| Summary: | The role of marine organisms in the redox cycling of iodine in the ocean is not well understood presently. Previous studies have suggested that phytoplankton play an important role in the biogeochemical cycling of iodine, and were responsible for the appearance of the non thermodynamically favoured species iodide in the euphotic zone. A key question that arises however is how this reduction occurs; Is it driven by primary production, via direct biologically mediated uptake, or alternatively is it driven chemically by redox reactions related to the passive release of reduced substances from the decay of biological materials? To directly address this question we have recently performed laboratory experiments and field measurements (Tropical Atlantic and Pacific, Southern Ocean) for this purpose. In culture experiments, including a variety of phytoplankton taxa (diatoms, dinoflagellates and prymnesiophytes), we observed changes in the speciation of iodine over the course of an experiment indicating the apparent ability to reduce iodate to iodide. Production rates were found to be species specific and not related to biomass. In all but one species tested the iodide production commenced in the stationary growth phase and peaked in the senescent phase of the algae. This indicates that iodide production is connected to cell senescence and suggests that iodate reduction results from increased cell permeability. We hypothesize that this is due to subsequent reactions of iodate with reduced sulphur species exuded from the cell. Combined with our field observations we suggest that cell senescence and other related processes that cause cell breakage (e.g. grazing, viral lysis) are responsible for the production of iodide. Our data additionally suggest that the iodine redox cycle is completed via biological processes also. We observed that an experimentally induced shift from senescence back to the exponential growth phase resulted in a decline in the iodide concentrations, suggesting reoxidation back to iodate. Our new data ...
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