Intracellular Sulphur Globule Formation in Halorhodospira halophila and a Putative Role for Outer Membrane Vesicles...

Condensed sulphur phases (e.g., S⁰) are key intermediates in the microbial oxidation of reduced sulphur compounds and influence sulphur cycling in natural systems. Biogenic S⁰, particularly, extracellular sulphur globules, can serve as an energy substrate for diverse microorganisms and interface with other biogeochemical cycles. However, the cellular processes that generate extracellular globules remain incompletely resolved. Here, using cryogenic electron tomography, scanning transmission electron microscopy and stage-resolved transcriptomics, we tracked sulphur globule formation in Halorhodospira halophila, a purple sulphur bacterium known to produce extracellular sulphur globules. We observed S⁰ globule nucleation within cells followed by the appearance of globules outside the cell, consistent with export from an intracellular site. Stage-resolved differential gene-expression analyses revealed the upregulation of genes associated with outer membrane remodelling and outer membrane vesicle (OMV) biogenesis during stages in which export is observed, providing indirect support for an OMV-associated export process. Coordinated transcriptional increases in sulphide-oxidation pathways, OMV-related functions, secretion systems, photosynthetic complexes and the Calvin-Benson-Bassham (CBB) cycle suggest integrated metabolic regulation linking sulphide oxidation, photosynthetic energy conservation and carbon fixation to membrane/vesicle production. Together, these observations support a model in which intracellular sulphur metabolism is linked to extracellular S⁰ reservoirs through an export process potentially involving OMVs.