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Green sulfur bacteria
Green sulfur bacteria
Green sulfur bacteria
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Green sulfur bacteria
Green sulfur bacteria in a Winogradsky column
Scientific classification Edit this classification
Domain: Bacteria
Kingdom: Pseudomonadati
Phylum: Chlorobiota
Iino et al. 2021[3]
Class: "Chlorobia"
Garrity and Holt 2001[2]
Order: Chlorobiales
Gibbons and Murray 1978 (Approved Lists 1980)[1]
Families and Genera
Synonyms
  • Chlorobiota:
    • Chlorobi Iino et al. 2010
    • "Chlorobi" Garrity and Holt 2001
    • "Chlorobaeota" Oren et al. 2015
    • "Chlorobiota" Whitman et al. 2018
  • Chlorobiota:
    • "Chlorobia" Whitman et al. 2018
    • Chlorobea Cavalier-Smith 2002
    • "Chlorobiia" Cavalier-Smith 2020
  • Chlorobiales:
    • "Chlorobiales" Garrity and Holt 2001

The green sulfur bacteria are a phylum, Chlorobiota,[4] of obligately anaerobic photoautotrophic bacteria that metabolize sulfur.[5]

Green sulfur bacteria are nonmotile (except Chloroherpeton thalassium, which may glide) and capable of anoxygenic photosynthesis.[5][6] They live in anaerobic aquatic environments.[7] In contrast to plants, green sulfur bacteria mainly use sulfide ions as electron donors.[8] They are autotrophs that utilize the reverse tricarboxylic acid cycle to perform carbon fixation.[9] They are also mixotrophs and reduce nitrogen.[10][11]

Characteristics

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Green sulfur bacteria are gram-negative rod or spherical shaped bacteria. Some types of green sulfur bacteria have gas vacuoles that allow for movement. They are photolithoautotrophs, and use light energy and reduced sulfur compounds as the electron source.[12] Electron donors include H2, H2S, S. The major photosynthetic pigment in these bacteria is Bacteriochlorophylls c or d in green species and e in brown species, and is located in the chlorosomes and plasma membranes.[7] Chlorosomes are a unique feature that allow them to capture light in low-light conditions.[13]

Habitat

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The majority of green sulfur bacteria are mesophilic, preferring moderate temperatures, and all live in aquatic environments. They require anaerobic conditions and reduced sulfur; they are usually found in the top millimeters of sediment. They are capable of photosynthesis in low light conditions.[7]

The Black Sea, an extremely anoxic environment, was found to house a large population of green sulfur bacteria at about 100 m depth. Due to the lack of light available in this region of the sea, most bacteria were photosynthetically inactive. The photosynthetic activity detected in the sulfide chemocline suggests that the bacteria need very little energy for cellular maintenance.[14]

A species of green sulfur bacteria has been found living near a black smoker off the coast of Mexico at a depth of 2,500 m in the Pacific Ocean. At this depth, the bacterium, designated GSB1, lives off the dim glow of the thermal vent since no sunlight can penetrate to that depth.[15]

Green sulfur bacteria has also been found living on coral reef colonies in Taiwan, they make up the majority of a "green layer" on these colonies. They likely play a role in the coral system, and there could be a symbiotic relationship between the bacteria and the coral host.[16] The coral could provide an anaerobic environment and  a source of carbon for the bacteria. The bacteria can provide nutrients and detoxify the coral by oxidizing sulfide.[17]

One type of green sulfur bacteria, Chlorobaculum tepidum, has been found in sulfur springs. These organisms are thermophilic, unlike most other green sulfur bacteria.[7]

Phylogeny

[edit]
16S rRNA based LTP_10_2024[18][19][20] 120 marker proteins based GTDB 09-RS220[21][22][23]
Chlorobiaceae

Chloroherpeton thalassium

Chlorobium phaeovibrioides

Prosthecochloris

P. marina

P. aestuarii

P. vibrioformis

Pelodictyon clathratiforme (Szafer 1911) Lauterborn 1913

Chlorobium phaeobacteroides

Chlorobium limicola (type sp.)

Chlorobium luteolum

Chlorobaculum

C. thiosulfatiphilum

C. tepidum

"C. chlorovibrioides" (Gorlenko et al. 1974) Imhoff 2003

C. parvum

"Chloroherpetaceae"

Chloroherpeton thalassium Gibson et al. 1985

"Thermochlorobacteraceae"

"Ca. Thermochlorobacter aerophilum" Liu et al. 2012b

Chlorobiaceae
Prosthecochloris

P. marina Bryantseva et al. 2020

P. vibrioformis (Pelsh 1936) Imhoff 2003

P. aestuarii Gorlenko 1970 (type sp.)

P. ethylica Shaposhnikov, Kondrateva & Federov 1959 ex Kyndt, Van Beeumen & Meyer 2020

Chlorobaculum

C. parvum Imhoff 2003

C. tepidum (Wahlund et al. 1996) Imhoff 2003 (type sp.)

C. limnaeum Imhoff 2003

C. thiosulfatiphilum Imhoff 2022

Chlorobium

C. limicola Nadson 1906 emend. Imhoff 2003 (type sp.)

C. phaeobacteroides Pfennig 1968

C. luteolum (Schmidle 1901) emend. Imhoff 2003

"C. antarcticum" Panwar et al. 2021

C. phaeovibrioides Pfennig 1968

"C. chlorochromatii" Meschner 1957 ex Vogl et al. 2006

C. clathratiforme (Szafer 1911) Imhoff 2003

"C. ferrooxidans" Heising et al. 1998

"Ca. C. masyuteum" Lambrecht et al. 2021

Taxonomy

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Specific characteristics of genera

