Ocean acidification reduces iodide production by the marine diatom Chaetoceros sp. (CCMP 1690)

Phytoplankton in marine surface waters play a key role in the global iodine cycle. The biologically-mediated iodide production under future scenarios is limited. Here we compare growth, iodate to iodide conversion rate and membrane permeability in the diatom Chaetoceros sp. (CCMP 1690) grown under seawater carbonate chemistry conditions projected for 2100 (1000 ppm) and pre-industrial (280 ppm) conditions. We found no effect of CO₂ on growth rates, but a significantly higher cell yield under high CO₂, suggesting sustained growth from relief from carbon limitation. Cell normalised iodate uptake (16.73 ± 0.92 amol IO₃⁻ cell⁻¹) and iodide production (8.61 ± 0.15 amol I⁻ cell⁻¹) was lower in cultures grown at high pCO₂ than those exposed to pre-industrial conditions (21.29 ± 2.37 amol IO₃⁻ cell⁻¹, 11.91 ± 1.49 amol I⁻ cell⁻¹, respectively). Correlating these measurements with membrane permeability, we were able to ascertain that iodide conversion rates were not linked to cell permeability and that the processes of mediated iodate loss and diatom-iodide formation are decoupled. These findings are the first to implicate OA in driving a potential shift in diatom-mediated iodate reduction. If our results are indicative of diatom-mediated iodine cycling in 2100, future surface ocean conditions could experience reduced rates of iodide production by Chaetoceros spp., potentially lowering iodide concentrations in ocean regions dominated by this group. These changes have the potential to impact ozone cycling and new particle formation in the atmosphere.