Abundance of sulfate reducers

Solid phase chemical analyses included determination of total organic C content, and the distribution of S between iron monosulfides (acid-volatile sulfur or AVS) and pyrite (the difference between total reducible sulfur and AVS). Total organic carbon was measured coulometrically following combustion at 1050°C (7). Acid-volatile sulfur and total reducible sulfur analyses followed the procedure of Canfield et al. (8). A microbiological assay of the abundance of sulfate reducing bacteria was performed according to (9). 

A microbiological assay of the abundance of sulfate reducing bacteria was performed to determine the distribution of colony... 

Figure 4. Pore fluid/sediment chemistries and abundance of sulfate reducers from the various cores.

A record was maintained of the size of each pyrite occurrence. Core 1 shows a gradual increase in the percentage of pyrite particles less than six microns in size whereas core 3 shows a decrease in the percentage of the same size pyrite particles (Table I). Core 2 shows no consistent pattern. The tidal creek with an abundance of sulfate in a reducing environment appears to produce more pyrite of smaller sizes. On the marsh panne, larger pyrite particles are formed in this hypersaline, high alkalinity environment. 

With abundant evidence for sulfate reduction in the hydrosphere, the question arises as to the actual site of the sulfate-reducing activity. This depends upon oxygen input to the system, organic matter concentration, and other factors. Suitable conditions are often encountered at, or just below, the water-sediment interface. Here, the population of sulfate-reducers is highest because of the availability of sulfate and organic matter. In some non-eutrophic lakes a secondary population maximum may arise at depths around 3 m in the sediments (Kuznetsov et al., 1963). Sulfate reduction occurs both within the water column and the sediments of the Black Sea and a number of the lakes examined by Ivanov (1964) and Sorokin (1970). 

Occurrences of elemental sulfur in peat, coal, and petroleum are described in Chapter 6.4. The role of sulfate reducers in these environments is suggested by the fact that fossil fuels formed in marine environments, where sulfate is in abundant supply, have significantly more sulfide and native sulfur than those formed under freshwater conditions. In fact, a general geological feature of native sedimentary sulfur deposits is their location in sulfate-carbonate rocks and proximity to oil-gas-bearing strata and hydrologic zones where sulfate waters mix with chloride brines (Ivanov, 1964). 

Archaean, by 2.7 Ga microbial life was diverse and abundant. However, tradng the record further back in time brings greater uncertainties, but there is good evidence of sulfate reducing bacteria as far back as 3.5 Ga and some evidence for photosynthe-sising cyanobacteria as old as 3.8 Ga in the Isua sediments. 

The potential necessary to protect buried steel is —0.85 V, however, in the presence of sulfates, reducing bacteria a minimum potential of-0.95 V with respect to copper sulfate electrode would be necessary. Approximately 15-100 mA/ft current is needed for protection of bare steel in sluggish water. In rapidly moving water, 1-10 mA/ft for bare steel in a soil would be necessary. Current requirements in various environments can be foxmd abundantly in the literature as well as cathodic protection specifications. For submarine pipeline, a current density of 5 mA/ft is required. 

The energetics depicted in this way are in accord with the microbial ecology observed at deep sea hydrothermal systems (e.g., Kelley el al., 2002 Huber el al., 2003 Schrenk et al, 2003). Sediments and black smoker walls invaded by hydrothermal fluids there contain sparse microbial populations of mostly thermophilic methanogens and sulfate reducers. Abundant populations of mesophilic aerobes dominated by sulfide reducers, in contrast, are found in the open ocean where hydrothermal fluids mix freely with seawater. 

Sulfur fulfills many diverse roles in lakes. As the sixth most abundant element in biomass, it is required as a major nutrient by all organisms. For most algae, S is abundant in the form of sulfate in the water column however, in dilute glacial lakes in Alaska (I) and in some central African lakes (2) low concentrations of sulfate may limit primary production. Sulfur also serves the dual role of electron acceptor for respiration and, in reduced forms, source of energy for chemolithotrophic secondary production. Net sulfate reduction can account for 10-80% of anaerobic carbon oxidation in lakes (3-5), and hence this process is important in carbon and energy flow. Sulfate reduction, whether associated with uptake of sulfate and incorpo-... 

Rooney-Varga, J.N., Devereux, R., Evans, R.S., and Hines, M.E. (1997) Seasonal changes in the relative abundance of uncultivated sulfate-reducing bacteria in salt marsh sediments and in the rhizosphere of Spartina altemiflora. Appl. Environ. Microbiol. 63, 3895-3901. 

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