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Green sulfur bacteria are family Chlorobiaceae. There are four genera; Chloroherpeton, Prosthecochloris, Chlorobium and Chlorobaculum. Characteristics used to distinguish between these genera include some metabolic properties, pigments, cell morphology and absorption spectra. However, it is difficult to distinguish these properties and therefore the taxonomic division is sometimes unclear.[24]

Generally, Chlorobium are rod or vibroid shaped and some species contain gas vesicles. They can develop as single or aggregate cells. They can be green or dark brown. The green strains use photosynthetic pigments Bchl c or d with chlorobactene carotenoids and the brown strains use photosynthetic pigment  Bchl e with isorenieratene carotenoids. Low amounts of salt are required for growth.[24]

Prosthecochloris are made up of vibroid, ovid or rod shaped cells. They start as single cells that form appendages that do not branch, referred to as non-branching prosthecae. They can also form gas vesicles. The photosynthetic pigments present include Bchl c, d or e. Furthermore, salt is necessary for growth.[24]

Chlorobaculum develop as single cells and are generally vibroid or rod-shaped. Some of these can form gas vesicles. The photosynthetic pigments in this genus are Bchl c, d or e. Some species require NaCl (sodium chloride) for growth. Members of this genus used to be a part of the genus Chlorobium, but have formed a separate lineage.[24]

The genus Chloroherpeton is unique because members of this genus are motile. They are flexing long rods, and can move by gliding. They are green in color and contain the photosynthetic pigment Bchl c as well as γ-carotene. Salt is required for growth.[24]

Metabolism

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Photosynthesis

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The green sulfur bacteria use a Type I reaction center for photosynthesis. Type I reaction centers are the bacterial homologue of photosystem I (PSI) in plants and cyanobacteria. The GSB reaction centers contain bacteriochlorophyll a and are known as P840 reaction centers due to the excitation wavelength of 840 nm that powers the flow of electrons. In green sulfur bacteria, the reaction center is associated with a large antena complex called the chlorosome that captures and funnels light energy to the reaction center. The chlorosomes have a peak absorption in the far red region of the spectrum between 720 and 750 nm because they contain bacteriochlorophyll c, d and e.[25] A protein complex called the Fenna-Matthews-Olson complex (FMO) is physically located between the chlorosomes and the P840 RC. The FMO complex helps efficiently transfer the energy absorbed by the antena to the reaction center.

PSI and Type I reaction centers are able to reduce ferredoxin (Fd), a strong reductant that can be used to reduce NAD+ and fix CO2. Once the reaction center (RC) has given an electron to Fd, it becomes an oxidizing agent (P840+) with a reduction potential of around +300 mV. While this is not positive enough to strip electrons from water to synthesize O2 (E0 = +820 mV), it can accept electrons from other sources like H2S, thiosulphate or Fe2+ ions.[26] This transport of electrons from donors like H2S to the acceptor Fd is called linear electron flow, or linear electron transport. The oxidation of sulfide ions leads to the production of sulfur as a waste product that accumulates as globules on the extracellular side of the membrane. These globules of sulfur give green sulfur bacteria their name. When sulfide is depleted, the sulfur globules are consumed and further oxidized to sulfate. However, the pathway of sulfur oxidation is not well-understood.[8]

Instead of passing the electrons onto Fd, the Fe-S clusters in the P840 reaction center can transfer the electrons to menaquinone (MQ:MQH2) which returns the electrons to the P840+ via an electron transport chain (ETC). On the way back to the RC, the electrons from MQH2 pass through complex III (cytochrome bc1 complex) that pumps  H+ ions across the membrane. The electrochemical potential of the protons across the membrane is used to synthesize ATP by the FoF1 ATP synthase. This cyclic electron transport is responsible for converting light energy into cellular energy in the form of ATP.[25]

Sulfur metabolism

[edit]

Green sulfur bacteria oxidize inorganic sulfur compounds to use as electron donors for anaerobic photosynthesis, specifically in carbon dioxide fixation. They usually prefer to utilize sulfide over other sulfur compounds as an electron donor, however they can utilize thiosulfate or H2.[27] The intermediate is usually sulfur, which is deposited outside of the cell,[28] and the end product is sulfate. The sulfur, which is deposited extracellularly, is in the form of sulfur globules, which can be later oxidized completely.[27]

