Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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Other In the soil, ammonium is often held at cation-exchange sites of clay minerals.

Nitriﬁcation mobilizes this nitrogen, making it easier for plants to absorb if the ammo-

nium in their rhizosphere has been depleted, but also easier for it to leach into the ground-

water. On the other hand, assimilation of ammonium or nitrate by bacteria can make this

nitrogen immobilized and unavailable to plants. Thus, if waste materials with a high car-

bon/nitrogen ratio, (such as some crop residues and some solid wastes) are applied to soil,

the microbial activity on the carbonaceous materials can lead to a depletion of available

soil N for plants. This is referred to as nitrogen robbing.

Some potable waters naturally contain ammonia, and others may add it intentionally.

This results in the formation of chloramines, which although weaker disinfectants, are less

likely to react with organic compounds in the water to produce trihalomethanes and other

potential carcinogens. However, nitriﬁers have been found growing as bioﬁlms attached to

the walls of the water distribution pipes in some of these systems, where their relative

resistance to chlorination aids in their survival. Concerns arising from these observations

include the production of nitrite (which inactivates chlorine), organic material (promoting

growth of heterotrophs), and acidity (potentially contributing to corrosion, which might

further increase concentrations of lead in the water).

During anaerobic digestion of sewage sludges, concentrations of ammonium (from

ammoniﬁcation), magnesium, and phosphate will sometimes become high enough to lead

to the precipitation of magnesium ammonium phosphate, or struvite (MgNH

Heavy deposition of this mineral can clog pipes, coat heat exchangers, and otherwise

interfere with digester operation.

Because of the limited metabolic diversity of nitriﬁers, some compounds act as selec-

tive inhibitors, limiting nitriﬁcation while having little or no effect on other organisms.

Allyl thiourea was one compound used early for this purpose in both agriculture (to pre-

vent loss of nitrogen through nitriﬁcation followed by leaching or denitriﬁcation) and the

BOD test (to prevent interference in the measurement of the C-BOD, Section 13.1.3),

but it biodegrades fairly rapidly. Other compounds have also been used, most containing

sulfur and/or nitrogen. The compound now recommended for the C-BOD test is 2-chloro-

6-(trichloromethyl) pyridine, also called N-serve, nitrapyrin, and TCMP. For research

purposes, nitrite oxidation has been inhibited using potassium perchlorate, KClO

, but

this compound may also inhibit nitrate reduction. It is also possible that a compound

entering a wastewater treatment plant may interfere with nitriﬁcation without having a

noticeable effect on other treatment processes.

Chlorine Dose

Chlorine Residual

combined chlorine

free chlorine

chlorine

demand

no demand

Figure 13.23 Breakpoint chlorination.

428

MICROBIAL TRANSFORMATIONS

13.3 SULFUR

The biogeochemical cycling of sulfur involves substantial abiotic chemical as well as the

biochemical conversions. There are ﬁve sulfur oxidation states of appreciable biochemical

importance in the environment. With an atomic number of 16, sulfur’s outer orbital shell

has six electrons. By either depleting or ﬁlling this shell, therefore, sulfur may reach

extreme oxidation states of 2 (sulﬁde, S

2

) and þ6 (sulfate, SO

2

) (Figure 13.24),

as well as residing at 0 (elemental sulfur, S

), þ2 (thiosulfate, S

2

), and þ4 (sulﬁte,

2

). (Note: In pyrite, FeS

, the sulfur has an average oxidation state of 1, although

this may result from a combination of S

2

and S

As with reduced nitrogen in the form of ammonia, reduced sulﬁde can also be volatile

(as hydrogen sulﬁde, H

S). However, these two gases have opposite responses to pH.

Whereas ammonia gas is a weak base and tends to form at higher pH levels (pK

9:3), hydrogen sulﬁde is a weak acid (pK

¼ 7:2 for ﬁrst H

, 11.9 for the second).

Thus, it tends to be nonionized, and hence volatile, only at neutral or lower pH. Reduced

sulfur in alkaline solutions tends to remain ionized, mostly as HS



(or S

2

at very high

pH). Sulﬁde also tends to react with metals to form insoluble precipitates. Ferrous sulﬁde

(FeS), in particular, gives many anaerobic sediments and bioﬁlms their black color.

