Sulphate Reducing Bacteria and Mackinawite Stability

Introduction: The formation of sedimentary iron monosulphides (mackinawite, greigite) and pyrite is strongly interlinked with bacterial sulphate reduction, and thus with the global geochemical Fe, S and C cycles [1, 2]. The inorganic formation and stability of FeS phases at T < 100 ̊C has been extensively studied due to their vital importance in the sedimentary pyrite formation processes [e.g., 1-6]. However, whether sulphate-reducing bacteria (SRB) are actively [7-8] or passively [3, 9-11] involved in this process is equivocal. The main goal of this investigation was, therefore, to study the formation and long term stability of FeS phases in the presence of ubiquitous SRB’s. Methods: A cultures of the SRB Desulfovibrio desulfuricans (ATCC # 29578) was grown in fastidiously anaerobic, minimal seawater-type medium supplemented with lactate. Culture tubes with fresh, sterile media were inoculated and equilibrated at 34 ̊C. In the nutrient-deficient medium, growth was slower than predicted [9] and maximum growth was reached after 10–14 days [10]. The bacterial stock solution was used to inoculate fresh sterile media that subsequently was injected with 1 mL of Fe or Fe solution. This step promoted the immediate precipitation of a black iron sulphide phase. At various time intervals bacterial growth rates, fluid and solid composition and bacterialand precipitate morphologies were monitored. Growth rates were determined by measuring the number of cells per mL of solution using a flow cytometer after treating with BacLight DNA dye. The solution composition was monitored for pH25 ̊C (H2S-tolerant glass electrode), ∑Fe(aq) (inductively-coupled plasma spectroscopy), ∑SO4(aq) (ion chromatograph) and ∑S(aq, reduced) (S-coulometer). The solids were characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM-EDS) and Transmission Electron Microscopy (TEM). The ratio between iron monosulphides (Acid-Volatile Sulphides, AVS; mackinawite and greigite) and pyrite (chromium reducible sulphur) was determined via a sequential extraction method using an S-coulometer [details in 6 and 10]. Results and Discussion: For all cultures (at maximum growth stage) a total count of 1-2 x 10 cells pre mL was determined. The consumption rate of SO4 corresponds to the production rate of H2S with a maximum H2S concentration reached after ~ 2 weeks [10]. XRD and SEM-EDS analysis of the solids revealed that even after 6 months the main solid reaction product is mackinawite. In some tubes greigite was present, however, it is assumed that the redox-state of these tubes was compromised (partial oxidation during sampling) because in several parallel experiments this was not observed. Pyrite did not form in any experiment even after 6 months of reaction time. These results were also corroborated by TEM analysis, which revealed both intact and lysed cells (Fig. 1).