Photochemically induced redox reactions in seawater, II. Nitrogen and iodine

Experiments have been conducted to determine whether the speciation of the nitrate-nitrite and iodate-iodide redox couples is influenced by light and organic matter in both deionised and sea waters. The results obtained indicate that photochemical production of nitrite occurs and that the rate of this reaction is dependent on the concentration of organic matter present in the system. The findings suggest that nitrate undergoes not only direct photochemical reaction, acting itself as a primary chromophore, but also a photochemically induced reduction through organic matter. This reaction is, however, unlikely to influence productivity to any great extent as most phytoplankton can use both species as nutrients. It may, however, be a source of the highly reactive hydroxyl radical and of NO which has the potential to degas to the atmosphere and there help to regulate ozone concentration. Experiments conducted on the iodine redox system similarly indicate that photochemically induced production of iodide occurs. The results show that the reaction is not biologically mediated and that the presence of organic matter is essential for iodide photoproduction. At the concentrations of added iodate used in the experiments no direct 1:1 relationship between iodate and iodide is seen. It is suggested that, under these conditions, organic matter is not only essential for the photochemical production of iodide but that it is also the limiting species for the reduction reaction.