thumb|[[Chromatium okenii in the microscope at 600x magnification]]

The purple sulfur bacteria (PSB) are part of a group of Pseudomonadota capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in stratified water environments including hot springs, stagnant water bodies, as well as microbial mats in intertidal zones.

Ecology

Habitat

Purple sulfur bacteria are generally found in illuminated anoxic zones of lakes and other aquatic habitats where hydrogen sulfide accumulates and also in "sulfur springs" where geochemically or biologically produced hydrogen sulfide can trigger the formation of blooms of purple sulfur bacteria. Anoxic conditions are required for photosynthesis; these bacteria cannot thrive in oxygenated environments.

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 fresh water) nearer the surface. Growth of purple sulfur bacteria is also supported by the layering in holomictic lakes.

Ecological significance

Purple sulfur bacteria are able to affect their environment by contributing to nutrient cycling, and by using their metabolism to alter their surroundings. They are able to play a significant role in primary production suggesting that these organisms affect the carbon cycle through carbon fixation. Purple sulfur bacteria also contribute to the phosphorus cycle in their habitat, Through upwelling of these organisms, phosphorus, a limiting nutrient in the oxic layer of lakes, is recycled and provided to heterotrophic bacteria for use. This indicates that although purple sulfur bacteria are found in the anoxic layer of their habitat, they are able to promote the growth of many heterotrophic organisms by supplying inorganic nutrients to the above oxic layer. Another form of recycling of inorganic nutrients and dissolved organic matter by purple sulfur bacteria is through the food chain; they act as a source of food to other organisms. Due to the inefficiency of their carotenoids, or light-harvesting centres, the organisms are able to release excess light energy as heat energy. This anoxic environment with light and sufficient sulfide availability is ideal for purple sulfur bacteria. The authors of the study concluded that, based on the presence of purple sulfur bacteria's biomarker, the Paleoproterozoic ocean must have been anoxic and sulfidic at depth. This finding provides evidence for the Canfield Ocean hypothesis.

Bioremediation

Purple sulfur bacteria can contribute to a reduction of environmentally harmful organic compounds and odour emission in manure wastewater lagoons where they are known to grow. Harmful compounds such as methane, a greenhouse gas, and hydrogen sulfide, a pungent, toxic compound, can be found in wastewater lagoons. PSB can help lower the concentration of both, and others.

Harmful organic compounds can be removed through photoassimilation, the uptake of carbon by organisms through photosynthesis. When PSB in the lagoons perform photosynthesis they can utilize the carbon from harmful compounds, such as methane, as their carbon source. This removes methane, a greenhouse gas, from the lagoon and reduces the lagoons' atmospheric pollution affect.

H<sub>2</sub>S can act as a sulfur source for PSB during these same photosynthetic processes that remove the organic compounds. The use of H<sub>2</sub>S as a reducing agent by PSB removes it from the lagoon and leads to a reduction of odour and toxicity in the lagoons.

See also

  • Anoxic event
  • Anoxygenic photosynthesis
  • Green Lake (New York)
  • Green sulfur bacteria
  • Okenane
  • Sulfur-reducing bacteria

References