I started my PhD project with Prof. Filip Meysman and Prof. Yue Gao (Department of Analytical, Environmental and Geochemistry, Vrije Universiteit Brussel, Belgium) in October 2013, after completing my bachelor’s degree in Chemistry and master’s in Environmental Chemistry at the Vrije Universiteit Brussel (VUB, Brussels, Belgium). My PhD research is funded by the Flanders FWO foundation (‘FWO Aspirant’ grant).
The discovery of electrogenic sulphur oxidation (e-SOx) in marine sediments has implied a revolution in the field of geochemistry (Nielsen et al., 2010). In this newly discovered redox pathway, the reduction of oxygen was shown to be coupled to the oxidation of sulphide via direct electron transfer over spatial scales of several centimetres (so-called Long Distance Electron Transport). Two years later, long filamentous bacteria, known as cable bacteria, were identified as the drivers of this process (Pfeffer et al., 2012). These bacteria form chains of more than 10,000 individual cells that cooperate to mediate the Long Distance Electron Transport. These chains extend vertically through the sediment with the bottom cells of the cable bacterium (the cells in the anoxic part of the sediment) taking electrons from sulphide oxidation, and transferring them to the cells in the oxic zone, where oxygen is reduced. This process of electrogenic sulphur oxidation allows cable bacteria to bypass the classical redox sequence in the seafloor and gain an advantage over other bacteria that need direct contact between sulphide and oxygen molecules.
The decoupling of redox reactions strongly affects the pH in the sediment, by forming an alkaline peak (pH > 8) near the sediment-water interface (corresponding to the reduction of oxygen) and generating an acidic zone below (corresponding to the oxidation of sulphide). This low pH leads to the dissolution of acid-sensitive minerals (iron sulphides and carbonates), which in its turn drives the geochemical cycling of several important elements, such as iron and calcium (Risgaard-Petersen et al., 2012).
Long Distance Electron Transport in marine sediments forms the topic of my PhD. My project specifically focuses on the impact of electrogenic sulphur oxidation on the geochemical cycling in coastal sediments, with special attention to trace elements (arsenic, cobalt, molybdenum etc.). This problem is approached through a combination of field sampling campaigns, laboratory experiments, as well as theoretical modelling. Cable bacteria have been discovered to be widespread across the globe, and in varying habitat types. Numerous field campaigns to identify their whereabouts and to document the geochemical cycling have been conducted. Current field sites for my project include the Belgian Coastal Zone (Belgium), Lake Grevelingen (The Netherlands), East Gotland Basin (Sweden) and Blakeney salt marshes (UK).
Apart from Long Distance Electron Transport, I am also interested in other important processes affecting marine sediment geochemistry, both natural (e.g. bioturbation) as well as anthropogenic (e.g. dredging, bottom trawling).
The photographs show sampling is shown in the Belgian Coastal Zone on board the RV Simon Stevin.
by Sebastiaan Van de Velde, PhD Student, Department Analytical, Environmental and Geochemistry, VUB - Vrije Universiteit Brussels