The mechanisms of sulfur oxidation in green sulfur bacteria are not well characterized. Some enzymes thought to be involved in sulfide oxidation include flavocytochrome c, sulfide:quinone oxidoreductase and the SOx system. Flavocytochrome can catalyze the transfer of electrons to cytochromes from sulfide, and these cytochromes could then move the electrons to the photosynthetic reaction center. However, not all green sulfur bacteria produce this enzyme, demonstrating that it is not needed for the oxidation of sulfide. Sulfide:quinone oxidoreductase (SQR) also helps with electron transport, but, when alone, has been found to produce decreased rates of sulfide oxidation in green sulfur bacteria, suggesting that there is a different, more effective mechanism.[27] However, most green sulfur bacteria contain a homolog of the SQR gene.[29] The oxidation of thiosulfate to sulfate could be catalyzed by the enzymes in the SOx system.[27]

It is thought that the enzymes and genes related to sulfur metabolism were obtained via horizontal gene transfer during the evolution of green sulfur bacteria.[29]

Carbon fixation

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Green sulfur bacteria are photoautotrophs: they not only get energy from light, they can grow using carbon dioxide as their sole source of carbon. They fix carbon dioxide using the reverse tricarboxylic acid cycle (rTCA) cycle[9] where energy is consumed to reduce carbon dioxide, rather than oxidize as seen in the forward TCA cycle,[9] in order to synthesize pyruvate and acetate. These molecules are used as the raw materials to synthesize all the building blocks a cell needs to generate macromolecules. The rTCA cycle is highly energy efficient enabling the bacteria to grow under low light conditions.[30] However it has several oxygen sensitive enzymes that limits its efficiency in aerobic conditions.[30]

Reductive TCA Cycle Diagram

The reactions of reversal of the oxidative tricarboxylic acid cycle are catalyzed by four enzymes:[9]

  1. pyruvate:ferredoxin (Fd) oxidoreductase:
    acetyl-CoA + CO2 + 2Fdred + 2H+ ⇌ pyruvate + CoA + 2Fdox
  2. ATP citrate lyase:
    ACL, acetyl-CoA + oxaloacetate + ADP + Pi ⇌ citrate + CoA + ATP
  3. α-keto-glutarate:ferredoxin oxidoreductase:
    succinyl-CoA + CO2 + 2Fdred + 2H+ ⇌ α-ketoglutarate + CoA + 2Fdox
  4. fumarare reductase
    succinate + acceptor ⇌ fumarate + reduced acceptor

However, the oxidative TCA cycle (OTCA) still is present in green sulfur bacteria. The OTCA can assimilate acetate, however the OTCA appears to be incomplete in green sulfur bacteria due to the location and down regulation of the gene during phototrophic growth.[9]

Mixotrophy

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Green sulfur bacteria are often referred to as obligate photoautotrophs as they cannot grow in the absence of light even if they are provided with organic matter.[9][26] However they exhibit a form of mixotrophy where they can consume simple organic compounds in the presence of light and CO2.[9] In the presence of CO2 or HCO−3, some green sulfur bacteria can utilize acetate or pyruvate.[9]

Mixotrophy in green sulfur bacteria is best modeled by the representative green sulfur bacterium Chlorobaculum tepidum.[31] Mixotrophy occurs during amino acid biosynthesis/carbon utilization and energy metabolism.[11] The bacterium uses electrons, generated from the oxidation of sulfur, and the energy it captures from light to run the rTCA. C. tepidum also exhibits use of both pyruvate and acetate as an organic carbon source.[11]

An example of mixotrophy in C. tepidum that combines autotrophy and heterotrophy is in its synthesis of acetyl-CoA. C. tepidum can autotrophically generate acetyl-CoA through the rTCA cycle, or it can heterotrophically generate it from the uptake of acetate. Similar mixotrophic activity occurs when pyruvate is used for amino acid biosynthesis, but mixotrophic growth using acetate yields higher growth rates.[31][11]

In energy metabolism, C. tepidum relies on light reactions to produce energy (NADPH and NADH) because the pathways typically responsible for energy production (oxidative pentose phosphate pathway and normal TCA cycle) are only partly functional.[11] Photons absorbed from the light are used to produce NADPH and NADH, the cofactors of energy metabolism. C. tepidum also generates energy in the form of ATP using the proton motive force derived from sulfide oxidation.[31] Energy production from both sulfide oxidation and photon absorption via bacteriochlorophylls.[11]

Nitrogen fixation

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The majority of green sulfur bacteria are diazotrophs: they can reduce nitrogen to ammonia which is then used to synthesize amino acids.[32] Nitrogen fixation among green sulfur bacteria is generally typical of an anoxygenic phototroph, and requires the presence of light. Green sulfur bacteria exhibit activity from a Type-1 secretion system and a ferredoxin-NADP+ oxidoreductase to generate reduced iron, a trait that evolved to support nitrogen fixation.[33] Like purple sulfur bacteria, they can regulate the activity of nitrogenase post-translationally in response to ammonia concentrations. Their possession of nif genes, even though evolutionarily distinct, may suggest their nitrogen fixation abilities arose in two different events or through a shared very distant ancestor.[32]

Examples of green sulfur bacteria capable of nitrogen fixation include the genus Chlorobium and Pelodictyon, excluding P. phaeoclathratiforme. Prosthecochloris aestuarii and Chloroherpeton thalassium also fall into this category.[32] Their N2 fixation is widespread and plays an important role in overall nitrogen availability for ecosystems. Green sulfur bacteria living in coral reefs, such as Prosthecochloris, are crucial in generating available nitrogen in the already nutrient-limited environment.[16]

