X-ray absorption spectroscopy of bacterial sulfur globules

This article consists of two paired scientific communications debating the chemical composition of bacterial sulfur globules using X-ray absorption spectroscopy. George et al. argue that Prange et al.'s conclusions about different sulfur species in different bacteria are erroneous, stemming from experimental artifacts in transmittance detection that cause spectrum distortion through 'pinhole effects.' They contend that proper electron yield detection reveals sulfur globules contain a form resembling cyclo-octasulfur (S₈) across diverse bacterial species. Prange et al. counter that their transmittance data are valid and supported by biological evidence, including buoyant density measurements, Raman spectroscopy, and solubility properties indicating sulfur globules contain hydrated sulfur or organyl sulfanes with hydrophilic end groups, not pure S₈. They argue different sulfur species correlate with bacterial growth conditions—anaerobic phototrophic bacteria contain sulfur chains, while aerobic bacteria accumulate sulfur rings or polythionates. Both groups dispute the reliability of the other's detection methods and reference standards, with Prange et al. noting all detection modes carry potential artifacts. The debate highlights methodological challenges in measuring sulfur biochemistry in intact biological samples.

Key findings

• George et al. demonstrate that transmittance X-ray absorption spectra are distorted by pinhole effects from sample heterogeneity, causing incorrect quantification of sulfur species in bacterial globules
• Prange et al. report biochemical evidence that sulfur globules contain different chemical species depending on bacterial growth conditions: sulfur chains in anaerobic phototrophs versus sulfur rings/polythionates in aerobic organisms
• The two groups fundamentally disagree on whether bacterial sulfur globules universally contain cyclo-octasulfur (S₈) or variable hydrated/polymeric sulfur forms, reflecting broader methodological disputes about X-ray spectroscopy detection techniques
• Both electron yield and transmittance detection methods have inherent limitations and potential artifacts that complicate interpretation of sulfur speciation data