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58<br />

SO4<br />

Galdino Andrade<br />

Assimilatory<br />

sulfate<br />

Reduction<br />

Dissimilatory<br />

Sulfate<br />

reduction<br />

Living organisms<br />

Proteins<br />

Sulphur<br />

aminoacids<br />

H2S<br />

Excretion<br />

Celular death<br />

Proteases<br />

Peptidases<br />

Dissimilatory<br />

Sulfate<br />

Reduction<br />

Fig. 5. The activity of some functional<br />

groups of microorganisms in the sulphur<br />

cycle<br />

phatases where elements such as nitrogen and sulphur-linked carbon mineralised<br />

by microorganisms oxidize the organic carbon compounds to obtain<br />

energy. The heterotrophic soil microorganisms decompose organic sulphur to<br />

form sulphide. In the case of indirect mineralisation, those elements that exist<br />

as sulphate esters are hydrolysed by endo or exoenzymes. This process occurs<br />

by positive feedback or negative control (Sylvia et al. 1998).<br />

The activity of these microorganisms may be aerobic or anaerobic. Anaerobic<br />

microorganisms exist in fairly low numbers in the rhizosphere of <strong>plant</strong>s<br />

which live in non-flooded soils. Bearing in mind that sulphate is fundamental<br />

for <strong>plant</strong> metabolism and that the turnover of organic to inorganic sulphate<br />

implies availability of the nutrient for <strong>plant</strong> growth, the study of these populations<br />

may complement the analysis of functional microorganism groups as<br />

indicators of environmental impact or of biotic fertility indexes in sustainable<br />

agricultural systems or natural areas.

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