Iodate reduction by Isochrysis galbana is relatively insensitive to de-activation of nitrate reductase activity—are phytoplankton really responsible for iodate reduction in seawater?

The reduction of iodate to iodide in laboratory cultures of the marine microalga Isochrysis galbana was found to be essentially independent on the presence or absence of nitrate reductase activity. This places serious doubt on the hypothesis that iodate is inadvertently reduced by NR, when it is mistaken for nitrate. NR activity was de-activated by growing the algae with tungsten instead of molybdenum, and supplying ammonium-N. An alternative mechanism is proposed linking iodate to photo-inhibition in phytoplankton. When observed, iodide production was found to be close to linear with time. A preliminary fit of the Michaelis–Menten model to the graph of iodide production versus iodate substrate concentration is promising, yielding Vmax and Km (with 95% confidence limits) of 62 (±21) nM day−1 and 87 (±80) μM, respectively. Iodide production was only observed when the iodate concentration was higher than that found naturally. This was also found to be the case with cultures of Skelotenema costatum. Further, no significant iodide production was observed from 0.5 μM iodate in cultures of Cyclotella cryptica and Synechococcus sp. At the higher concentrations of iodate and after rationalisation to the prevailing concentrations of chlorophyll a and iodate, iodide production was typically 0.003 nM-I− μg chlorophyll a−1 day−1 μM−1-IO3−. At most, these dense cultures were only able to reduce 0.3% of the prevailing iodate, and it is argued that this is too low to explain the 0.2 μM reduction commonly experienced across the temperate continental shelf or within the photic zone of the tropical water column.