Purple sulfur bacteria (PSB) are photolithoautotrophs that perform anoxygenic photosynthesis by coupling light-driven electron transfer to the oxidation of reduced sulfur species such as sulfide. In these organisms, absorbed photons initiate electron transport chains that extract electrons from dissolved inorganic sulfur compounds, thereby linking photophysics with sulfur metabolism and biosynthesis. Sulfide is the preferred electron donor for many PSB. However, sulfide oxidation presents a physicochemical challenge: it inevitably produces condensed sulfur phases, including polysulfides and elemental sulfur, which can alter electron availability and transport pathways. In this talk, I will describe how two model strains of PSB manage this challenge and exploit sulfide oxidation to sustain efficient electron flow and energy conversion. Their strategies illustrate how microbial systems integrate light-driven electron transfer with complex sulfur chemistry, providing a natural example of coupled photochemical and redox processes in a heterogeneous chemical environment.