The organic sulfur of the amino acids methionine and cysteine (Table 3.6) is also at an

oxidation state of 2 and thus has two bonds with adjacent atoms (Section 3.2). One of

these is to a carbon atom, but the other may be to a hydrogen (sulfhydryl bond) or to the

sulfur (disulﬁde bond) of a second amino acid, an important factor in the three-dimen-

sional conﬁgurations of proteins.

In comparison to some of the other elements, which are considerably more limiting,

sulfur tends to be readily available in most environments. Fresh water usually contains

at least 10 mg/L of sulfate, and sometimes much more (especially near the coast), and

sewage usually has 30 mg/L more sulfate than the drinking water from which it is

derived. The concentration in seawater is 2700 mg/L. The limit for sulfate in drinkin g

water is 250 mg/L, but this is a secondary standard (protecting public welfare rather than

health) because of taste and a laxative effect.

The reactions of the sulfur cycle are depicted schematically in Figure 13.25. As with

the nitrogen cycle, some of the steps are carried out only by prokaryotes. Bacteria and

S HH

−2

Sulfate

−

Hydrogen

H S

Sulfide

- -

SULFUR

-- --

Figure 13.24 Sulfur oxidation state extremes.

SULFUR 429

archaea associated with some of the nonassimilative sulfur reductions (use as an electron

acceptor) and oxidations (use as an energy source) are listed in Table 13.5.

Also as with nitrogen, mineralization of organic sulfur does not involve oxidation or

reduction of the sulfur, which is released as sulﬁde. However, assimilation of sulﬁde is

uncommon, largely because of its toxicity. Instead, most organisms utilize sulfate (requir-

ing a reduction) and/or organic sulfur.

13.3.1 Sulfur Reduction

Some bacteria and archaea, including chemoorganotrophs and chemolithotrophs, use sul-

fate or other oxidized forms of sulfur as an electron acceptor for anaerobic respiration,

producing further reduced int ermediates (sulﬁte, thiosulfate, elemental sulfur) or fully

reduced sulﬁde. In addition, many organisms can assimilate sulfate through a reductive

process.

Dissimilatory Sulfate and Sulfur Reduction The complete reduction of sulfate to

hydrogen sulﬁde, during its use as an electron acceptor for anaerobic respiration, can

be achieved by specialized but common and widespread groups of strictly anaerobic bac-

teria, referred to functionally as sulfate-reducing bacteria (SRBs). The best known of

these are Desulfovibrio, a proteobacteria, and Desulfotomaculum, a gram-positive endo-

sporeformer, although other genera are now recognized as well. Interestingly, one archaea,

Archaeoglobus, is also known that can carry out this reaction. Some other anaerobic bac-

teria and archaea can reduce elemental sulfur to sulﬁde.

It was initially believed that SRBs could only utilize a few fermentation products, such

as H

, lacta te, and pyruvate. However, it is now realized that some can utilize acetate and

Thio-

sulfate

Sulfate

Sulfite

-2

+5+4

-1 +6

-2

+5+4

-1 +6

-2

+5+4

-1 +6

-2

+5+4

-1 +6

Elemental sulfur oxidizersSulfide Oxidizers

Sulfur Oxidation States

Thiosulfate

reducers

Sulfide generators

Hydrogen

Sulfide

Sulfur

Organic

Sulfur

Sulfite reducers

Sulfate reducers

Sulfate Assimilation

mineralization

Figure 13.25 Biochemical sulfur transformations.

430

MICROBIAL TRANSFORMATIONS

other short- and long-cha in fatty acids, ethanol and other alcohols, benzoate (which has

an aromatic ring), and hexadecane (a hydrocarbon), among other compounds.

Sulfate reduction does not involve the electron transport chain utilized by aerobes and

denitriﬁers. In fact, unlike those reactions, it requires an initial activation of the sulfur

with adenosine triphosphate (ATP) and thus involves an initial expenditure of energy.

The sulfate within the adenosine phosphosulfate (APS) formed is then reduced (with

the addition of two electrons) to sulﬁte. Sulﬁte reductase then catalyzes the sequential

loss of three more electron pairs, yielding H

S, which is released to the environment.

2

þ ATP ! APS þ pyrophosphate (pyrophosphate is two phospha tes joined )

APS þ 2e



! AM P þ SO

2

þ 6e



! H

Assimilatory Sulfate Reduction The assimilatory reduction of sulfate can be carried out

by most bacteria, archaea, fungi , algae, and plants, while many protozoa and animals

require organic sulfur. This reduction also starts with the production of APS, but a second

ATP is then used to attach another phosphate , forming phosphoadenosine 5

-phosphosul-

fate (PAPS). The sulfur is then reduced to sulﬁte, followed by further reduction to sulﬁde.

