Iodate Transformation By Marine Phytoplankton

IODATE TRANSFORMATION BY MARINE PHYTOPLANKTON Ajcharaporn Ud.omk. it Old Dominion University, 1994 Director : Dr. William M. Dunstan The speciation and distribution of iodine in the oceans are partly under biological control. Phytoplankton are suspected to mediate the transformation of iodate to iodide via reduction by the enzyme nitrate reductase. However, there has been no direct evidence to support this hypothesis. The influence of phytoplankton on the speciation of iodine was examined with emphasis on the transformation of iodate to iodide. Six cultures of marine phytoplankton: Skeletonema costatum, Dunaliella tertiolecta, Amphidinium carterae, Tetraselmis levis, Emiliania huxleyi, and Synechococcus sp. , have been examined for their ability to take up and reduce iodate under a reduced nitrate environment. In both natural and elevated iodate environments, all phytoplankton took up iodate and produced iodide. Iodate loss from the medium was not always equivalent to iodide production indicating either the accumulation of iodine in phytoplankton cells or the presence of other reduced forms of iodine besides iodide. Under an ambient iodate concentration of 359 nM, the uptake Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of iodate decreased in the order of A. carterae > Synechococcus sp. > T. levis > D. tertiolecta > E. huxleyi > S. costatum. The highest rate of iodate uptake, 0.93 nM-jig chi a-1-d-1, was observed in A. carterae, a coastal dinoflagellate. The oceanic cyanobacteria, Synechococcus sp. , took up only 0.32 nM-jig chi a-1d-1 of iodate and released 0.31 nM*(ig chi a-1-d-1 as iodide. This iodide release was the highest rate among the phytoplankton tested. Because of its abundance, this cyanobacteria could act as a major producer of iodide in the ocean. On the other hand, in coastal waters the spring bloom of diatoms and dinoflagellates may be responsible for the low concentration of iodate and the presence of organic iodine. There was no evidence of inhibitory effects of high concentrations of iodate on growth and development of phytoplankton. In addition, in these experiments there was no evidence that bacterial activities were responsible for the uptake and reduction of iodate. Studies on the transformation of iodate in the diatom S. costatum revealed that the changes in concentration of iodate had a significant inverse relationship with the increase of phytoplankton cell density (R = -0.98, P-value < 0.001, N = 6) . The variation in iodide was best explained by the change in phaeo-pigments which are the indicator of senescent cells (R = 0.95, P-value = 0.003, N = 5) . The ratio I: C calculated from the changes in the sum of iodate and iodide and the chlorophyll-specific photosynthetic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. rate(Pchl) was close to those values previously reported in hydrographic data as well as in planktonic tissue by other investigators. To examine the effect of nitrogen sources on the uptake of iodate, S. costatum was grown in two different media based on nitrate and ammonium as nitrogen sources. The time course variations in iodate and iodide concentration were monitored for 9 days. The decrease in iodate concentration was more intense in the culture with nitrate than in ammonium-enriched culture. The change in iodate concentration related to nitrate was highly significant (R = 0.89). The presence of ammonium ion in the media suppressed the transformation of iodate to iodide. The result implied the close relationship between iodate reduction and nitrate reduction in phytoplankton. The processes of iodate transformation may occur at the surface of or inside the phytoplankton cell. Iodate removal rate by S. costatum ranged from 0.10 to 0.57 nM*(lg chi a-1-d-1 depending on the growth stages. The removal rate was higher in the exponential phase than in the stationary phase. On the other hand, the production of iodide occurred mostly after the cell approached the stationary phase. The rate of iodide production in this species ranged from 0.01 to 0.07 nM*ng chi a_1-d-1. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.