LECTURE 23. THE SULFUR CYCLE

I. The Sulfur Cycle

An important distinction between cycling of sulfur and cycling of nitrogen and carbon is that sulfur is "already fixed". That is, plenty of sulfate anions (SO₄^2-) are available for living organisms to utilize. By contrast, the major biological reservoirs of nitrogen atoms (N₂) and carbon atoms (CO₂) are gases that must be pulled out of the atmosphere.

Overview: Important reactions of the sulfur cycle (Figure 17.35) include:

• Assimilative sulfate reduction - sulfate (SO₄^2-) is reduced to organic sulfhydryl groups (R-SH) by plants, fungi and various prokaryotes. The oxidation states of sulfur are +6 in sulfate and -2 in R-SH.
  
• Desulfuration - organic molecules containing sulfur can be desulfurated, producing hydrogen sulfide gas (H₂S), oxidation state = -2. Note the similarity to deamination.
  
• Oxidation of hydrogen sulfide produces elemental sulfur (S^o), oxidation state = 0. This reaction is done by the photosynthetic green and purple sulfur bacteria and some chemolithotrophs.
  
• Further oxidation of elemental sulfur by sulfur oxidizers produces sulfate.
  
• Dissimilative sulfur reduction - elemental sulfur can be reduced to hydrogen sulfide.
  
• Dissimilative sulfate reduction - sulfate reducers generate hydrogen sulfide from sulfate.

Reservoirs of sulfur atoms:

• The largest physical reservoir is the Earth's crust wherein sulfur is found in gypsum (CaSO₄) and pyrite (FeS₂).
  
• The largest reservoir of biological useful sulfur is found in the ocean as sulfate anions (very concentrated at 2.6 g/L), dissolved hydrogen sulfide gas, and elemental sulfur.
  
• Other reservoirs include:
  
• Freshwater - contains sulfate, hydrogen sulfide and elemental sulfur;
• Land - contains sulfate;
• Atmosphere - contains sulfur oxide (SO₂) and methane sulfonic acid (CH₃SO₃^-); volcanic activity releases some hydrogen sulfide into the air.

A. Given that Sulfur Is "Already Fixed", Why Bother Studying the Sulfur Cycle?

1. Environmental impacts are diverse and important locally even on a human time scale:

a. Some of the reactions that occur in the sulfur cycle open up new environments to life. They support biological communities in unlikely places such as deep sea thermal vents, areas of low pH and areas of high temperature.

b. On the other hand, certain reactions remove needed metabolites or produce wastes that make environments uninhabitable to some organisms.

2. Interesting microbial chemistries, that no other organisms do, are found in cycles such as the sulfur cycle. They have been exploited in:

a. Mining,

b. Bioremediation,

c. Synthesis of industrial chemicals.

B. Sulfur Oxidation

1. The light-induced oxidation of hydrogen sulfide for harvesting electrons during photosynthesis has already been discussed:

a. Organisms? The green and purple sulfur bacteria oxidize hydrogen sulfide for photosynthesis.

b. Habitats?

(i) Obviously, these organisms must live in the light. Therefore they cannot exist deep in the oceans where light does not penetrate.

(ii) The environment must contain a source of hydrogen sulfide, usually arising from desulfuration of decaying organic material or from sulfate reduction.

e.g. These organisms are often found in waters "one level" above decaying organics or sulfate reducers where they acquire hydrogen sulfide bubbling up from below and are simultaneously illuminated by the sunlight.


2. Chemolithotrophic oxidation of hydrogen sulfide generates energy:


a. Hydrogen sulfide can be oxidized to elemental sulfur:

H₂S + 1/2 O₂ -----> S^o + H₂O + energy

b. Elemental sulfur in turn can be oxidized to sulfate:

S^o + 1 1/2 O₂ + H₂O ---> SO₄^2- + 2 H⁺ + energy

c. Habitats/Requirements?

(i) Oxic or anoxic? Bacteria that oxide sulfur-containing materials occur in both oxic and anoxic environments. Those that live in oxic environments perform the reactions shown above. A different electron acceptor, such as nitrate, is utilized in anoxic environments since the "favorite" acceptor, oxygen, is unavailable.

(ii) pH? Note that the oxidation of sulfur in oxic habitats produces sulfuric acid (SO₄^2- + 2 H⁺ = H₂SO₄). Organisms doing these reactions must be acidophiles that can tolerate the resultant acidic habitats.

(iii) Source of hydrogen sulfide?

• Desulfuration of decaying organic material releases hydrogen sulfide;
• Sulfate reducers can generate hydrogen sulfide;
• Volcanic activity releases hydrogen sulfide. For example, chemolithotrophs near thermal vents in the deep sea harvest the energy from this source. Thus they form the foundation of whole communities in the deep sea where light cannot penetrate.

(iv) When is elemental sulfur (S^o) oxidized? Organisms will oxidize hydrogen sulfide (H₂S) until it runs out and then begin utilizing elemental sulfur. This is logical, since more energy can be acquired from oxidizing hydrogen sulfide compared to elemental sulfur. As we have seen before, use of an alternate substrate requires the expression of genes not previously expressed.

d. Organisms:

(i) Beggiatoa - historically important because it was the first chemolithotroph identified.

