Chemolithotrophy sulfur oxidation metabolism

Chemolithotrophy- Sulfur

Oxidizing bacteria

Name- Deepika RanaRoll no.-1601

Department-Microbiology(2nd semester)

M.D. University, Rohtak

CHEMOLITHOTROPHY•Chemolithotrophs-These microbes obtain electrons for the electron transport chain from the oxidation of inorganic molecules rather than NADH generated by the oxidation of organic nutrients.

•The acceptor is usually O2, but sulfate and nitrate are also used.

•The most common electron donors are hydrogen, reduced nitrogen compounds, reduced sulfur compounds, and ferrous iron (Fe2).

Chemolithotrophs• Energy yield is always lower than that for a glucose molecule.• Much less energy is available from oxidation of inorganic

molecules than from the complete oxidation of glucose to CO2(∆G=686 kcal/mole). This is because the NADH that donates electrons to the chain has a more negative reduction potential than most inorganic substrates.

•Thus the P/O(Phosphate/Oxygen)ratios for oxidative phosphorylation in chemo-lithotrophs are probably around 1.0 (although in the oxidation of hydrogen it is considerably higher).• Because of low ATP yield, chemolithotrophs must oxidize a large quantity of inorganic material to grow and reproduce.•This is particularly true of autotrophic chemolithotrophs, which fix CO2 into carbohydrates. For each molecule of CO2 fixed, these microbes expend three ATP and two NADPH molecules. •Because they must consume a large amount of inorganic material, chemolithotrophs have significant ecological impact.

Chemolithotrophs• Hydrogen Oxidizers:

– Most efficient (P/O > 1); εH2 <εNADH– Hydrogenase may donate electrons to NAD+

• Sulfur Oxidizers:– ATP by Substrate level phosphorylation in addition to oxidative

phosphorylation– Substrate level phosphorylation is via adenosine 5’-phosphosulfate (APS)

• Iron Oxidizers – Acidophilic Thiobacillus ferrooxidans Fe+2 → Fe+3

– Acid Mine Drainage if pyrite is exposed to O2 and H2O!– Circumneutral Gallionella ferruginea Fe+2 → Fe+3

• Nitrifying Bacteria:– Ammonium Oxidizers (NH4

+ → NO2-)

– Nitrate Oxidizers (NO2- → NO3

-)

– Process of “Nitrification” (NH4+ → NO3

-)

Aerobic Chemolithotrophs-Sulfur oxidizers

• Sulfur-oxidizing bacteria are Gram-negative rods or spirals

• Grow in filaments• Obtain energy through

oxidation of reduced sulfur– Including hydrogen sulfide,

elemental sulfur and thiosulfate

– Molecular oxygen serves as terminal electron acceptor

• This produces sulfuric acid

• Filamentous sulfur oxidizers live in sulfur springs, sewage polluted waters and on surface of aquatic sediments

• Causes bulking in sewage treatment facilities– Interferes with the separation

of solid sludge and liquid effluent

• Unicellular sulfur oxidizers found in both terrestrial and aquatic environments

• Responsible for bioleaching through oxidation of metal sulfides producing sulfuric acid and liquid metal– Some species produce enough

acid to lower pH to 1.0

Sulfur Oxidizing Bacteria

– Two broad classes• Neutrophiles

• Acidophiles

• enzymatic and molecular basis of sulfur oxidation in archaea is totally different from those of bacteria

• Some obligate chemolithotrophs possess special structures that house Calvin cycle enyzmes (carboxysomes)

– Thiobacillus and close relatives are best studied

• Rod-shaped• Sulfur compounds most commonly

used as electron donors are H2S, So, S2O3

2-; generates sulfuric acid

– Achromatium• Common in freshwater sediments• Spherical cells• Pylogenetically related to purple

bacteria Chromatium• A classic example of a sulfur-

oxidizing bacterium is Beggiatoa, a microbe originally described by Sergei Winogradsky, one of the founders of environmental microbiology. Another example is Paracoccus.

(a)Halothiobacillus neapolitanus

(b)Achromatium sp.

Beggiatoa• Filamentous, gliding bacteria

• Found in habitats rich in H2S

– e.g., sulfur springs, decaying

seaweed beds, mud layers of

lakes, sewage polluted

waters, and hydrothermal

vents

• Most grow mixotrophically

– with reduced sulfur

compounds as electron

donors

– and organic compounds as

carbon sources ( lack Calvin ∵cycle enzymes)

– Thioploca• Large, filamentous sulfur-

oxidizing bacteria that form cell bundles surrounded by a common sheath

• Thick mats found on ocean floor off Chile and Peru

• Couple anoxic oxidation of H2S with reduction of NO3

- to NH4+

Thioploca sp.

