purple sulfur

The purple sulfur bacteria are photosynthetic anaerobic or microaerophilic Proteobacteria found in meromictic lakes and sulfur (hot) springs. These bacteria utilize hydrogen sulfide (H2S) rather than water as their electron donor, so they perform nonoxygenic photosynthesis. (By contrast, oxygenic photosynthesis in plants employs water as the electron donor and produces oxygen.)

Members of the Chromatiaceae family produce internal sulfur granules, while members of the Ectothiorhodospiraceae family produce external sulfur granules on oxidation of H2S:

CO2 + 2H2S → CH2O + H2O + 2S

Meromictic lakes are permanently stratified lakes with denser (typically saline) water at the bottom. Under conditions that support purple sulfur bacteria, sulfide is produced in the bottom sediments and diffuses upward into the anoxic bottom waters, where the purple sulfur bacteria form blooms, usually in association with green phototrophic bacteria.

Sulfur springs possess geochemically or biologically produced hydrogen sulfide, which can trigger the formation of blooms of purple sulfur bacteria.

Nonoxygenic photosynthesis
Purple sulfur families:
Chromatiaceae Order Chromatiales (chlorophylls bchl a or b)
electron donor for photoautotrophy = S– or S^o or H2 (S^o globules formed inside cell from S–)
photoheterotrophy? some spp. chemotrophy? some spp.

Ectothiorhodospiraceae (chlorophylls bchl a or b)
electron donor for photoautotrophy = S– or H2 (^o globules formed outside cell from S–)
photoheterotrophy? possibly all spp. chemotrophy? some spp.