To avoid toxicity, as the sulﬁde is formed it is quickly combined to form an organic sulfur

compound such as cysteine.

Sulfur Disproportionation Some SRBs can simultaneously oxidize and reduce an inter-

mediate sulfur compound such as sulﬁte or thiosulfate to provide energy. For example,

TABLE 13.5 Examples of Prokaryotes Able to Oxidize or Reduce (Dissimilatory)

Inorganic Sulfur

Forms of Sulfur

Genus Kingdom: Class Substrate

Reduce Desulfovibrio d-Proteobacteria SO

2

Desulfotomaculum Firmicutes: Clostridia SO

2

Thermodesulfobacterium Aquaﬁcae SO

2

Pyrolobus Crenarchaeota: Thermoprotei S

2

Pyrodictium Crenarchaeota: Thermoprotei S

Thermoplasma Euryarchaeota: Thermoplasma S

Archaeoglobus Euryarchaeota: Thermococci SO

2

Thermococcus, Pyrococcus Euryarchaeota: Thermococci S

Oxidize Chloroﬂexus Thermomicrobia: Chloroﬂexi H

(green nonsulfur)

Chlorobium Chlorobia (green sulfur) H

S, S

Rhodospirillum a-Proteobacteria (purple nonsulfur) H

Chromatium g-Proteobacteria (purple sulfur) H

S, S

Thiobacillus b-Proteobacteria H

S, S, S

2

Beggiatoa, Thiothrix g-Proteobacteria H

S; S; S

2

Aquafex Aquaﬁcae S; S

2

Sulfolobus Crenarchaeota: Sulfolobi H

S; S

Ferroglobus Euryarchaeota: Thermococci H

SULFUR 431

three molecules of sulﬁte are oxidized to sulfate at the same time that one molecule is

reduced to hydrogen sulﬁde.

! 3H

þ H

13.3.2 Sulfur Oxidation

A wide range of bacteria and archaea are able to oxidize reduced sulfur, including photo-

trophs and lithotrophs (Table 13.5). Thiobacillus, for example, is an aerobic or denitrify-

ing (one species) lithotroph that can grow on sulﬁdes. It is of particular interest because of

the acid tolerance of several species; one has a pH optimum of 2! This is particularly use-

ful to the organism, as the product of complete sulﬁde oxidation (sulfate) is actually sul-

furic acid.

Beggiatoa, a ﬁlamentous proteobacteria, has a neutral pH range but is of interest

because of its gliding motility. This allows it to move so that it can stay right at the bound-

ary between an anaerobic environment (where the sulﬁde is formed) and the aerobic

one from which it gets oxygen. When growing on sulﬁde, it deposit s, internally, elemental

sulfur that is readily visible as yellow granules under the microscope (see Figure 10.22).

These may serve as a substrate reserve to be used if sulﬁde is depleted. However, they

may also have the beneﬁt of minimizing sulfuric acid production.

There are also a number of phototrophic bacteria that oxidize reduced sulfur (Table 10.4).

The green and purple nonsulfur bacteria oxidize hydrogen sulﬁde but do not use or deposit

elemental sulfur. The green and purple sulfur bacteria, on the other hand, utilize both H

and elemental sulfur, which they also deposit (externally for the green, internally for the

purple).

13.3.3 Sulfur in Environmental Engineering and Science

Sulfur plays an important role in several areas of concern to environmental engineer s and

scientists, from corrosion to odors. Its important contribution to acid mine drainage is dis-

cussed below as part of the iron cycle (Section 13.4.3). Sulfur oxides are also a major

component of acid rain, or acid deposition (see Section 15.7), but this stems mainly

from combustion rather than microbial transformations.

