Purple sulfur bacteria are a group of bacteria capable of photosynthesis. They are anaerobic (don’t require oxygen) or microaerophilic (need only a small amount of oxygen), and are often found in hot springs or stagnant water. Unlike plants, algae, and cyanobacteria, they do not use water as their reducing agent, and so do not produce oxygen. Instead they use hydrogen sulfide, which is oxidized to produce granules of elemental sulfur. This in turn may be oxidized to form sulfuric acid.
Purple sulfur bacteria are generally found in illuminated anoxic zones of lakes and other aquatic habitats where hydrogen sulfide accumulates and they are also found in “sulfur springs” where geochemically or biologically produced hydrogen sulfide can trigger the formation of blooms. Anoxic conditions are required for photosynthesis; these bacteria do not thrive in oxygenated environments.
Purple bacteria were the first bacteria discovered to photosynthesize without having an oxygen by-product. Instead, their by-product is sulfur. This was demonstrated by first establishing the bacteria’s reactions to different concentrations of oxygen. It was found that the bacteria moved quickly away from even the slightest trace of oxygen. Next, a light was focused on one part of a dish containing the bacteria, leaving the rest dark. As the bacteria cannot survive without light, all the bacteria moved into the circle of light, becoming very crowded. If the bacteria’s by-product was oxygen, the distances between individuals would become larger and larger as more oxygen was produced. But because of the bacteria’s behavior in the focused light, it was concluded that the bacteria’s photosynthetic by-product could not be oxygen.
The most favorable lakes for the development of purple sulfur bacteria are meromictic (permanently stratified) lakes. Meromictic lakes stratify because they have denser (usually saline) water in the bottom and less dense (usually freshwater) nearer the surface. If sufficient sulfate is present to support sulfate reduction, the sulfide, produced in the sediments, diffuses upward into the anoxic bottom waters, and here purple sulfur bacteria can form dense cell masses, called blooms, usually in association with green phototrophic bacteria.
Photo credit: T. Northcote and K. Hall
Article originally published in the BC Lake Stewardship Society Quarterly Newsletter Volume 17 Issue 3.