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The purple sulfur bacteria (PSB) are part of a group of
Pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The ...
capable of
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
, collectively referred to as
purple bacteria Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll ''a'' or ''b'', together with various ...
. They are
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
or
microaerophilic A microaerophile is a microorganism that requires environments containing lower levels of dioxygen than that are present in the atmosphere (i.e. < 21% O2; typically 2–10% O2) for optimal growth. A more re ...
, and are often found in stratified water environments including
hot spring A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by ci ...
s,
stagnant water Water stagnation occurs when water stops flowing. Stagnant water can be a major environmental hazard. Dangers Malaria and dengue are among the main dangers of stagnant water, which can become a breeding ground for the mosquitoes that transmi ...
bodies, as well as microbial mats in intertidal zones.Hunter, C.N., Daldal, F., Thurnauer, M.C., Beatty, J.T
"The Purple Phototropic Bacteria"
'' Springer-Dordrecht'', 2008.
Unlike
plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
,
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular m ...
, and
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blue ...
, purple sulfur bacteria do not use
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
as their
reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth met ...
, and therefore do not produce
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
. Instead, they can use sulfur in the form of sulfide, or
thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, ...
(as well, some species can use H2, Fe2+, or NO2) as the electron donor in their
photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
pathways. The sulfur is
oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
to produce granules of elemental
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
. This, in turn, may be oxidized to form
sulfuric acid Sulfuric acid ( American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
. The purple sulfur bacteria are largely divided into two families, the
Chromatiaceae The Chromatiaceae are one of the two families of purple sulfur bacteria, together with the Ectothiorhodospiraceae. They belong to the order Chromatiales of the class Gammaproteobacteria, which is composed by unicellular Gram-negative organisms. ...
and the Ectothiorhodospiraceae, which produce internal and external sulfur granules respectively, and show differences in the structure of their internal membranes. They make up part of the order Chromatiales, included in the Gammaproteobacteria. The genus '' Halothiobacillus'' is also included in the Chromatiales, in its own family, but it is not photosynthetic.


Characteristics of purple sulfur bacteria

Major photosynthetic pigments:
Bacteriochlorophyll Bacteriochlorophylls (BChl) are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by C. B. van Niel in 1932. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacter ...
s a or b Location of photosynthetic pigments: Plasma membrane and chromatophore (lamellar membrane complexes that are continuous with the plasma membrane) Photosynthetic electron donors: H2, H2S, S Sulfur deposition: Inside the cell Metabolic type: Photolithoautotroph


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 A meromictic lake is a lake which has layers of water that do not intermix. In ordinary, holomictic lakes, at least once each year, there is a physical mixing of the surface and the deep waters. The term ''meromictic'' was coined by the Austri ...
(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. These lakes are thermally stratified; in the spring and summer time, water at the surface is warmed making it less dense than underlying colder water which provides sufficiently stable stratification for purple sulfur bacteria growth. If sufficient sulfate is present to support sulfate reduction, the sulfide, produced in the sediments, diffuses upward into the anoxic bottom waters, where purple sulfur bacteria can form dense cell masses, called blooms, usually in association with green phototrophic bacteria. Purple sulfur bacteria can also be found and are a prominent component in intertidal microbial mats. Mats, such as the Sippewissett Microbial Mat, have dynamic environments due to the flow of tides and incoming fresh water leading to similarly stratified environments as meromictic lakes. Purple sulfur bacteria growth is enabled as sulfur is supplied from the death and decomposition of microorganisms located above them within these intertidal pools. The stratification and sulfur source allows the PSB to grow in these intertidal pools where the mats occur. The PSB can help stabilize these microbial mat environment sediments through the secretion of extracellular polymeric substances that can bind the sediments in the pools.Hubas, C. et al
"Proliferation of Purple Sulphur Bacteria at the Sediment Surface Affects Intertidal Mat Diversity and Functionality"
'' PLOS One'', December 5, 2013. Retrieved February 12, 2020.