Corrosion Water reacts spontaneously with iron (Fe

) to produce a thin layer of hydro-

gen, H

, and ferrous hydroxide, Fe(OH)

. Still, it was originally thought that iron and steel

would not corrode under anaerobic conditions, since no oxygen would be present to

oxidize the H

and thereby allow the reaction to continue. However, if sulfate is present,

sulfate-reducing bacteria can utilize the H

, perpetuating the reaction and leading to exten-

sive deterioration:

4Fe

þ 4H

O þ SO

2

! FeS þ 3FeðOHÞ

þ 2OH



ð13:20Þ

The microbial reactions of the sulfur cycle are also responsible for the crown corro-

sion of sewers, particularly those made of concrete. Unlike water distribution pipes, which

are pressurized, ﬂow in sewers is usually by gravity, with the pipes often less than half

full. During periods of low ﬂow, wastewater velocities may drop to the point where orga-

nic matter deposits on the bottom. This is particularly common in ﬂat areas, such as shore

432 MICROBIAL TRANSFORMATIONS

communities built on the coastal plain. After sufﬁcient time, these bottom deposits and the

wastewater itself may become septic, or go anaerobic, and if sufﬁcient sulfate is present

(as is also typical in shore communities), hydrogen sulﬁde will be formed by sulfate-

reducing bacteria. A portion of this H

S volatilizes from the liquid phase and is then

absorbed in the moist bioﬁlm at the crown (top) of the sewer. Here, under aerobic condi-

tions, it can be reoxidized by bacteria such as Thiobacillus to sulfate, in the form of sul-

furic acid (H

), which then reacts with and dissolves the concrete. In fact, one of the

sulﬁde-oxidizing bacteria involved is called T. concretivorus (‘‘concrete eater’’). Prior to

an understanding of this process, it seemed mysterious that sewers corroded at the crown

rather than at the bottom.

Odors Decomposition of proteins can lead to the release of mercaptans, such as ethyl

mercaptan (C

SH), from sulfur-containing amino acids. These volatile organic sulfur

compounds are highly odorous. Both protein decomposition and sulfate reduction als o

lead to the production of H

S, with its penetrating rotten egg odor. Thus, very strong

and unpleasant odors can be released from a variety of ‘‘natural’’ anaerobic processes,

including decomposition of vegetative debris in salt marshes and piles of manure. Odor

problems from sulﬁdes and mercaptans (as well as amines, organic acids, and other com-

pounds) are also a major area of complaint for a variety of waste treatment facilities,

including pumping stations on sewer lines, slud ge tanks, composting facilities, land appli-

cation sites, and landﬁlls. In addition to aesthetic considerations, odors also appear to be

associated with health effects in some people. Thus, control of odors is a major expense at

some sites. Sulﬁdes and mercaptans can be removed from gas streams by alkaline scrub-

bers (since at high pH the molecules ionize and are then nonvolatile), chemical oxidation

(e.g., with chlorine), biological oxidation (e.g., in bioﬁlters), or reaction with metals (e.g.,

iron). Alternatively, the formation or release of odorous compounds can be minimized by

controlling oxygen levels, maintaining alkaline pH, and adding iron (to bind sulﬁdes) or

nitrate (leading to denitriﬁcation instead of sulfate reduction).

Toxicity Hydrogen sulﬁde may be highly toxic, in both aquatic and soil systems, to both

prokaryotes and eukaryotes. Once inside the cell, it binds rapidly to metals, inactivating

cytochromes and other critical cell constituents. Low concentrations of H

S in air are

readily detectable by the odor. However, at higher concentrations it deadens the olfactory

cells and can no longer be smelled. The buildup of hydrogen sulﬁde in conﬁned spaces is

one of the reasons (others include lack of oxygen and buildup of combustible gases) that

entry into such areas without proper precautions is so dangerous, potentially resulting in

death. Accidents involving H

S frequently produce multiple fatalities, as would-be res-

cuers rush into contaminated spaces, only to become victims themselves. The gas is so

toxic that even professional responders have been felled by gas leaking around their

gas masks.

Wastewater Treatment The sulﬁde present in wastewater can be oxidized to sulfate

under aerobic conditions. Usually, this is a minor part of the secondary treatment process.

However, some wastewaters may contain considerable amounts of sulﬁde, either from

industrial inputs or from extensive sulfate reduction due to anaerobic conditions in the

sewer lines or in earlier treatment steps. Under these conditions the sulﬁde may represent

a considerable oxygen demand and in fact would be measured as C-BOD in standard

tests. In activated sludge treatment (Section 16 .1.3) under such conditions, ﬁlamentous

SULFUR 433

sulﬁde-oxidizing bacteria such as Thiothrix may grow sufﬁciently to interfere with the

settling that is an essential part of the process.

In another secondary treatment process involving rotating biological contactors (RBCs;

Section 16.1.2), an anaerobic bioﬁlm layer may develop that leads to H

S production. The

motile ﬁlamentous sulﬁde-oxidizing bacteria Beggiatoa may then grow at the interface

between this region and the overlying aerobic zone. Growths of Beggiatoa under these

conditions are sometimes so massive that they lead to the physical collapse of the system

(e.g., shaft failure).

