PhD student: Leonardo Contreira, Thesis director: Nadine Le Bris
Thesis defended the 4th of december 2012

Sulfur cycling is of major importance in the functioning of marine ecosystems. Particularly, microbial and chemically catalyzed redox processes involving sulfide formation or oxidation play a key role in energy transfer, including chemoautotrophy and organic matter transformation. The concentration and chemical speciation of this reduced sulfur compounds can change rapidly as a result of chemical, biological and physical processes. Systems driven by turbulent mixing of sulfidic fluids and seawater, such as hydrothermal vent plumes are obvious examples of these variable environments, but not the only ones. Any oxic-sulfidic interface under hydrodynamic control is potentially experiencing such instabilities. To date, however, most instruments available for in situ sulfide measurement are however limited to short-term use, and tools are still largely lacking for continuous in situ measurements over durations ranging from several days to weeks or beyond. In this context, electrochemical instruments offer several advantages, although recognized to suffer from some analytical limitations. In this study, the well-known potentiometric Ag/Ag2S electrode and a new bare-silver voltammetric method, that were previously adapted for short-term in situ sensing, have been applied for autonomous continuous sulfide monitoring in deep-sea and shallow waters. Their performances and advantages for new scientific approaches have been investigated. The limitations and advantages inherent of each technique are discussed and compared, with respect to different scientific purposes. The results are discussed in the light of the capacities of alternative in situ sensing techniques.