LECTURE 24. THE IRON AND THE MERCURY CYCLES

I. The Iron Cycle

Overview: Important reactions of the iron cycle include:

• Oxidation of ferrous iron (Fe²⁺) to ferric iron (Fe³⁺). Oxygen functions as the electron acceptor. The reaction occurs spontaneously in oxic environments. In addition, oxidation of ferrous iron is done by Thiobacillus spp. living in acidic environments to obtain energy.
  
• Microbial reduction of ferric iron to ferrous iron. The use of Fe³⁺ as an electron acceptor only occurs under anoxic conditions, when the "favorite" acceptor, oxygen, is unavailable.

A. The impact of cyanobacteria on ferrous iron in the oceans:

1. Before the advent of oxygenic photosynthesis, the oceans were full of ferrous iron.

2. When oxygen appeared on Earth, ferrous iron spontaneously reacted with it, forming ferric iron:

Fe²⁺+ 1/4 O₂ + H⁺ -----> Fe³⁺+ 1/2 H₂O

The ferric iron subsequently reacted with hydroxide ions, forming the insoluble ferric hydroxide:

Fe³⁺+ 3 OH^- -----> Fe(OH)₃

Some additional points:

a. Thus the ocean's supply of aqueous ferrous iron was converted to solid ferric hydroxide. A brown band in the Earth's crust containing ferric hydroxide marks when oxygen first appeared on the planet.

b. Removal of hydroxide ions by precipitation with ferric iron amounts to an acidification.

c. The oxidation of ferrous iron occurs fairly slowly. However, at neutral to basic pH, the reaction is driven forward by the removal of the ferric iron product via precipitation.

B. How can metabolic energy be drawn from a chemically rapid spontaneous reaction?

We have seen that the oxidation of reduced compounds (sugars, organic acids, CH₄, NH₃, H₂S) generates energy that may be harvested by organisms. Can energy be similarly obtained from the oxidation of ferrous iron, given that this reaction proceeds rapidly in nature anyway?

1. It can't! There is no opportunity to extract energy from the oxidation of ferrous iron if nature has already depleted the substrate by spontaneous chemical reaction.

2. However, the oxidation of ferrous iron is not spontaneous under acidic conditions. The reason for this is obvious - there is not much hydroxide available to react with ferric iron. As a result, the slow oxidation reaction is not driven forward by precipitation of product ferric iron as described above.

3. Thus, if you are a microorganism that likes acid and lives in an oxic environment, oxidation of ferrous iron can be used as an energy source. Bacteria of the genus Thiobacillus, especially Thiobacillus ferrooxidans, acquire metabolic energy by oxidizing ferrous iron.

C. How can ATP energy be gained by oxidizing ferrous iron?

The problem is that the Fe²⁺/Fe³⁺couple yields very little energy. In other words, ferrous iron has a high reduction potential. It is a poor electron donor that doesn't really "want" to be oxidized. As a result, not much energy is gained when electrons are transferred from ferrous iron to oxygen.

1. The solution is to use the little bit of energy generated by each oxidation to enhance a pre-existing proton (H⁺) gradient that stores the energy. Thus, to draw this energy these organisms must oxidize a lot of ferrous iron. A natural proton gradient exists in these bacteria. Since the cells are soaked in an acidic solution, the periplasm, which is equilibrated with outside, has a very low pH (~ 2) relative to the cytoplasm (pH ~ 6). The key point is that cytoplasmic protons are consumed when oxygen accepts electrons derived from ferrous iron (Figure 16.22b). Therefore, each time ferrous iron is oxidized, the proton gradient becomes more extreme and thus acquires more energy.

In periplasm: 2 Fe²⁺ -----> 2 Fe³⁺ + 2 e^-

In cytoplasm: 2 e^- + 2 H⁺ + 1/2 O₂ -----> H₂O

2. As protons flow into the cytoplasm from the periplasm, an ATPase is "powered", i.e. ATP is produced by the phosphorylation of ADP using inorganic phosphate (P_i). In other words, as protons travel down their concentration gradient, the energy "released" as the gradient is diminished is conserved as the chemical energy in ATP.

D. Acid-Mine Drainage

The microbial oxidation of ferrous iron is part of a series of ecologically important reactions that can result in the acidification of some habitats (Figure 17.39):

1. Initiator reaction - A great deal of pyrite (FeS₂) is exposed to air during mining of coal. Exposure of the pyrite to oxygen causes its solubilization and the spontaneous oxidation of the sulfur to sulfate, producing ferrous iron and sulfuric acid:

2 FeS₂ + 7 O₂ + 2 H₂O -----> 2 Fe²⁺+ 4 H₂SO₄

2. The products of this reaction favor the growth of the acidophile Thiobacillus that oxidizes the ferrous iron derived from pyrite to ferric iron:

Fe²⁺+ 1/4 O₂ + H⁺ -----> Fe³⁺+ 1/2 H₂O

3. Propagation cycle - The ferric iron reacts with more pyrite, producing even more ferrous iron:

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

The products of the propagation cycle [ferrous iron (Fe²⁺) and sulfuric acid (H₂SO₄)] are fed back into the second reaction, further promoting Thiobacillus growth. As long as there is pyrite, these chain reactions continue.