Methanogenic anaerobic treatment systems are commonly used for wastewater sludges

(anaerobic digestion, Section 16.2.1) and occasionally, for high-strength wastewaters.

However, if too much H

S is produced (from amino acid mineralization or sulfate reduc-

tion), it can be toxic to the methanogens. One control measure is to add iron, which binds

with the sulﬁde as FeS.

Sulﬁde can also interfere with disinfection. Active chlorine is rapidly consumed by

reaction with sulﬁdes and other reduced sulfur compounds.

Example 13.8 A wastewater contains 2 mg/L of hydrogen sulﬁde. How much chlorine

will this consume?

Answer Although the active form of chlorine in water is usually hypochlorous acid,

HOCl, or chloramines, NH

Cl (especially in wastewater), by convention (since 1 mol of

either comes from 1 mol of molecular chlorine), chlorination calculations are based on the

equivalent amount of Cl

(molecular weight ¼ 71):

S þ 4Cl

þ 4H

O ! SO

2

þ 8Cl



þ 10H

chlorine consumed ¼ 2mg=LH

1 mol H

34 g H



4 mol Cl

1 mol H



71 g Cl

1 mol Cl



¼ 2ð8:35 mg Cl

=mg H

SÞ¼17 mg=LCl

Dimethyl Sulﬁde The biogenic production, fate, and impact of dimethyl sulﬁde (DMS),

SCH

, in remote oceans has only recently come under investigation. This reduced

(oxidation state ¼2) volatile compound is considered to be the most abundant form of

organic sulfur in nature and the largest source of sulfur in the atmosp here. The environ-

mental signiﬁcance of DMS stems from its eventual contribution to the formation of

atmospheric aerosols and cloud-forming particles that can impose a large-scale effect

on the incidence of solar radiation over remote ocean areas.

Metabolic production of DMS has been estimated to amount to 4:5  10

metric tons

per year. It results from the transformation of dimethylsulfoni um propionate (DMSP),

which is used by various marine algae, such as dinoﬂagellates, as a means of controlling

their osmotic pressure. Bacteria metabo lize released DMSP as a carbon and energy

source, forming DMS as one of the end products. DMS then can be released into the

atmosphere (Figure 13.26), where it undergoes photochemical oxidation to methane sul-

fonic acid (CH

H), sulfur dioxide (SO

), and non-sea-salt sulfate (as opposed to aero-

solized sulfate from ocean spray).

−

DMSP

434 MICROBIAL TRANSFORMATIONS

These aerosols themselves, along with the clouds that form when they serve as cloud

condensation nuclei, affect the sola r radiation reaching the ocean surface through scatter-

ing of incoming light and direct shading. In addition to the effects on the global radiation

(and heat) balance, it may be that large-scale oceanic algal blooms promote atmospheric

shading indirectly, thereby decreasing their own activity.

A variety of chemoorganotrophic and chemolithotrophic bacteria are able to oxidize

DMS remaining in the water phase, producing dimethyl sulfoxide (DMSO), which is

also frequently generated as a waste contaminant in paper manufacturing operations. Bac-

teria such as Hyphomicrobium are then capable of using DMSO aerobically as a sole

source of carbon, energy, and sulfur. Alternatively, DMSO can be reduced under anaero-

bic conditions by organisms utilizing it as an electron acceptor, produc ing DMS again.

DMSO

13.4 IRON

In addition to its metallic elemental form (Fe

), iron has only the þ2 ferrous, Fe(II), and

þ3 ferric, Fe(III), oxidation states, separated by a one-electron difference. Most iron

cycling involves oxidations and reductions between these two states, via both abiotic

and biochemical mechanisms.

Iron is one of the most abundant elements on Earth, but it is conﬁned primarily to the

lithosphere (soil averages about 7% Fe). It is nonvolatile, and its concentration in air is

+3+2

−10

−1− 2+5

DMS

Marine

Algae

with

DMSP

and

cleavage

DMS release

DMS Photochemical oxidation

DMS

volatilization

and shading by

and cloud

Light reflecting

aerosol particles

condensation nuclei

Sulfur Oxidation State

Methane

sulfonic

acid

Sulfur

NSS

Sulfate

dioxide

Figure 13.26 Dimethyl sulﬁde reactions over remote ocean waters. (DMSP, dimethylsulfonium

propionate).

