Cycling of iodine by microalgae: Iodine uptake and release by a microalgae biofilm in a groundwater holding pond

Biological iodine cycling has been widely studied in marine environments, but rarely considered in terrestrial waters. In this work, a microalgae biofilm, with an iodine content of 350 ± 29 mg kg−1, is shown to potentially be a significant iodine cycler in a groundwater holding pond. Cultivation of the biofilm in extracted groundwater was carried out at bench scale. In parallel, iodine release from an iodine-rich biofilm kept in the dark was monitored. Iodine uptake and release were found to be proportional to algal growth and decay, with the maximum growth rate and specific decay rate being 0.53 ± 0.05 g m−2 d−1 and 0.12 ± 0.02 d−1 respectively. Iodine update and release in an engineered groundwater holding pond (1 m3:1 m2 × 1 m) was simulated. The results indicated that bioaccumulation causes non-steady state conditions in such ponds, and in extreme circumstances decay events can elevate iodine concentrations to problematic levels.