─Thiothrix• Filamentous sulfur-

oxidizing bacteria in which filaments group together at their ends by a holdfast to form cellular arrangements called rosettes

• Obligate aerobic mixotrophs

Thiothrix

Sulfolobus•Members of the genus Sulfolobus stain gram negative, and are aerobic, irregularly lobed spherical archaea with a temperature optimum around 70 to 80°C and a pH optimum of 2 to 3.•For this reason, they are thermoacidophiles, so called because they grow best at acid pH values and high temperatures. •Their cell wall contains lipoprotein and carbohydrate. •They grow lithotrophically in sulfur granules in hot acid springs and soils while oxidizing the sulfur to sulfuric acid.•Oxygen is the normal electron acceptor, but ferric iron may beused.• Sugars and amino acids such as glutamate also serve as carbon and energy sources.

Physiological Characteristics of Sulfur Oxidizers

Sulfur oxidationReduced sulfur compounds are oxidized by most organisms, including higher animals and higher plants.

Some organisms can conserve energy (i.e., produce ATP) from the oxidation of sulfur.

Sulfur is the sole energy source for some lithotrophic bacteria and archaea.

Reduced sulfur compounds, such as hydrogen sulfide, elemental sulfur, sulfite, thiosulfate, and various polythionates (e.g.,tetrathionate), are used by various lithotrophic bacteria and are all oxidized by Acidithiobacillus.

Sulfur oxidizers utilize enzymes such as sulfur oxygenase and sulfite oxidase to oxidize sulfur compounds to sulfate. Lithotrophs that can produce sugars through chemosynthesis make up the base of some food chains.

Food chains have formed in the absence of sunlight around hydrothermal vents, which emit hydrogen sulfide and carbon dioxide. Chemosynthetic archaea use hydrogen sulfide as an energy source for carbon fixation, producing sugars.

•Biological oxidation of hydrogen sulfide to sulfate is one of the major reactions of the global sulfur cycle. Reduced inorganic sulfur compounds are exclusively oxidized by prokaryotes, and sulfate is the major oxidation product. Sulfur oxidation in members of the Eukarya is mediated by lithoautotrophic bacterial endosymbionts .

•The sulfur-oxidizing prokaryotes are phylogenetically diverse. In the domain Archaea aerobic sulfur oxidation is restricted to members of the order Sulfolobales, and in the domain Bacteria sulfur is oxidized by aerobic lithotrophs or by anaerobic phototrophs. The non-phototrophic obligate anaerobe Wolinella succinogenes oxidizes hydrogen sulfide to polysulfide during fumarate respiration.

A Simplified Sulfur Cycle.Sulfur oxidation can be carried out by a wide range of aerobic chemotrophs and by aerobic and anaerobic phototrophs.

•The metabolism of Thiobacillus has been best studied. These bacteria oxidize sulfur (So), hydrogen sulfide (H2S), thiosulfate (H2S2O3), and other reduced sulfur compounds to sulfuric acid; therefore they have a significant ecological impact.

•Interestingly they generate ATP by both oxidative phosphorylation and substrate level phosphorylation involving adenosine 5 -phosphosulfate (APS). APS is a high-′energy molecule formed from sulfite and adenosine monophosphate.

•Some sulfur-oxidizing procaryotes are extraordinarily flexible metabolically.

•For example, Sulfolobus brierleyi, an archaeon, and some bacteria can grow aerobically by oxidizing sulfur with oxygen as the electron acceptor; in the absence of O2, they carry out anaerobic respiration and oxidize organic material with sulfur as the electron acceptor.

Sulfur chemolithotrophy as the earliest self-sustaining metabolism.

Correspondingly, it is noteworthy that photolithotrophic sulfur oxidation has not yet been reported from any member of Archaea, or for that matter any other hyperthermophile. However, diverse groups of optimally adapted anoxygenic photolithotrophic bacteria thrive in moderately extreme temperature, pH or salinity conditions, and act as primary producers in such unusual habitats.

•Efficacy of energy conservation from the same sulfur substrates by different organisms at their respective pH and temperature optima is also variable.