See also

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Green sulfur bacteria

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from Grokipedia
Green sulfur bacteria, members of the family Chlorobiaceae within the phylum Chlorobi, are strictly anaerobic, obligate phototrophic microorganisms that perform using reduced sulfur compounds such as (H₂S) as electron donors, depositing elemental sulfur extracellularly as globules. These bacteria are Gram-negative, non-motile (or with limited motility in some species), and characterized by their ovoid, rod-shaped, or vibrioid cells, often forming consortia with other microbes in stratified environments. They fix (CO₂) via the reductive tricarboxylic acid (TCA) cycle, enabling autotrophic growth, and are phylogenetically distinct, with freshwater and marine lineages showing genetic separation. A defining feature of green sulfur bacteria is their highly efficient light-harvesting apparatus, the chlorosome, a unique antenna complex containing up to 250,000 molecules of (BChl) c, d, or e (distinguishing green from brown species), along with like chlorobactene or isorenieratene. Energy from these pigments is transferred via the Fenna-Matthews-Olson (FMO) protein to type I reaction centers, supporting noncyclic electron transport that generates NADPH without producing oxygen. Unlike oxygenic phototrophs, they cannot use as an and instead oxidize to via enzymes like sulfide-quinone reductase (SQR) and dissimilatory sulfite reductase (DSR), with some species also utilizing through the system. This allows them to thrive in low-light conditions (as little as 25–80 ), with chlorosomes up to 10 times larger than light-harvesting complexes in , providing a nearly double for CO₂ fixation compared to . Green sulfur bacteria inhabit anoxic, sulfide-rich aquatic environments, including the hypolimnia of stratified lakes, microbial mats, sulfur springs, coastal sediments, and even deep marine waters such as the at depths of 80 meters. Notable species include Chlorobium tepidum (a thermophilic growing at 45–55°C), Chlorobium vibrioforme, Chlorobaculum thiosulfatophilum, and Prosthecochloris aestuarii, distributed across genera like Chlorobium, Chlorobaculum, Prosthecochloris, and Chloroherpeton. Ecologically, they play a crucial role in global carbon and sulfur cycling as primary producers in euxinic (sulfide-containing) systems, regulating H₂S levels to prevent for other organisms and often forming syntrophic partnerships with (Chromatiaceae). Their δ¹³C enrichment of 10–11‰ in serves as a for ancient anoxic environments, and their adaptations to extreme low light make them key indicators of environmental stratification.

Taxonomy and Phylogeny

Classification

Green sulfur bacteria belong to the phylum Chlorobiota (formerly Chlorobi), class Chlorobia, order Chlorobiales, and family Chlorobiaceae. This taxonomic placement reflects their phylogenetic position within the Bacteria domain, based on 16S rRNA gene sequences and other molecular markers. The reclassification from phylum Chlorobi to Chlorobiota was proposed by Iino et al. in 2021 to align with updated nomenclatural standards for bacterial phyla ending in -ota, and it was validated by Oren and Garrity later that year. Prior to this, the group was commonly referred to as green sulfur bacteria within the phylum Chlorobi, a name established in earlier taxonomic frameworks. The family Chlorobiaceae comprises four main genera: Chlorobium, Chlorobaculum, Prosthecochloris, and Chloroherpeton. Species within these genera are delineated primarily using 16S rRNA gene sequences, with sequence similarities below 97-98.7% thresholds indicating distinct species; currently, there are over 15 validly published species, though the total including synonyms and provisional names exceeds 20. The following table summarizes the key genera, representative species, and distinguishing morphological traits:
GenusRepresentative SpeciesDistinguishing Traits
ChlorobiumC. limicola, C. phaeobacteroidesOvoid to short rod-shaped cells; typically mesophilic and non-motile.
ChlorobaculumC. tepidum, C. thiosulfatophilumRod- or vibrioid-shaped cells; often thermophilic, some form gas vacuoles.
ProsthecochlorisP. aestuariiRod-shaped cells with prosthecae ( protrusions); associated with marine or hypersaline environments.
ChloroherpetonC. thalassiumFilamentous, multicellular rods; capable of .

