Structural Stability, Transitions, and Interactions within SoxYZCD-Thiosulphate from Sulfurimonas denitrificans: An In Silico Molecular Outlook for Maintaining Environmental Sulphur Cycle

Thiosulphate oxidation (an essential mechanism) serves to maintain the global sulphur cycle. Earlier experimental and computational studies dealt with environmental thiosulphate oxidation but none dealt with thiosulphate oxidation from deep ocean belts. Wet-laboratory experimental research shows that epsilon-proteobacteria Sulfurimonas denitrificans possess sox (sulphur-oxidizing) operon and perform thiosulphate oxidation efficiently underneath the oceans. From this specific sox operon, SoxCD complex recycles the thiosulphate-bound SoxY from SoxYZ complex to balance the environmental sulphur cycle. So, four chief proteins were variedly modeled and relevant simulated interactive structures were obtained. The final simulated tetraprotein complex (SoxYZCD) from docked SoxYZ and SoxCD complexes was disclosed to be a highly interactive one with predominant ionic residues. Free energy of folding, solvent accessibility, and conformational shifts (coil-like conformation to helices and sheets) were observed in SoxYZ complex after interacting with SoxCD. The stability of the complex (SoxYZCD) after simulation was also observed through the electrostatic surface potential values. These evaluations were rationalized via biostatistics. This aids SoxCD for recycling SoxY along with thiosulphate, which remains interconnected by four H-bonds with SoxY. Therefore, this novel exploration is endowed with the detailed molecular viewpoint for maintaining the sulphur cycle (globally) including the ocean belts.

Net uptake of CO2 Driven by Sulphide and Thiosulphate Oxidation in the Bacterial Symbiont-Containing Clam Solemya Reidi

Solemva reidi Bernard is a gutless clam that lives in burrows in reducing sediments, and harbours intracellular sulphur-oxidizing bacteria in its gills. Clams were incubated in various concentrations of sulphide and thiosulphate for up to 65 h in a flow-through respirometer. Fluxes were determined by continuous sampling of the respiratory medium with analysis of CO2, O2 and sulphide by gas chromatography and analysis of thiosulphate, sulphite (and sulphide) by HPLC using monobromobimane-denvatized discrete samples. Net CO2 uptake was shown to occur with exposure to 50–100μmol1−1 sulphide and greater than 225μmoll−1 thiosulphate; sulphide oxidation and thiosulphate uptake were also demonstrated. 45CaCO3 deposition in the shells of. S. reidi was found to be insignificant compared to the net CO2 flux measured in the presence of low levels of sulphide. In experiments conducted under various O2 conditions, O2 limitation, produced by a combination of low [O2] and low water flow, was shown to inhibit sulphide oxidation and to prevent Co2 uptake. However, if O2 supply was not limited by low flow rates, in the presence of low [O2] (25–40 μmoll−1) S. reidi showed rates of O2 and sulphide consumption and CO2 uptake near the maximum levels determined under high [O2] conditions, indicating the potential for net Co2 uptake in the low [O2] conditions presumed to exist in the animal's burrows. Thiosulphate levels in the blood of S. reidi were analysed and shown to increase rapidly during incubation in sulphide. These levels reached an apparent steady state (approx. 300μmoll−1) in recently captured clams after 1 h of incubation. However, both O2 limitation and time in captivity (>43 days after capture) caused a marked increase in the blood thiosulphate levels, which exceeded 2.5mmoll−1 after 16 h of exposure to sulphide. These results indicate that blood thiosulphate is transported to the bacteria and further oxidized, and that sulphide and thiosulphate oxidation are oxygen-dependent. In analyses of gill tissues for elemental sulphur, we found a wide range in the levels of sulphur stores. Calculations indicated these to be a small fraction of the total flux of sulphur maintained during continuous sulphide oxidation. Estimates of Co2:o2: sulphide ratios suggest CO2 fixation efficiencies similar to those of chemolithoautotrophic bacteria. Assuming translocation and oxidation of symbiont-fixed organic compounds, the net uptake of CO2 by S. reidi in the presence of reduced sulphur compounds suggests that this intact symbiosis may be able to meet its organic carbon needs through autotrophy.

A New Method of Sulphur Denitrification for Sewage Treatment by a Fluidized Bed Reactor

Anaerobic treatment of sewage has a problem of denitrification, because that organic carbon and hydrogen are recoverd mainly in the form of methane and not used for denitrification. Other electron donors must be introduced. Promising donors are reduced forms of sulphur. In this study, thiosulphate and elemental sulphur were used in fluidized bed reactor systems. Denitrification by oxidation of sulphur was unstable due to poor solubility of sulphur into sewage. Thiosulphate denitrification was very stable. Stoichiometric relationship of the denitrification and thiosulphate oxidation for the reactor was obtained. The denitrification process was described by the first order consecutive reactions. The rate constant of nitrate to nitrite reduction, kl varied between 2.05×10−2 and 1.09×10−1 (1/gVSS/l*min.). The rate constant of nitrite to nitrogen reduction, k2 varied between 1.41×10−1 and 9.08×10−1 (l/gVSS/l*min.).