Spherical or lemon 28-35 Rod 37

Filaments 28-32

Oval rods or 33

coccoid, packages

Straight to curved rods

Most 25-40 Some 40-65

Bacterial thermophilic

ThermodesulfobacteriumVibroid to rod 65-70

Archaeal thermophilic

Archaeoglobus Coccoid 64-92

Incomplete oxidation of acetate Incomplete oxidation of acetate Complete oxidation of benzoate and fatty acids Incomplete oxidation of lactate; fatty acids may be completely oxidized Incomplete oxidation of lactate and propionate Acetate completely oxidized Acetate completely oxidized Acetate completely oxidized Complete oxidation of fatty acids

Lack desulfoviridin, the usual replacement is P-582

Incomplete oxidation of acetate

Incomplete oxidation of acetate a With permission from Castro et al. (2000).

to growth. In such cases, only certain organic substrates such as lactic and pyruvic acids are utilized as electron donors. These reactions are as follows:

lactic acid, 2 CH3CHOHCOOH + SO4~ ^

The pathway for dissimilatory reduction uses the same activation of SO4~ to APS as found in assimilatory reduction (see Microbial Transformations of Sulfur— Immobilization). The dissimilatory reduction scheme includes the production of HSO3 without the involvement of PAPS, and the other difference is the large production of HS~ without the formation of organic S.

Aside from their obvious importance in the S cycle, sulfate-reducing bacteria are important regulators of a variety of processes in anaerobic upland and wetland soils, including organic matter turnover, biodégradation of chlorinated aromatic pollutants, and mercury methylation.


The decomposition of organic S compounds in poorly drained soils, sediments, manures, and organic wastes can lead to the formation of volatile organic S compounds. These compounds include mercaptans such as methyl mercaptan (CH3SH) and alkyl sulfides such as dimethyl sulfide (CH3SCH3). Volatile S compounds may be of importance because they can inhibit plant growth, nitrification, and other biochemical processes. These compounds are also undesirable atmospheric pollutants causing unpleasant odors and adversely affecting climate.

micronutrients and trace metals

Micronutrients are chemical elements that are required by plants and microorganisms in small amounts. These micronutrients are vital constituents of enzymes or growth hormones. The essential trace elements for plants are iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), boron (B), molybdenum (Mo), and nickel (Ni). Microorganisms and higher animals also require copper (Cu), cobalt (Co), chromium (Cr), selenium (Se), and tin (Sn). Other metal elements such as mercury (Hg), arsenic (As), lead (Pb), and cadmium (Cd) are not essential to organisms, but are found in soils as potentially toxic contaminants. This section will focus on select micronu-trients and nonessential metal elements, which will be referred to collectively as trace metals for simplicity, even though some of these elements (particularly Fe) can exist in soils at much higher than trace concentrations.

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