Reduction of iodate to iodide by cold water diatom cultures

The presence of a surface iodide maximum in seawater is commonly attributed to biological activity, however laboratory studies on the influence of phytoplankton on inorganic iodine speciation have yielded somewhat contradictory results. Here, we report changes in the speciation of inorganic dissolved iodine in nutrient-enriched seawater during the growth of a variety of phytoplankton taxa, including the cold water pennate diatoms Nitzschia and Navicula. Unialgal batch cultures were grown under axenic conditions in f/20 media; iodate was kept at ambient seawater levels or raised to 10 μM. The ability of algal cultures to reduce iodate to iodide varied considerably between the different algal species studied. Nitzschia sp. cultures removed close to 100% of iodate from growth media and produced iodide at rates of 0.03 and 3 nmol I μg chl-a− 1 day− 1 at ambient and elevated iodate levels, respectively. At elevated iodate levels, iodate depletion in Nitzschia cultures was matched by iodide production such that applying a linear regression to the concentrations of the two iodine species gave slopes of − 1 with R2 values greater than 0.9. At ambient iodate levels, only 50 to 100 nM of iodide was produced in Nitzschia cultures despite more than 200 nM of iodate being consumed, suggesting production of some other iodine species. Additionally, iodide levels peaked at the end of the exponential growth phase suggesting that an iodide consuming mechanism began to operate in the stationary growth phase. Cultures of the cold water diatom Navicula sp. and the temperate phytoplankton Emiliania huxleyi, Thalassiosira pseudonana and Dunaliella tertiolecta also exhibited iodate uptake, although the extent of this was variable and always less than in the Nitzschia cultures. Navicula and E. huxleyi also showed iodide production at both ambient and elevated iodate levels, but T. pseudonana and D. tertiolecta did not. In both the cold water algal cultures 50% to 100% of the lost iodate appeared to have been converted to an organic or particulate form, or lost by volatilisation.