Evolutionary Relationships

Green sulfur bacteria, classified within the phylum Chlorobiota (class Chlorobia), occupy a distinct position in the bacterial domain as part of the FCB superphylum (Fibrobacterota-Chlorobiota-), a grouping supported by phylogenomic analyses utilizing 120 conserved bacterial marker proteins. This placement reflects their deep-branching phylum alongside within the FCB superphylum in standardized genomic taxonomies, where Chlorobiota forms a monophyletic with related groups. Their closest relatives include the (including Bacteroidia classes) and the Ignavibacteriota, with robust support from concatenated protein alignments showing shared molecular signatures and early divergence within the FCB assembly. Phylogenetic reconstructions, based on whole-genome sequences and genes, illustrate the divergence of Chlorobiota from other anoxygenic phototrophs, such as the within the Proteobacteria phylum, early in evolution. These trees position Chlorobiota as a basal lineage among phototrophic , separate from the alpha-, beta-, and gammaproteobacterial clades that encompass , highlighting independent evolutionary trajectories for their light-harvesting systems. The ancient origins of Chlorobiota are tied to Earth's early anoxic conditions, with estimates suggesting the emergence of in their ancestors around 3.5 billion years ago, contemporaneous with the eon when reducing atmospheres prevailed. Evidence from indicates that key photosynthetic genes in Chlorobiota were acquired through (HGT) from other bacterial donors, particularly proteobacterial lineages, facilitating the assembly of their type-I reaction centers and chlorosome structures. This HGT event likely occurred after the initial radiation of anoxygenic phototrophs, allowing Chlorobiota to adapt chlorophototrophy in sulfide-rich niches without relying on oxygenic mechanisms. Within the Chlorobiota, phylogenetic branching reveals a core structure comprising several orders, including Chlorobiales with the family Chlorobiaceae (e.g., genera Chlorobium and Prosthecochloris), and Ignavibacteriota as the nearest non-photosynthetic sister (e.g., Ignavibacterium spp.). This phylogenetic structure, derived from 16S rRNA and multi-protein phylogenies, underscores the of photosynthetic Chlorobiota while incorporating Ignavibacteriota as the nearest non-photosynthetic relatives. Recent metagenomic studies from have expanded the known diversity of Chlorobiota through phylogenomic analyses of uncultured lineages, revealing novel clades in extreme environments such as thermophilic hot springs in and , where aerobic photoheterotrophic variants dominate mats under low-oxygen conditions. Similarly, 2023-2024 investigations of deep-sea metagenomes have identified divergent Chlorobiota-related sequences in sulfidic sediments, suggesting adaptations to chemolithoautotrophy in dark, high-pressure settings and broadening the ecological range of this beyond illuminated niches.

Morphology and Cellular Characteristics

Structural Features

Green sulfur bacteria are Gram-negative prokaryotes characterized by diverse cell morphologies, including rods, vibroids, or spheres, with typical dimensions of 0.5–1.5 μm in width and 1–5 μm in length. These cells generally lack flagella and are non-motile, though certain species, such as Chloroherpeton thalassium, exhibit and form long, flexible unicellular filaments or multicellular aggregates. The cell envelope follows the typical Gram-negative architecture, consisting of an inner cytoplasmic membrane, a thin layer in the , and an outer membrane embedded with porins that facilitate nutrient transport. Unlike , green sulfur bacteria do not possess extensive intracytoplasmic membrane systems for ; instead, their light-harvesting structures, known as chlorosomes, are ovoid organelles (70–180 nm long and 30–60 nm wide) that attach directly to the inner surface of the cytoplasmic membrane via a protein baseplate. Electron microscopy reveals these chlorosomes as densely packed, sac-like entities filled with self-assembled aggregates, contributing to the cells' greenish hue. Some species contain gas vacuoles, composed of hollow protein cylinders that provide , enabling vertical migration within stratified aquatic environments. For instance, Pelodictyon clathratiforme forms net-like colonies with gas vacuoles that aid in positioning cells at optimal light intensities. Additionally, ovoid inclusions serve as storage sites for elemental , appearing as refractile globules primarily on the extracellular side of the outer during oxidation.

Pigments and Light-Harvesting Complexes

Green sulfur bacteria possess specialized systems that enable efficient light absorption in low-light anaerobic environments, primarily through bacteriochlorophylls (BChls) integrated into unique light-harvesting structures. The primary light-harvesting pigments are BChl c, d, or e, which are aggregated within antenna complexes and exhibit absorption maxima in the 650-760 nm range, allowing capture of far-red light that penetrates deeper into water columns. Specifically, BChl c absorbs at approximately 750-760 nm, BChl d at 725-735 nm, and BChl e at 650-660 nm, with variations depending on the specific homologs and aggregation state. In contrast, BChl a serves as the core pigment in the reaction centers, absorbing around 663 nm (often referred to as BChl 663), where it facilitates the initial charge separation during . The principal light-harvesting complexes in green sulfur bacteria are chlorosomes, which are large, sac-like organelles attached to the cytoplasmic membrane and containing 150,000 to 250,000 molecules of BChl c, d, or e per chlorosome, organized into self-assembling aggregates without the need for proteins. These aggregates form rod-like or lamellar structures that enhance light capture through excitonic interactions, enabling the bacteria to thrive at light intensities as low as 0.2% of full sunlight. The chlorosome baseplate, a protein-rich layer interfacing with the membrane, incorporates BChl a bound to proteins such as CsmA, which mediates energy transfer from the bulk pigments to the reaction center. Energy transfer within chlorosomes achieves efficiencies of up to 95-100%, owing to the highly ordered pigment arrangement that minimizes losses through rapid exciton migration. Accessory pigments, including , complement the BChls by providing photoprotection and broadening the absorption spectrum. In green-colored species, such as those in the Chlorobium, chlorobactene is the predominant , absorbing in the 450-550 nm range to shield against excess light and oxidative damage. Brown-colored strains, like Chlorobaculum limnaeum, instead feature isorenieratene, which similarly aids in quenching triplet states but shifts the overall pigmentation. Pigment profiles vary across genera: Chlorobium species predominantly contain BChl c with chlorobactene, enabling absorption peaks around 745-755 nm; Prosthecochloris may incorporate BChl d for slightly shorter wavelengths (around 710-720 nm); and brown genera like Ancalochloris rely on BChl e with isorenieratene, peaking at 650-660 nm for adaptation to different qualities in stratified aquatic habitats. These compositional differences reflect evolutionary adaptations to specific ecological niches, with spectroscopic analyses confirming the fine-tuned absorption properties that optimize .