•Species-dependent biochemical differences also pertain to the oxidative enzymes, pathways and electron transport mechanisms that different groups of bacteria use to metabolize any given sulfur compound.

• On the other hand, chemolithotrophic sulfur oxidation by archaea is relatively less understood in comparison to the bacterial counterparts.

•Generally, the OXIDATION OF SULFIDE occurs in stages, with inorganic sulfur being stored either inside or outside of the cell until needed.

•This two step process occurs because energetically sulfide is a better electron donor than inorganic sulfur or thiosulfate, allowing for a greater number of protons to be translocated across the membrane.

•Sulfur-oxidizing organisms generate reducing power for carbon dioxide fixation via the Calvin cycle using reverse electron flow, an energy-requiring process that pushes the electrons against their thermodynamic gradient to produce NADH.

• Biochemically, reduced sulfur compounds are converted to sulfite (SO32-) and subsequently

converted to sulfate (SO42-) by the enzyme sulfite oxidase

•Some organisms, however, accomplish the same oxidation using a reversal of the APS reductase system used by sulfate-reducing bacteria.

•In all cases the energy liberated is transferred to the electron transport chain for ATP and NADH production. In addition to aerobic sulfur oxidation, some organisms (e.g. Thiobacillus denitrificans use nitrate (NO−3) as a terminal electron acceptor and therefore grow anaerobically.

Energy Generation by Sulfur Oxidation.(a) Sulfite can be directly oxidized to provide electrons for electron transport and oxidative phosphorylation. (b) Sulfite can also be oxidized and converted to adenosine 5 -′phosphosulfate (APS). This route produces electrons for use in electron transport and ATP by substrate-level phosphorylation with APS. (c) The structure of APS.

•Again, more than one sulfur-oxidizing enzyme system have also been envisaged even within a single bacterium such as Thiobacillus denitrificans, which can adapt to varying physicochemical conditions in diverse environments.

•Thiobacillus denitrificans , which is also capable of carrying out the unique anaerobic (nitrate-dependent) oxidation of uranium oxide minerals [U(IV) to U(VI)], differs from all other sulfur chemolithotrophs by its ability to conserve energy from the oxidation of inorganic sulfur compounds under both aerobic and denitrifying conditions.

•Although sulfur chemolithotrophs are mostly aerobic, using molecular oxygen as terminal electron acceptor, species of Beggiatoa and Thioploca, the haloalkaliphilic gammaproteobacterium Thioalkalivibrio and the moderately halophilic Thiohalomonas, and Sulfurimonas denitrificans (formerly Thiomicrospira denitrificans)can also anaerobically oxidize sulfur by coupling it to nitrate reduction

• Like these aforesaid bacteria, several sulfur-oxidizing chemolithoautotrophic hyperthermophilic archaea can also use an extraordinary array of electron donors, including H2, Fe2+, H2S, S, S2O3

2−, S4O62−, sulfide minerals, CH4, various carboxylic acids,

alcohols, amino acids and complex organic substrates, while electron acceptors include O2, Fe3+, CO2, CO, NO3

−, NO2−, NO, N2O, SO4

2−, SO32−, S2O3

2− and S

•Besides S. denitrificans, newly described members of Epsilonproteobacteria such as Sulfurimonas autotrophica and Sulfurovum lithotrophicum , Arcobacter sp. strain FWKO B, Thiomicrospira sp. strain CVO and Sulfuricurvum kujiense are also facultatively anaerobic, nitrate-reducing, sulfur-oxidizing chemolithotrophs.

• Thiobacillus denitrificans is the best studied among the obligately sulfur-chemolithoautotrophic bacteria known to couple denitrification to sulfur compound oxidation.

Anaerobic Chemo-lithotrophyWhen NO3- an electron acceptor it is converted to NO2- and water

References

1. Prescott,Harley, and Klein’s Microbiology Seventh Edition Joanne M.WilleyHofstra University Linda M. Sherwood Montana State University Christopher J.Woolverton Kent State University

2. Brock Biology of Microorganisms 13th edition Michael Madigan 2009 Pearson Education Inc.

3. Sulfur metabolism From Wikipedia, the free encyclopedia

4. Microbial Cell Factories Review Open AccessCharacteristics and adaptability of iron- and sulfur-oxidizing microorganisms used for the recovery of metals from minerals and their concentrates Douglas E Rawlings*Address: Department of Microbiology, University of Stellenbosch, Private BagX1, Matieland, 7602, South Africa Received: 06 April 2005 Accepted: 06 May 2005

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