Ecological significance

Purple sulfur bacteria are able to affect their environment by contributing to
nutrient cycling A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyc ...
, and by using their metabolism to alter their surroundings. They are able to play a significant role in
primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs throug ...
suggesting that these organisms affect the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major component ...
through
carbon fixation Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms. The compounds are then used to store energy and as ...
. Purple sulfur bacteria also contribute to the phosphorus cycle in their habitat, and the
iron cycle The iron cycle (Fe) is the biogeochemical cycle of iron through the atmosphere, hydrosphere, biosphere and lithosphere. While Fe is highly abundant in the Earth's crust, it is less common in oxygenated surface waters. Iron is a key micronutrient ...
. Through upwelling of these organisms, phosphorus, a limiting nutrient in the oxic layer of lakes, is recycled and provided to
heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
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 Dissolved organic carbon (DOC) is the fraction of organic carbon operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometers. The fraction remaining on the filter is called partic ...
by purple sulfur bacteria is through the
food chain A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), det ...
; they act as a source of food to other organisms. Some purple sulfur bacteria have evolved to optimize their environmental conditions for their own growth. For example, in the South Andros Black Hole in the Bahamas, purple sulfur bacteria adopted a new characteristic in which they are able to use their
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...
to radiate heat energy into their surroundings. Due to the inefficiency of their carotenoids, or light-harvesting centres, the organisms are able to release excess light energy as heat energy. This adaptation allows them to compete more effectively within their environment. By raising the temperature of the surrounding water, they create an
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...
which supports their own growth, while also allowing them to outcompete other non-thermotolerant organisms.


Growth in meromictic lakes

Meromictic lake A meromictic lake is a lake which has layers of water that do not intermix. In ordinary, holomictic lakes, at least once each year, there is a physical mixing of the surface and the deep waters. The term ''meromictic'' was coined by the Austri ...
s are permanently stratified lakes produced by a gradient of saline concentrations. The highly salinated bottom layer is separated from the top layer of fresh water by the
chemocline A chemocline is a type of cline, a layer of fluid with different properties, characterized by a strong, vertical chemistry gradient within a body of water. In bodies of water where chemoclines occur, the cline separates the upper and lower layers, ...
, where the salinity changes drastically. Due to the large difference in density, the upper and lower layers do not mix, resulting in an anoxic environment below the chemocline. This anoxic environment with light and sufficient sulfide availability is ideal for purple sulfur bacteria. A study done at the Mahoney Lake suggested that purple sulfur bacteria contributes to the recycling of the inorganic nutrient, phosphorus. The upwelling of purple sulfur bacteria into the top layer of water creates a source of bound phosphorus, and
phosphatase In biochemistry, a phosphatase is an enzyme that uses water to cleave a phosphoric acid monoester into a phosphate ion and an alcohol. Because a phosphatase enzyme catalyzes the hydrolysis of its substrate, it is a subcategory of hydrolases. P ...
activity releases this phosphorus into the water. The soluble phosphorus is then incorporated into
heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
ic bacteria for use in developmental processes. In this way, purple sulfur bacteria participates in the phosphorus cycle and minimizes nutrient loss.


Biomarkers

Purple sulfur bacteria make conjugated
pigments A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
called
carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...
s that function in the light harvesting complex. When these organisms die and sink, some pigment molecules are preserved in modified form in the sediments. One carotenoid molecule produced, okenone, is diagenetically altered to the biomarker okenane. The discovery of okenane in marine sediments implies the presence of purple sulfur bacteria during the time of burial. Okenane has been identified in one sedimentary outcrop from Northern Australia dating to 1640 million years ago. The authors of the study concluded that, based on the presence of purple sulfur bacteria's biomarker, the
Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6  Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
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 Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on ...
, 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 Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on ...
, as their carbon source. This removes methane, a greenhouse gas, from the lagoon and reduces the lagoons' atmospheric pollution affect. H2S can act as a sulfur source for PSB during these same photosynthetic processes that remove the organic compounds. The use of H2S 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 Oceanic anoxic events or anoxic events ( anoxia conditions) describe periods wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters. Although anoxic events have not ...
* Anoxygenic photosynthesis * Green Lake (New York) * Okenane *
Sulfur-reducing bacteria Sulfur-reducing bacteria are microorganisms able to reduce elemental sulfur (S0) to hydrogen sulfide (H2S). These microbes use inorganic sulfur compounds as electron acceptors to sustain several activities such as respiration, conserving energy an ...


References

{{Taxonbar, from=Q134239 Gammaproteobacteria