Habitats and Ecology

Environmental Distribution

Green sulfur are obligate anaerobes that predominantly inhabit sulfide-rich, stratified aquatic environments where light penetration is limited, such as meromictic lakes, coastal sediments, and hypersaline . These thrive in the anoxic zones of these systems, where accumulates below the oxic layer, providing both an for and a protective barrier against oxygen. In such habitats, they often form dense populations at the chemocline, the transition zone between oxygenated and sulfidic waters, contributing to the stratification and biogeochemical cycling of and carbon. Prominent examples include the chemocline, where green sulfur bacteria dominate at depths around 100 meters, reaching cell densities of up to 8 × 10^4 cells per milliliter. In geothermal settings like Yellowstone National Park's hot springs, thermophilic strains form dense, dark green microbial mats at temperatures exceeding 50°C. Additionally, these bacteria have been isolated from deep-sea hydrothermal vents, such as those at approximately 2,400 meters in the , where they exploit geothermal light for amid high concentrations. These diverse locales highlight their adaptability to extreme aquatic niches while maintaining strict anaerobic requirements. Most green sulfur bacteria are mesophilic, with optimal growth temperatures between 15°C and 40°C, though thermophilic species like Chlorobaculum tepidum extend this range up to 52°C. They tolerate pH levels from 6 to 8 and low light intensities of 1 to 10 μmol photons m⁻² s⁻¹, enabling survival in dimly lit profundal zones. Their global distribution spans freshwater and marine ecosystems, including polar regions such as Antarctic meromictic lakes like Ace Lake, where they dominate phototrophic communities. Recent surveys in 2023 revealed expanded diversity in geothermal hot springs across the , , and , underscoring their widespread ecological presence. In these environments, they occasionally interact with other microbes, such as , at the boundaries of overlapping niches.

Ecological Interactions

Green sulfur bacteria serve as primary producers in the anoxic zones of stratified aquatic environments, such as meromictic lakes and coastal basins, where they perform by oxidizing (H₂S) to elemental or , thereby mitigating toxicity that could diffuse into overlying aerobic layers and harm oxygen-dependent organisms. This process supports carbon fixation via the reductive tricarboxylic acid cycle and maintains balance at the oxic-anoxic interface. In habitats like the , small populations of these bacteria contribute significantly to anaerobic H₂S oxidation under low-light conditions, highlighting their ecological influence despite low . Symbiotic associations involving green sulfur bacteria have been documented in coral microbiomes, with a 2021 genomic study identifying coral-associated Prosthecochloris species dominant in the skeletons of Isopora palifera from reefs near Lyudao, Taiwan, where they facilitate sulfide detoxification in anaerobic tissue microenvironments through oxidation of H₂S, sulfite, and thiosulfate via dissimilatory sulfate reduction and Sox systems. These bacteria engage in potential mutualism with sulfate-reducing partners, such as Candidatus Halodesulfovibrio lyudaonia, via syntrophic exchange of sulfur compounds—sulfide produced by reducers is oxidized by green sulfur bacteria, while oxidized forms are recycled back—enhancing nutrient cycling and resilience in sulfide-prone coral niches. Such interactions extend to broader syntrophic consortia with sulfur- and sulfate-reducing bacteria in low-light, sulfide-rich settings, where green sulfur bacteria's efficient chlorosome-based photosynthesis drives stable sulfur recycling. In microbial mats and sediments, green sulfur bacteria occupy key trophic roles as prey for grazers like protozoans and metazoans, linking primary production to higher trophic levels and facilitating energy transfer in benthic ecosystems. They contribute substantially to sulfur cycling, oxidizing a major portion of H₂S generated by sulfate reducers—up to 50% in some stratified systems—thus regulating sulfide fluxes and preventing anoxic toxicity. Community dynamics often feature dense blooms of these bacteria at chemoclines, where they dominate the phototrophic layer in meromictic lakes like Cadagno, Switzerland, co-occurring with to partition light and sulfide resources. Overall, green sulfur bacteria exert significant biogeochemical impacts by driving carbon fixation and sulfur transformations in stratified waters, influencing global fluxes in euxinic basins like the , though mechanisms of their coral symbioses remain incompletely understood, with gaps in elucidating host-microbe metabolite exchanges noted in current literature.

