism of hydrogen peroxide production was tested using a simplified three factor representation of the system based on hydrogen sulfide, Fe(II) and Fe(III). We show that changes in marine pH associated with predicted ocean acidification are sufficient to quench hydrogen peroxide formation, potentially reducing it by an order of magnitude relative to current marine conditions (e.g. from 18.3x10-6 M to 2.01x10-6 M over the range of conditions studied)