4. Ultimate outcome:

a. FeS₂? The pyrite supply is exhausted.

b. pH? The sulfuric acid produced by these reactions causes the pH of nearby waters to drop dramatically. Ten thousand miles of waterways in the United States, mostly in Ohio and bordering states have been acidified by this mechanism!

c. Humans, fishes? The acidified water kills aquatic life and cannot be used for human recreation or consumption.

d. Time scale? These waters are useless during the 50-150 years that it takes for the pyrite to be completely exhausted.

Where is the opportunity behind this difficulty? Microbial oxidation of metals can be exploited in controlled mining operations without harming the environment.

E. Copper Recovery

Thiobacillus is valuable for mining. For example, this organism is used to liberate metallic copper (Cu^o) from the low-grade insoluble copper ores chalcocite (Cu₂S) and covellite (CuS). Analogously to the reactions that lead to acid mine drainage, the process consists of a series of self-supporting chemical and microbial-catalyzed reactions (Figure 17.43):

1. Microbial reactions:

a. The spontaneous oxidation of metallic iron (Fe^o) generates ferrous iron. The ferrous iron is further oxidized to ferric iron byThiobacillus if acid is added to encourage the growth of this organism.

Spontaneous: Fe^o -----> Fe²⁺ + 2 e^-

Thiobacillus: Fe²⁺ -----> Fe³⁺ + e^-

b. Thiobacillus oxidizes the cuprous copper in chalcocite (oxidation state = +1), generating soluble cupric copper (oxidation state = +2), and covellite (copper oxidation state = +2):

Cu₂S + O₂ -----> CuS + Cu²⁺ + H₂O

Thus we see that Thiobacillus is versatile, i.e. it can oxidize reduced forms of sulfur, iron, copper and other metals.

2. Propagation cycle

Additional soluble cupric ions are solubilized from covellite by the oxidation of the sulfur in covellite to sulfate. The electron acceptor in this spontaneous chemical reaction being the ferric iron produced by Thiobacillus:

CuS + 8 Fe³⁺+ H₂O -----> Cu²⁺ + 8 Fe²⁺+ SO₄^2- + 8 H⁺

The ferrous iron produced is returned to Step 1b (above) for microbial oxidation to ferric iron, allowing for a chain reaction.

3. Recovery of metallic copper from cupric ions

Metallic iron spontaneously reduces cupric copper ions, generating metallic copper.

Cu²⁺ + Fe^o -----> Cu^o + Fe²⁺

The ferrous iron produced here is also returned to Step 1b (above) for microbial oxidation to ferric iron, allowing for a chain reaction.

II. The Man-Made Mercury "Cycle"

Approximately 40,000 tons of mercury (Hg^o) per year are dumped into the environment.

Overview: Important reactions of the mercury cycle (Figure 17.44) include:

• Volatile elemental mercury (Hg^o) is spontaneously oxidized in the presence of light, producing the mercuric cation (Hg²⁺).
  
• Conversely, some microorganisms harboring heavy-metal resistance plasmids can detoxify the toxic mercuric cation by reducing it, regenerating elemental mercury.
  
• Many microorganisms transfer methyl groups to the mercuric cation, using enzymes that have vitamin B12 as their cofactor, producing methylmercury (CH₃Hg⁺).
  
• Methanogens can utilize methylmercury as a substrate, producing methane (CH₄) and elemental mercury as a bi-product.
  
• Hydrogen sulfide spontaneously reacts with the mercuric cation, reducing the mercury atom and producing insoluble mercuric sulfide (HgS). Recall that sulfate-reducing bacteria can provide the substrate, hydrogen sulfide.
  
• Conversely, Thiobacillus can convert mercuric sulfide to the mercuric cation.

Note in the last three reactions the ability of microorganisms to metabolize "unusual" substances. Bacteria are more robust, more adaptable and better chemists than we are!

A. Toxicity of Mercury and Related Substances

1. Elemental mercury (oxidation state = 0) is relatively non-toxic.


2. Mercuric cation (+2) is toxic, but it is rapidly excreted. However, if dumped into the environment, it can be converted to methylmercury by microorganisms.


3. Methylmercury (+1) is very toxic - It is 100-fold more toxic than the mercuric cation. In humans, it is a potent neurotoxin that has a long half-life (~ 70 days). In addition, methylmercury is concentrated ~ a million times by aquatic food chains, e.g. in fish and filter-feeding shellfish. See the optional reading above on the Minamata disaster and the famous W. Eugene Smith photograph "Tomoko in her bath."

B. Anoxic Detoxification of Mercury by Microorganisms

1. Methanogens are helpful in disposing of methylmercury by returning the mercury atom to elemental mercury.

CH₃Hg⁺ -----> CH₄ + Hg^o

2. Microorganisms harboring heavy-metal resistance plasmids can reduce mercuric cation to elemental mercury.

Hg²⁺ -----> Hg^o + 2 e^-

3. In anoxic environments, sulfate-reducing bacteria are helpful in the disposal of mercuric cations. They generate hydrogen sulfide that spontaneously reacts with mercuric cations, precipitating them as insoluble sulfides of mercury and removing them from solutions.

Hg²⁺+ H₂S -----> HgS, Hg₂S

C. Toxifying Effects of Microorganisms

1. If the mercuric sulfides (in Section B.2) work their way into oxic environments, they can be converted back to mercuric cations by Thiobacillus spp.

HgS -----> Hg²⁺+ H₂S

2. Methyl groups are transferred to mercuric cation by many microorganisms, using B12 enzymes, producing the extremely toxic methylmercury (CH₃Hg⁺).

Hg²⁺ + "CH₃^- " -----> CH₃Hg⁺