Metabolism and Physiology

Anoxygenic Photosynthesis

Green sulfur bacteria conduct , a light-driven that generates reducing power and ATP without evolving oxygen, in contrast to the oxygenic photosynthesis performed by and . This metabolic strategy enables these organisms to thrive in anaerobic environments rich in reduced compounds. The photochemical reaction occurs in specialized type I reaction centers, denoted as P840, which are homodimeric complexes composed of two PscA subunits, each containing eight bacteriochlorophyll a molecules and two chlorophyll a molecules. The primary electron donor, P840, is a heterodimer of bacteriochlorophyll a molecules that absorbs at approximately 840 nm, initiating charge separation upon excitation. Iron-sulfur clusters (FX, FA, and FB) serve as terminal electron acceptors within the reaction center, facilitating efficient to soluble . Light energy is captured primarily by chlorosomes, large, sac-like antenna complexes attached to the cytoplasmic membrane, which house thousands of bacteriochlorophyll c, d, or e molecules organized into self-assembled aggregates. These pigments absorb far-red light in the 700–800 nm range and transfer excitation energy nearly 100% efficiently through the Fenna-Matthews-Olson (FMO) protein trimer to the P840 reaction center, achieving overall quantum efficiencies of 40–75% despite some losses due to asymmetrical energy coupling. The cyclic electron flow in GSB involves electrons from the iron-sulfur centers reducing ferredoxin, which then donates to menaquinone (vitamin K) in the membrane. Menaquinone reduces a Rieske iron-sulfur protein, leading to cytochrome c oxidation and eventual return to the photooxidized P840, thereby establishing a proton motive force for ATP synthesis. For NAD+ reduction, reverse electron transport utilizes sulfide-derived electrons via a ferredoxin:NAD+ oxidoreductase, bypassing the need for oxygenic water splitting. A simplified representation of the donor-side reaction is: 2H2S+[light](/page/Light)→2S+4H++4e−2 \mathrm{H_2S} + \text{[light](/page/Light)} \rightarrow 2 \mathrm{S} + 4 \mathrm{H^+} + 4 e^- This process highlights the integration of capture with electron donation from , producing elemental as a . Adaptations for low- conditions are central to GSB photosynthesis efficiency, with chlorosomes enabling growth at irradiances below 4 μmol photons m−2 s−1, far lower than those required by many other phototrophs. The large antenna arrays in chlorosomes maximize photon capture in dim, sulfidic environments, such as deep-water columns, by increasing bacteriochlorophyll content and prostheca formation under reduced intensity. A 2024 review emphasizes how these features allow GSB to maintain high photosynthetic yields in anoxic, low- niches, underscoring their ecological significance in stratified aquatic systems.

Sulfur Oxidation Pathways

Green sulfur bacteria oxidize (H₂S) to elemental (S⁰), which is stored as extracellular globules, or completely to (SO₄²⁻), with serving as intermediates in some strains during the initial oxidation steps. This process generates reducing equivalents that donate electrons to the photosynthetic , supporting in anaerobic environments. The primary enzymes involved include the membrane-bound sulfide:quinone oxidoreductase (, variants such as SqrD, SqrE, and SqrF), which catalyzes the oxidation of H₂S to S⁰ or , and the periplasmic flavocytochrome c (Fcc), which further oxidizes sulfur compounds to or S⁰. Complete oxidation to proceeds via the dissimilatory sulfite reductase (Dsr) system, which converts stored S⁰ or to , while the system—common in —is absent in green sulfur bacteria, relying instead on Dsr for and handling. The overall stoichiometry for complete oxidation mirrors the aerobic reaction H₂S + 2 O₂ → SO₄²⁻ + 2 H⁺, but in these anaerobic phototrophs, it occurs via the , yielding approximately 150 kJ/mol H₂S in conserved for cellular processes. to S⁰ (H₂S → S⁰ + 2 H⁺ + 2 e⁻) provides less but allows storage and controlled release for further oxidation. Variations exist across genera: in Chlorobium species, such as Chlorobium limicola, S⁰ aggregates as extracellular globules, often limiting complete oxidation unless Dsr is active, while Chlorobaculum strains, like Chlorobaculum tepidum, efficiently oxidize alongside H₂S and S⁰ using Dsr and partial Sox components. Recent studies from the , including proteomic analyses of C. tepidum, highlight gaps in pathway elucidation, such as the roles of uncharacterized proteins in S⁰ transport and conversion, suggesting additional cytoplasmic enzymes like heterodisulfide reductase (Hdr) may contribute to mobilization. By rapidly consuming H₂S in stratified aquatic environments, green sulfur bacteria prevent its diffusion into overlying oxic layers, thereby maintaining redox gradients and mitigating toxicity to oxygenic phototrophs.

