Kinetics of superoxide reactions with dissolved organic matter in tropical Atlantic surface waters near Cape Verde (TENATSO)

The decay kinetics of superoxide (O₂^-) reacting with organic matter was examined in oligotrophic waters at, and nearby, the TENATSO ocean observatory adjacent to the Cape Verde archipelago. Superoxide is the short-lived primary photochemical product of colored dissolved organic matter (CDOM) photolysis and also reacts with CDOM or trace metals (Cu, Fe) to form H₂O₂. In the present work we focused our investigations on reactions between CDOM and superoxide. O₂^- decay kinetics experiments were performed by adding KO₂ to diethylenetriaminepentaacetic acid (DTPA) amended seawater and utilizing an established chemiluminescence technique for the detection of O₂ ^- at nM levels. In Cape Verdean waters we found a significant reactivity of superoxide with CDOM with maximal rates adjacent to the chlorophyll maximum, presumably from production of new CDOM from bacteria/phytoplankton. This work highlights a poorly understood process which impacts on the biogeochemical cycling of CDOM and trace metals in the open ocean.