IRON 435

virtually zero except for the small amounts present in airborne soil particles. In water, the

solubility of ferric iron is low (<0.02 mg/L at pH 6), except under very acidic conditions

or in some organic complexes. Ferrous iron is more soluble (about 20 mg/L at pH 6, but

depending strongly on alkalinity). However, under the anaerobic conditions in which it is

usually found, it is likely to form a precipitate with sulﬁde. Although iron is an essential

element, it typically is present in organisms on ly at low concentrations (e.g., 0.2% of the

dry weight of E. coli, Table 11.1). Thus, although iron is abundant and only small amounts

are needed, it still may be available only at levels that limit growth in some environments,

especially the open ocean.

13.4.1 Iron Reduction

In the absence of oxygen and nitrate, ferric iron can be used for respiration by a variety of

aerobic bacteria. In the process, the Fe(III) is reduced to Fe(II). Both organotrophic and

lithotrophic iron reducers are known. The process seems to be carried out by many of the

same organisms that can reduce nitrate.

There are a number of natural oxygen-deﬁcient conditions under which reduction

might take place. These include anoxic bioﬁlm layers, lake bottoms and sediments,

water-saturated soils, groundwa ters, and bogs. Reduction is a principal mea ns of solubi-

lizing iron-bearing solids. Iron reduction and resolubilization also occur in landﬁlls and

anaerobic digesters. Where sulﬁdes are present, FeS will form, precipitating the iron while

helping to limit odor and toxicity problems associated with H

13.4.2 Iron Oxidation

A diverse assortment of aerobic bacteria and archaea can oxidize ferrous iron, apparently

using it as an energy source. Because only a small amount of energy is available from this

transformation, large amounts of iron must be processed to support their growth.

At neutral pH values, ferrous iron is unstable under aerobic conditions and will be oxi-

dized rapidly and spontaneously (abiotically) to the þ3 form. Thus, microorganisms

utilizing iron at such pH values must be located at the interface between aerobic and anae-

robic zones, depending on this specialized niche so that they may capture this energy

before it is lost. On the other hand, at low pH, ferrous iron is sufﬁciently stable that micro-

organisms can utilize it in aerobic systems.

Thus, iron oxidizers can be subdivided into two groups based on their pH preferences.

The acidophiles include several Thiobacillus, Leptospirillum (a xenobacteria), and Sulfo-

lobus (an archaea). Many of the same organisms can also oxidize reduced sulfur, an

advantage since ferrous iron and sulﬁdes often occur together. T. ferrooxidans, an auto-

troph, is the best known; it has a pH range of 0.5 to 6, with an optimum at 2! It utilizes a

copper-containing protein, rusticyanin, for transferring the electron from iron during its

oxidation (Figure 13.27). The amount of energy available from the oxidation is too

small to use directly for ATP synthesis; instead, the proton gradient is utilized for this

purpose.

Gallionella (Fig ure 13.28) grows at neutral pH, usually at a point where anaerobic

groundwater comes in contact with oxygen (such as in wells). It is commonly heavily

coated with ferric hydroxide, Fe(OH)

. Sphaerotilus and Leptothrix also deposit oxidized

iron on their sheath, but it is not certain that they are able to capture the energy from iron-

oxidation. Other iron, oxidizing bacteria include Planctomyces and Hyphomicrobium;

436 MICROBIAL TRANSFORMATIONS

interestingly, these bacteria both have stalks, although they are not related phylogeneti-

cally. Ferroglobus, an archaea, is also of interest in that it oxidizes Fe(II) to Fe(III)

under anoxic conditions, using nitrate as the electron acceptor.

13.4.3 Iron in Environmental Engineering and Science

Iron cycling plays a role in a number of issues of relevance to environmental engineers

and scientists. These include problems of acidiﬁcation, particularly acid mine drainage,

and drinking water.

ATP

Periplasm

Cytoplasm

Membrane

Rusticyanin

Cytochrome a

Cytochrome c

Proton Gradient

Energy Source

Proton Gradient

Energy Source

Elevated H

at low pH conditions

Cytoplasmic H

sink

(more neutral

ATPsynthase

Figure 13.27 Energetic proton gradient for acidophilic iron-oxidizing chemolithotrophs.

Figure 13.28 Gallionella-type bacteria causing iron precipitation in wells.

IRON 437