Abstract
Abstract. The reaction between ozone and iodide at the sea surface is now known to be an important part of atmospheric ozone cycling, causing ozone deposition and the release of ozone-depleting reactive iodine to the atmosphere. The importance of this reaction is reflected by its inclusion in chemical transport models (CTMs). Such models depend on accurate sea surface iodide fields, but measurements are spatially and temporally limited. Hence, the ability to predict current and future sea surface iodide fields, i.e. sea surface iodide concentration on a narrow global grid, requires the development of process-based models. These models require a thorough understanding of the key processes that control sea surface iodide. The aim of this study was to explore if there are common features of iodate-to-iodide reduction amongst diverse marine phytoplankton in order to develop models that focus on sea surface iodine and iodine release to the troposphere. In order to achieve this, rates and patterns of changes in inorganic iodine speciation were determined in 10 phytoplankton cultures grown at ambient iodate concentrations. Where possible these data were analysed alongside results from previous studies. Iodate loss and some iodide production were observed in all cultures studied, confirming that this is a widespread feature amongst marine phytoplankton. We found no significant difference in log-phase, cell-normalised iodide production rates between key phytoplankton groups (diatoms, prymnesiophytes including coccolithophores and phaeocystales), suggesting that a phytoplankton functional type (PFT) approach would not be appropriate for building an ocean iodine cycling model. Iodate loss was greater than iodide formation in the majority of the cultures studied, indicating the presence of an as-yet-unidentified “missing iodine” fraction. Iodide yield at the end of the experiment was significantly greater in cultures that had reached a later senescence stage. This suggests that models should incorporate a lag between peak phytoplankton biomass and maximum iodide production and that cell mortality terms in biogeochemical models could be used to parameterise iodide production.
Highlights
Interest in marine inorganic iodine has increased in recent years due to the realisation that ozone deposition to iodide (I−) at the sea surface plays an important role in ozone cycling and the release of reactive iodine into the troposphere (Carpenter et al, 2013)
The incorporation of phytoplankton functional types (PFTs) into the ecosystem dynamics of ocean biogeochemical models has led to improved performance and accuracy (Gregg et al, 2003), but our results suggest that this approach would not be suitable for models of inorganic iodine cycling in seawater
By combining our results with those of previous studies, we have shown that there is no significant difference in cell-normalised iodide production rates between key phytoplankton groups or phytoplankton functional types (PFTs; e.g. diatoms versus coccolithophores)
Summary
Interest in marine inorganic iodine has increased in recent years due to the realisation that ozone deposition to iodide (I−) at the sea surface plays an important role in ozone cycling and the release of reactive iodine into the troposphere (Carpenter et al, 2013). Once tropospheric ozone reacts with iodide, both hypoiodous acid and molecular iodine are produced in the sea surface microlayer and are released into the atmosphere (Carpenter et al, 2013; MacDonald et al, 2014): H+ + I− + O3 → HOI + O2, (1). Ozone deposition to iodide at the sea surface is considered to be an important component of atmospheric chemistry and is incorporated into large-scale chemical transport models (CTMs; Luhar et al, 2017; Sherwen et al, 2016)
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