Analysis of sulfur biochemistry of sulfur bacteria using X-ray absorption spectroscopy

Many sulfide-oxidizing organisms, including the photosynthetic sulfur bacteria, store sulfur in "sulfur globules" that are readily detected microscopically. The chemical form of sulfur in these globules is currently the focus of a debate, because they have been described as "liquid" by some observers, although no known allotrope of sulfur is liquid at physiological temperatures. In the present work we have used sulfur K-edge X-ray absorption spectroscopy to identify and quantify the chemical forms of sulfur in a variety of bacterial cells, including photosynthetic sulfur bacteria. We have also taken advantage of X-ray fluorescence self-absorption to derive estimates of the size and density of the sulfur globules in photosynthetic bacteria. We find that the form of sulfur that most resembles the globule sulfur is simply solid Ss, rather than more exotic forms previously proposed.