SEGH Articles

Long distance electron transport in the seafloor

21 July 2016
At the SEGH 32nd International Conference recently in Brussels, Sebastiaan Van de Velde won the best prize for PhD student oral presentation. Here he tells us a little about his research.

 

 

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.

More information can be found at my personal webpage: http://sevdevel.webs.com/ or the homepage of the cable bacteria project: http://www.microbial-electricity.eu/

 

by Sebastiaan Van de Velde, PhD Student, Department Analytical, Environmental and Geochemistry, VUB - Vrije Universiteit Brussels

 


 

Keep up to date

Submit Content

Members can keep in touch with their colleagues through short news and events articles of interest to the SEGH community.

Science in the News

Latest on-line papers from the SEGH journal: Environmental Geochemistry and Health

  • Spatial variability and geochemistry of rare earth elements in soils from the largest uranium–phosphate deposit of Brazil 2018-02-22

    Abstract

    The Itataia uranium–phosphate deposit is the largest uranium reserve in Brazil. Rare earth elements (REEs) are commonly associated with phosphate deposits; however, there are no studies on the concentrations of REEs in soils of the Itataia deposit region. Thus, the objective of the research was to evaluate the concentration and spatial variability of REEs in topsoils of Itataia phosphate deposit region. In addition, the influence of soil properties on the geochemistry of REEs was investigated. Results showed that relatively high mean concentrations (mg kg−1) of heavy REEs (Gd 6.01; Tb 1.25; Ho 1.15; Er 4.05; Tm 0.64; Yb 4.61; Lu 0.65) were found in surface soils samples. Soil properties showed weak influence on the geochemical behavior of REEs in soils, except for the clay content. On the other hand, parent material characteristics, such as P and U, had strong influence on REEs concentrations. Spatial distribution patterns of REEs in soils are clearly associated with P and U contents. Therefore, geochemical surveys aiming at the delineation of ore-bearing zones in the region can benefit from our data. The results of this work reinforce the perspective for co-mining of P, U and REEs in this important P–U reserve.

  • 2017 Outstanding Reviewers 2018-02-21
  • Seasonal occurrence, source evaluation and ecological risk assessment of polycyclic aromatic hydrocarbons in industrial and agricultural effluents discharged in Wadi El Bey (Tunisia) 2018-02-13

    Abstract

    Polycyclic aromatic hydrocarbons are of great concern due to their persistence, bioaccumulation and toxic properties. The occurrence, source and ecological risk assessment of 26 polycyclic aromatic hydrocarbons in industrial and agricultural effluents affecting the Wadi El Bey watershed were investigated by means of gas chromatographic/mass spectrometric analysis (GC/MS). Total PAHs (∑ 26 PAH) ranged from 1.21 to 91.7 µg/L. The 4- and 5-ring compounds were the principal PAHs detected in most of 5 sites examined. Diagnostic concentration ratios and molecular indices were performed to identify the PAH sources. Results show that PAHs could originate from petrogenic, pyrolytic and mixed sources. According to the ecotoxicological assessment, the potential risk associated with PAHs affecting agricultural and industrial effluents ranged from moderate to high for both aquatic ecosystem and human health. The toxic equivalency factor (TEF) approach indicated that benzo[a]pyrene and benz[a]anthracene were the principal responsible for carcinogenic power of samples.