Carbon and Nutrient Assimilation

Green sulfur bacteria primarily assimilate carbon through autotrophic fixation via the reductive tricarboxylic acid (rTCA) cycle, a highly efficient pathway adapted for anaerobic conditions. This cycle operates in the reverse direction of the oxidative TCA cycle, enabling the net synthesis of organic compounds from CO₂ using reducing equivalents generated from or sulfur oxidation. Key enzymes unique to the rTCA pathway include ATP-citrate lyase, which cleaves citrate to and oxaloacetate, and 2-oxoglutarate: oxidoreductase, which facilitates the reversible carboxylation of to 2-oxoglutarate using as an electron carrier. The overall simplified reaction for carbon fixation in the rTCA cycle is: 2 CO2+8 H++8 e−→(CH2O)+H2O2 \, \mathrm{CO_2} + 8 \, \mathrm{H^+} + 8 \, e^- \rightarrow (\mathrm{CH_2O}) + \mathrm{H_2O} This process fixes two molecules of CO₂ per cycle turn, demonstrating high thermodynamic efficiency in CO₂ incorporation under low-energy conditions typical of anaerobic phototrophs. Many green sulfur bacteria exhibit mixotrophic capabilities, allowing simultaneous autotrophic and heterotrophic carbon assimilation, particularly under light limitation. In species such as Chlorobaculum tepidum, acetate or pyruvate is taken up and metabolized via pathways that integrate with the rTCA cycle, enhancing growth rates when organic substrates are available. Facultative heterotrophy remains rare among green sulfur bacteria, with most strains relying predominantly on autotrophy. Nitrogen assimilation in green sulfur bacteria occurs through both fixation and uptake pathways, supporting growth in nitrogen-limited habitats. Diazotrophic species, such as those in the genus Chlorobium, possess nif gene clusters that enable biological , converting N₂ to NH₃ via under anaerobic conditions. Ammonium assimilation proceeds primarily via the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway, where incorporates NH₄⁺ into glutamate to form , and glutamate synthase then transfers the amide group to regenerate glutamate, fueling biosynthetic needs. Phosphate acquisition and storage involve the accumulation of granules, which serve as a reservoir for and in fluctuating conditions. In Chlorobaculum tepidum and related species, these electron-dense granules form intracellularly during excess, enabling rapid mobilization during scarcity and contributing to cellular stress responses.

Genomics and Molecular Biology

Genome Organization

The of green sulfur bacteria, belonging to the phylum Chlorobi, typically consist of a single circular ranging in size from approximately 2 to 3.1 megabase pairs (Mbp), with a moderate of 44–57%. For instance, the genome of Chlorobium tepidum TLS, a sequenced in 2002, spans 2.15 Mbp with a of 56.5%. These compact genomes reflect the streamlined lifestyle of these obligately anaerobic phototrophs, enabling efficient resource use in low-light, sulfidic environments. Key functional gene clusters are prominently organized into operons, facilitating coordinated expression. Photosynthetic genes, such as those encoding the Type I reaction center (pscA, pscB) and biosynthesis (bch genes), are clustered in operons that support . Similarly, genes for the reductive tricarboxylic acid (rTCA) cycle, essential for carbon fixation, are often contiguous, including unique ATP-dependent citrate lyase subunits that distinguish Chlorobi from other bacteria. Plasmids are rare among green sulfur bacteria, with most strains lacking extrachromosomal elements, though conjugative plasmid transfer has been demonstrated in select species like Chlorobaculum tepidum. Genomes exhibit high coding density exceeding 85%, as seen in C. tepidum at 88.9%, with few pseudogenes indicating minimal genomic decay and complete pathways for core metabolisms. Comparative genomics across Chlorobi species reveals a conserved core genome supporting phototrophy, including shared operons for chlorosome assembly and electron transport, while oxidation genes show greater variability, often clustered in 2–4 loci that differ by adaptations. Post-2020 sequencing efforts, including metagenome-assembled genomes from ferruginous lakes and euxinic basins, have expanded this view, uncovering novel strains with similar organizational features but enhanced genetic plasticity for iron-based .

Genetic Diversity and Recent Discoveries

Green sulfur bacteria (GSB), belonging to the phylum Chlorobi, exhibit substantial genetic diversity revealed through metagenomic approaches, particularly in extreme environments like s. A 2023 study utilizing 16S rRNA amplicon sequencing and identified several novel clades of uncultured GSB in thermophilic mats from and the , expanding the known diversity of moderately thermophilic Chlorobaculum species beyond the three previously cultured strains. These findings highlight uncultured lineages adapted to temperatures of 36–51°C, with genomic signatures indicating specialized pathways. Advancements in have led to the description of new GSB species and lineages, underscoring their adaptive versatility. In 2021, metagenomic reconstruction of a dominant GSB population in an identified Candidatus Chlorobium antarcticum, featuring genes for cold adaptation such as glycosyltransferases involved in modification and seasonal cobalamin biosynthesis, enabling persistence in polar light cycles with abundances fluctuating over 100-fold annually. Horizontal gene transfer (HGT) plays a key role in shaping GSB genomic diversity, particularly for photosynthetic machinery. Metagenomic assemblies from lake blooms have demonstrated HGT of synthesis genes from distant phyla, such as Proteobacteria, conferring advantages in low-light anoxic niches and explaining rapid ecological adaptations in natural populations. Complementing this, CRISPR-Cas systems are present in GSB genomes and provide defense against phages. Emerging research points to untapped applications of GSB in . Their oxidation capabilities position them for , and hydrogenases enable potential H₂ production from -rich substrates under anaerobic phototrophic conditions. Recent surveys from 2024–2025 have addressed gaps in GSB diversity, particularly in symbiotic contexts like coral holobionts, where uncovered limited but significant Chlorobi genomes encoding and genes, suggesting probiotic roles in sulfur-stressed reef environments. These updates, derived from global datasets, expand the phylum's known uncultured branches and emphasize ongoing genomic explorations in underrepresented habitats.

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