Chemolithotrophy sulfur oxidation metabolism

1. Chemolithotrophy-
Sulfur Oxidizing
bacteria
Name- Deepika Rana
Roll no.-1601
Department-Microbiology(2nd semester)
M.D. University, Rohtak

2. 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).

3. 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.

4. •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.

5. 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
-)

6. 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

7. • 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

8. 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)

9. – 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.

10. 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)

11. –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

12. 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.

13. Physiological Characteristics of Sulfur Oxidizers

14. Sulfur oxidation
Reduced 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.

15. •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.

16. •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.

17. 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.

18. •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.

19. •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 (SO3
2-) and subsequently
converted to sulfate (SO4
2-) 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.

20. 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.

22. •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

23. • 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−, S4O6
2−, 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−, SO3
2−, 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.

24. Anaerobi
c Chemo-
lithotrop
hy
When NO3- an
electron acceptor it is
converted to NO2-
and water

25. 1. Prescott,Harley, and Klein’s Microbiology Seventh Edition Joanne M.Willey
Hofstra 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 Access
Characteristics 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