The influence of marine phytoplankton on iodine speciation in the Tropical and Southern Atlantic Ocean

The role of halogene species like iodine in the ocean and how their speciation is affected by marine organisms is not well known. This lack of knowledge demands for more detailed field as well as experimental studies in order to unravel the role of iodine in marine ecosystems. My thesis comprises field work on the iodine speciation in polar and tropical marine environments complemented by a set of laboratory experiments on the role of phytoplankton species from the two regions studied for the iodine biogeochemical cycle. A large scale survey across the Atlantic sector of the Southern Ocean and three cruises to the Mauritanian upwelling region during both strong and weak upwelling seasons provide valuable information on iodine speciation over large spatial scales in case of the former survey, and on seasonal variability in case of the latter cruises. Furthermore, comparison of these two oceanic provinces will allow to decipher differences and conformities in iodine speciation between areas as far apart as the Southern Ocean and the Mauritanian upwelling region. In both provinces the total iodine (iodate + iodide) concentrations were in the same range between 450-480nmol L-1, while surface iodide values in the euphotic zone varied considerably and showed a steep vertical concentration gradient of less than 20nmol L-1 for antarctic and over 200nmol L-1 for tropical waters. In seawater the interconversion of the two inorganic forms of iodine, iodate and iodide, can be mediated by abiotic and/or biotic processes. The accumulation of iodide in the euphotic zone in both regions is suggested to be a more biologically mediated process and as observed in the experimental studies phytoplankton cells do influence the iodate reduction to iodide. However, highest iodide concentrations were not coupled to highest biological productivity instead we observed highest iodide values during post bloom periods in the respective regions indicating a strong relationship between iodide production and phytoplankton senescence during bloom collapse. Interestingly, productive regions with high phytoplankton biomass measured as chlorophyll-a show a decline in surface iodide and also the experimental study revealed an iodide decline in cultures with viable cells suggesting that an iodide oxidation or uptake mechanism is present when cells are in the exponential growth phase. Upwelled water was lower in surface iodide compared to water from a weak-upwelling scenario, and could on one hand be traced by its lower iodide concentrations while in the Weddell Sea Basin we observed elevated iodide in the deep Weddell Sea Bottom Water (WSBW) which could be traced on the other hand by its elevated iodide concentrations. From these results it appears that iodide can be used as a tracer for upwelled water on continental shelves and for newly formed WSBW. From the results gained in the laboratory we can say that iodide formation and senescence in phytoplankton cells are coupled. Iodide production was found to be species specific and not related to chlorophyll-a, cell size or cell numbers. Moreover iodide concentrations peaked in the stationary and/or senescence growth phase. A shift from senescence back to the exponential growth phase resulted in a decline in iodide concentrations indicating that phytoplankton mediated oxidation of iodide to iodate was triggering this shift. In summary, the results of my thesis show that the combined effects of abiotic and biotic processes resulting in iodate reduction are coupled via phytoplankton senescence. These findings challenge the conventional view, as described in other studies, that iodate reduction in the ocean is directly coupled to nutrient uptake and biological production.