(ii) Thiobacillus - an obligate acidophile, very tolerant of low pH; in addition to oxidizing hydrogen sulfide, this organism can extract iron from solid pyrite (FeS₂) in a two-step process in which sulfur atoms are oxidized.

First, the organism catalyzes the oxidation of ferrous iron, generating ferric iron

Fe²⁺+ 1/2 O₂ + 2 H⁺ -----> Fe³⁺+ H₂O


Secondly, the ferric iron produced spontaneously reacts with pyrite

FeS₂ + 14 Fe³⁺ + 8 H₂O -----> 15 Fe²⁺ + 2 SO₄^2- + 16 H⁺

Note: The reaction is self-supporting, since the ferrous iron produced in the second reaction can be fed back into the first reaction. Thus these chain reactions will continue until all of the pyrite is exhausted. These reactions also generate copious amounts of sulfuric acid (H₂SO₄) that acidify the waters near coal mines, where there is plenty of exposed pyrite (See the more extensive discussion of acid-mine drainage in the next lecture.).

(iii) The Thiovulum/Riftia symbiosis - Riftia is a tube worm, ~ 2 meters long, found near thermal vents in the deep sea. Riftia contains an organ called a trophosome that harbours Thiovolum and several other prokaryotic genera (~ 4 x 10⁹ cells/gram). The worm contains a unique hemoglobin that binds the hydrogen sulfide generated by volcanic activity and delivers it to the bacterial symbiont. Bacterial oxidation of the hydrogen sulfide generates the energy that is required to fix carbon. The worm receives the fixed carbon from the bacteria.

C. Sulfate reduction

Dissimilative sulfate reduction involves using sulfate as a terminal electron acceptor during the energy-generating oxidation of various materials (Table 16.6). A specific example of sulfate reduction involves the oxidation of molecular hydrogen (H₂) that occurs in several steps:

1. Sulfate, which is fairly stable, is activated by reaction with ATP, forming adenosine phosphosulfate (APS) (Figure 16.31):

SO₄^2- + ATP -----> APS + PP_i

2. A hydrogenase splits molecular hydrogen, and the electrons contained therein are used to reduce the sulfur atom of APS, releasing sulfite (SO₃^2-).

APS + H₂ -----> SO₃^2- + AMP + H₂O

This reaction involves an intermediate electron carrier, cytochrome c3, that is diagnostic for dissimilative sulfate reducers (Figure 16.32).

3. Using more electrons derived from molecular hydrogen, sulfite is reduced, producing hydrogen sulfide:

SO₃^2- + 6 H⁺+ 6 e^- ------> H₂S + H₂O + 2 OH^-

Two additional points:

a. Sulfite is toxic to most organisms, so it is reduced as soon as it is produced, i.e. organisms do not wait until sulfate is exhausted to begin utilizing sulfite. The hydrogen sulfide product is also toxic, but it is a gas that escapes into the atmosphere as it is generated.

b. The reaction generates hydroxide ions that elevate the pH and thus aid in de-acidification.

4. Habitats/Requirements?

a. Oxic or anoxic? The dissimilative sulfate reducers live in anoxic environments. Recall that organisms that utilize electron acceptors other than oxygen usually live in anoxic habitats.

b. Best sources of sulfate? Sulfate reducers occur in aquatic habitats, where sulfate is generally abundant. Some occur in the anoxic layers of soils where a lesser amount of sulfate resides.

c. Autotrophs vs. organotrophs? The dissimilative sulfate reducers are mostly organotrophs. Because of sulfate's low reduction potential, its reduction generates little energy. In other words, sulfate is a poor electron acceptor. Thus, it is not practical to fix carbon using sulfate reduction as an energy source. As a rule, dissimilative sulfate reducers require a carbon source, commonly acetate.

Note - Some organisms can use a variety of electron acceptors. They exhaust the preferred acceptor first and then switch to the next best acceptor, etc.

Rank of electron acceptors: O₂ > NO₃^- > SO₄^2-

d. Can the methanogens compete? Recall that acetoclastic methanogens consume molecular hydrogen and acetate, producing methane. Thus, a competition exists between the dissimilative sulfate reducers and those methanogens. In aquatic environments, where sulfate is abundant, the methanogens lose the competition. An additional advantage of the dissimilative sulfate reducers over the methanogens as a group is that the sulfate reducers have a greater affinity for molecular hydrogen.

e. Some examples of sulfate reducers and their habitats:

Note that the prefix "Desulfo" indicates a sulfate reducer.

(i) Desulfovibrio - found in water-logged soils.

(ii) Desulfotomaculum - cause of the "sulfide stinker", a type of spoilage of canned foods. This is indicated by swelling of the can as hydrogen sulfide gas is produced and an unpleasant odor on opening the can.

(iii) Desulfomonas - found in intestines.

(iv) Archaeglobus - a thermophilic Archea whose optimal growth temperature is 83^oC.