X-Ray Absorption Spectroscopy as a Probe of Microbial Sulfur Biochemistry: the Nature of Bacterial Sulfur Globules Revisited

Abstract

The chemical nature of the sulfur in bacterial sulfur globules has been the subject of controversy for a number of years. Sulfur K-edge X-ray absorption spectroscopy (XAS) is a powerful technique for probing the chemical forms of sulfur in situ, but two groups have used it with very different conclusions. The root of the controversy lies with the different detection strategies used by the two groups, which result in very different spectra. This paper seeks to resolve the controversy. We experimentally demonstrate that the use of transmittance detection for sulfur K-edge XAS measurements is highly prone to spectroscopic distortions and that much of the published work on sulfur bacteria is very likely based on distorted data. We also demonstrate that all three detection methods used for X-ray absorption experiments yield essentially identical spectra when the measurements are carried out under conditions where no experimental distortions are expected. Finally, we turn to the original question-the chemical nature of bacterial sulfur. We examine isolated sulfur globules of Allochromatium vinosum and intact cells of a strain of magnetotactic coccus and show that XAS indicates the presence of a chemical form of sulfur resembling S 8 .

Polysulfides as Intermediates in the Oxidation of Sulfide to Sulfate by Beggiatoa spp

“…In the literature there are conflicting reports of sulfur rings (10,46), polysulfides (11), or polythionates (5, 6) in these inclusions. Based on our results, we hypothesize that even within the same species of bacteria, the speciation of stored sulfur varies under different ecophysiological conditions.…”

Rhodanese Functions as Sulfur Supplier for Key Enzymes in Sulfur Energy Metabolism

“…These kind of sulfur structures are usually present either in the periplasmic compartment or in the extracellular environment (44). In any case, the chemical nature of sulfur in the globules is still a debated question, and it seems that three different forms could be found, depending on the approach used: cyclo-octasulfur (S 8 ), polythionates ([(S n SO 3 ) 2 ] 2Ϫ ), and sulfur chains (45). These three forms can all be used as substrates by SbdP.…”

Operando Characterization of Intermediates Produced in a Lithium-Sulfur Battery

“…These distortions can be explained by self-absorption of the X-rays, a common problem seen with sulfur samples. 32,[41][42][43] It is not possible to correct the distortions in operando spectra, in which neither the species nor their concentrations are known, 28,43 and therefore, no self-absorption corrections were applied. In principle, self-absorption effects can be eliminated by diluting the amount of sulfur in the sample and reducing the size of the sulfur particles below 1 μm.…”

“…The slightly dampened shape of all the peaks indicates the presence of self-absorption, which can be explained by the use of manual grinding procedure and the resulting micrometer-sized and not the needed sub-micrometer-sized particles and/or slightly too high amount of the sulfur compound used. 41 In the inset of the figure, we also include the spectrum of 4 mM S 8 in DOL-DME to compare a sample without any self-absorption distortions to our three standards. The observed presence of the self-absorption artifact does not restrict qualitative interpretation of the operando XANES spectra, which, as has been previously explained, also experiences self-absorption problems.…”