Presenter: Zoé Le Bras (ETH Zurich / Eawag)

Description:
Selenium (Se) is an essential trace element for humans and animals. The atmosphere represents an important reservoir of Se, which supplies Se to the Earth’s surface and thus presents a source of Se to ecosystems and food chains. The main natural atmospheric Se source is derived from marine biogenic emissions, mainly in the form of dimethyl selenide (DMSe), dimethyl diselenide (DMDSe) and dimethyl selenyl sulphide (DMSeS), present in trace levels (fM-pM) in seawater.  However, the spatial distribution and temporal variability of these volatile organic Se species are poorly constrained in marine environments due to a lack of adequate analytical methods. It is assumed that phytoplanktonic and/or bacterial communities are involved in the formation of seleno-metabolites that act as precursors of these volatile Se species. However, production mechanisms of volatile Se remain poorly understood. We developed a highly sensitive and selective analytical method using thermal desorption coupled with gas chromatography inductively coupled plasma mass spectrometry (TD-GC-ICP-MS) to quantify volatile Se as well as volatile organic sulfur, bromine and iodine species in seawater. This analytical method was used to investigate concentrations of volatile Se species over time at the coastal station of Endoume in the Mediterranean Sea between March and October 2021. In addition to studying daily and seasonal cycling of volatile Se in seawater, incubation experiments were carried out under various conditions, e.g., dark/light, different size fractions and using biological inhibitors, in which natural seawater was spiked with various (in)organic Se species to obtain insights into (a)biotic processes involved in the production of volatile Se species. We found that concentrations of volatile Se species varied by a factor of two to four during the period of investigation and that various pathways are responsible for volatile Se formation, which will be further presented here.