Oxygen sulfate-reducing bacteria, oxidative

Hydrogen sulfide is a gas typically found in well water that is devoid of oxygen. It is the result of sulfate-reducing bacteria oxidizing organic matter, which releases hydrogen sulfide gas. Hydrogen sulfide is a weak acid whose speciation depends on pH as follows ... 

The CRS AVS ratio reflects the trophic state of a lake (cf. Urban (28), Table V). This observation may be explained by the preceding kinetic considerations. The high organic matter supply in eutrophic lakes leads to an intensive mineralization rate by both iron- and sulfate-reducing bacteria. However, reoxidation of Fe2+ to ferric oxide or of sulfide to sulfate does not take place because an anoxic hypolimnion prevents penetration of oxygen. Therefore FeS can build up, but the sediment becomes depleted with respect to reactive iron. 

Until recently it has been assumed that sulfate-reducing bacteria always required a strictly anaerobic environment. These environments are found in deep coastal-plain areas, oil-field brines, and in black (organic-rich), waterlogged soils and muds associated with rivers, lakes, and swamps. Sulfate reduction has also been observed in local microenvironments such as those created by the decay of a fish buried in otherwise oxidizing sediments (Berner 1971). Contrary to traditional belief, active sulfate reduction has also been observed in the presence of dissolved oxygen in the photosynthetic zone of microbial mats (Canfield and Des Marais 1991). 

In addition, one of the commonly reported major elemental components of the ashes from woods in marine sediments is iron, which has very low concentrations in fresh wood (45). This iron is probably immobilized as pyrite (FeS2) and other reduced iron minerals as a result of sulfide released by sulfate-reducing bacteria (26, 59). These minerals are significant because they indicate in situ degradation and can affect density measurements in wood. However, from the perspective of the conservator, it is more important that when exposed to oxygen, iron-sulfur minerals tend to oxidize and release sulfuric acid (26). For example, Barbour measured a pH of 3.0 in the interstitial waters of a buried alder wood that had been stored for only 6 months at 2 °C (59). At such high acidities the hydrolysis of polysaccharides should be expected (4). Such mineral phases in buried wood from marine waters can have a deleterious effect any time the material is exposed to molecular oxygen. 

It is useful to note that it is necessary to have oxygen and reduced sulfur for Thiobacilli to act on and produce sulfuric acid. The growth of anaerobic sulfate-reducing bacteria in sewage produces H2S, which migrates to the air space at the top of the line, where it is oxidized to sulfuric acid in the water droplets at the crown of the pipe by Thiobacilli. The corrosion problem is because of the dissolution of alkaline mortar by the sulfuric acid produced by the bacterial action, followed by the corrosion of the ductile iron in the sewage pipe (50). 

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