Sulfur Oxidizing and Sulfur Reducing Bacteria

Although sulfur (S) accounts for <1% of the dry weight of most organisms, sulfur is an essential element for all organisms. Basic compounds that are necessary for all organisms are proteins, and sulfur is found in most proteins. Sulfur also is needed for the production of enzymes and enzyme cofactors such as thiamin and biotin. The sulfur requirement for organisms can be noted in their carbon-to-sulfur ratio. For most bacteria in wastewater treatment processes, the carbon-to-sulfur ratio is 100:1. For methane-forming bacteria, this requirement is higher.

Bacterial cells contain a large number and variety of proteins that fulfill many critical functions including structural and enzymatic roles. Proteins are made up from about 20 different amino acids. Two of these amino acids, cysteine and methionine (Figure 13.1), contain sulfur and are referred to as the sulfur amino acids. Sulfur in cysteine occurs in a thiol group (-SH). The thiol group is known also as the sul-hydral, sulhydryl, or hyrosulfide group. Nearly all proteins contain at least one of the sulfur amino acids.

The linkage of oxidized thiol groups of different cysteine molecules determines the configuration or shape of the protein molecule (Figure 13.2). The shape of the protein molecule is directly relevant to the structural or enzymatic role of the protein molecules. When thiol group reagents or toxicants such as lead (Pb) and mercury (Hg) react with the nonoxidized thiol groups, the activity of many enzymes is destroyed or inhibited.

Sulfur can exist in a number of oxidation states (Table 13.1). All bacteria contain sulfur, and most aerobic bacteria and facultative anaerobic bacteria obtain sulfur from the environment in the most oxidized form, sulfate (SO42-). However, most

FIGURE 13.1 Sulfur amino acids, cysteine and methionine. In addition to amino groups (-NH2) and carboxyl groups (-COOH), the sulfur amino acids contain the thiol group (-SH).

Methionine

FIGURE 13.1 Sulfur amino acids, cysteine and methionine. In addition to amino groups (-NH2) and carboxyl groups (-COOH), the sulfur amino acids contain the thiol group (-SH).

O

-SH-containing amino acid, cysteine amino acid without -SH

FIGURE 13.2 Linkage of thiol (-SH) groups. When the thiol groups (-SH) of sulfur amino acids (1) are ionized and joined together, they change the shape of the proteinaceous molecule (2).

TABLE 13.1 Oxidation States of Sulfur

Oxidation State

Name

Formula

Dithionite Thiosulfate Elemental sulfur Sulfide

Sulfate Dithionate

Sulfite Disulfite organic molecules that contain sulfur and are found in bacterial cells contain sulfur in the reduced form as the thiol group (-SH) or disulfide group (-S-S-). These reduced forms are found in cysteine and methionine, respectively. Therefore, bacteria that use sulfate must be able to reduce sulfate.

The sulfur nutrient for most aerobic bacteria and facultative anaerobic bacteria is sulfate (SO42-), while the sulfur nutrients for bacteria growing under anaerobic conditions include sulfide (S2-), elemental sulfur (S0), thiosulfate (S2O32-), sulfite (SO32-), and the sulfur amino acids.

Sulfate, the most oxidized state of sulfur, is an oxidizing agent and one of the most stable forms of sulfur. Sulfate is the most prevalent anion in natural water and the most abundant form of sulfur in the environment for use by organisms. Sulfite and thiosulfate, lower oxidation states of sulfur, are often found in waterlogged (anaerobic) soils. Sulfite and thiosulfate contribute to malodors associated with these soils.

The sulfur amino acids contain sulfur in the -2 oxidation state. This oxidation state is the same as that found in hydrogen sulfide (H2S) and sulfides. Therefore, if an organism uses an inorganic sulfur compound with an oxidation state greater than -2 for the synthesis of the sulfur amino acids, the sulfur must be reduced to the -2 oxidation state.

There are two metabolism processes for the reduction of sulfate. These processes are assimilatory sulfate reduction and dissimilatory sulfate reduction (Figures 13.3 and 13.4). During assimilatory sulfate reduction, inorganic sulfate (or any inorganic sulfur with an oxidation state greater than -2) is reduced to sulfide. Sulfide is then used to form the sulfur amino acids (Equation 13.1). Most aerobic bacteria use sulfate as their source for the sulfur nutrient. When sulfate is used, the bacteria release very little sulfide to the environment and do not store sulfide within their cells.

Sulfate + 2 electrons ^ Sulfite + 6 electrons ^ Sulfide ^ Sulfur amino acids (13.1)

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