The transformation of iodate to iodide in marine phytoplankton cultures

Abstract

Six species of phytoplankton, representing 6 major phylogenetic groups (2 oceanic species: a cyanobacteria, Synechococcus sp., and a coccolithophorid, Emiliania huxleyi; and 4 coastal species: a prasinophyte, Tetraselmis sp., the green algae Dunaliella tertiolecta, the diatom Skeletonema costatum and a dinoflagellate Amphidinium carterae) were tested for their ability to reduce iodate to iodide in batch cultures. They all did so to varying degrees. Thus, the reduction of iodate to iodide by phytoplankton may be a general phenomenon in the marine environment. At ambient concentrations of iodate, the rates of depletion of iodate and appearance of iodide varied between 0.8 and 0.02, and between 0.3 and 0.02 nmol µg chlorophyll a -1 d -1 , respectively. E. huxleyi was the least efficient while A. carterae was the most efficient in the depletion of iodate. However, in the formation of iodide, while E. huxleyi was also the least efficient, Synechococcus sp. were the most efficient. The rates of appearance of iodide were noticeably slower than the corresponding rates of depletion of iodate, suggesting that part of the iodate might have been converted to forms of iodine other than iodide in these cases. The slight mismatch in the rank order of the rates of depletion of iodate and appearance of iodide between the phytoplankton species was traced to this variable and incomplete conversion of iodate to iodide. These rates were increased by up to over an order of magnitude upon enriching the culture medium with 5 and 10 µM of iodate. The depletion of iodate and appearance of iodide occurred in all growth phases. However, the rates might vary with growth phase and the patterns of these variations might be species-specific. Phytoplankton growth was not impeded even under unnaturally high concentrations of iodate implying that there is little interaction between iodine processing and the metabolic activity of cell growth.

Transformation of iodate to iodide in marine phytoplankton driven by cell senescence

“…Our findings apparently contradict the earlier work of Wong et al (2002), who found no clear relationship between iodide production and the growth phase of the culture. Indeed, they ruled out cell lysis as a possible cause, though this may have been because they did not measure cell numbers or any proxy of cell permeability.…”

“…Average production rates per day were in agreement with those reported in previous studies (Wong et al 2002) and are shown in Table 3. In all cultures, except Scrippsiella trochoidea, the iodide production began in the stationary growth phase (Figs.…”

“…Highest rates per chl a were observed in Pseudo-nitzschia turgiduloides (0.26 nmol l -1 µg -1 chl a d -1 ), and this species was a clear example of the processes outlined above (Table 3). Emiliania huxleyi produced iodide with a maximum rate of 0.06 nmol l -1 µg -1 chl a d -1 , which is comparable with the findings of Wong et al (2002), but 4 times lower than those observed by Chance et al (2007) during their studies. This might be due to divergent calculations of the average chl a concentrations.…”

“…This might be due to divergent calculations of the average chl a concentrations. In our study we used the trapezoidal method, as did Wong et al (2002), and obtained comparable values. Chance et al (2007) normalised their values to the simple average chl a concentration over the selected period.…”

“…Other laboratory experiments with phytoplankton cultures have shown significant iodate uptake rates (Moisan et al 1994). The conversion of iodate to iodide has been shown in batch culture at ambient iodate concentrations (Wong et al 2002). Much of this work has been carried out to test the hypothesis put forward by Tsunogai & Sase (1969) that nitrate reductase can reduce iodate to iodide in the ocean when nitrate is limiting.…”