What is Anoxygenic photosynthesis?

The purple and green bacteria photosynthesise with a single photosystem and a bacteriochlorophyll that absorbs in the near infrared, using $\mathrm{H_2S}$, $\mathrm{H_2}$ or organic acids as electron donors instead of water: they release sulfur or sulfate, never oxygen, and they need light of wavelengths that pass through the green and cyanobacterial layer above them. Purple sulfur bacteria therefore grow in the purple band of the Winogradsky column, just where sulfide from below meets the light from above; green sulfur bacteria, which tolerate more sulfide and less light, grow beneath them. Oxygenic photosynthesis, with its two photosystems in series, is a later invention of one lineage — the cyanobacteria — that learned to take electrons from water.