Aerobic sulfate reduction

Canfield D. E. and Des Marais D. J. (1991) Aerobic sulfate reduction in microbial mats. Science 251, 1471-1473. 

Aerobic respiration Denitrification Mn(IV) reduction Fe(III) reduction Sulfate reduction Methane fermentation Nitrogen fixation... 

Fig. 22.7. Thermodynamic driving forces for various anaerobic (top) and aerobic (bottom) microbial metabolisms during mixing of a subsea hydrothermal fluid with seawater, as a function of temperature. Since the driving force is the negative free energy change of reaction, metabolisms with positive drives are favored thermodynamically those with negative drives cannot proceed. The drive for sulfide oxidation is the mirror image of that for hydrogentrophic sulfate reduction, since in the calculation 02(aq) and H2(aq) are in equilibrium.

While the above discussion describes testing of aerobic microbial activity, the same scenario is applicable for anaerobic bioreactions. The primary difference is the analytical parameter. The uptake of carbon dioxide, nitrate degradation, sulfate reduction, or iron reduction may be monitored instead of oxygen utilization. 

Janssen, G. M. C. M., and Temminghoff, E. J. M., 2004, In situ metal precipitation in a zinc-contaminated, aerobic sandy aquifer by means of biological sulfate reduction. Environ. Sci. Technol. 38 4002-4011. 

In sediments that lie in coastal waters, organic carbon levels are high enough to support denitrification, iron respiration, sulfate reduction and methanogenesis. As shown in the idealized profile presented in Figure 12.3b, the depth of O2 penetration in organic-rich sediments is typically so shallow as to make the zones of aerobic respiration. 

Fox, P. Venkatasubbiah, V. Coupled anaerobic/aerobic treatment of high-sulfate wastewater with sulfate reduction and biological sulfide oxidation. Water Sci. Technol. 1996, 34 (5/6), 359-366. 

Measured rates of sulfate reduction can be sustained only if rapid reoxidation of reduced S to sulfate occurs. A variety of mechanisms for oxidation of reduced S under aerobic and anaerobic conditions are known. Existing measurements of sulfide oxidation under aerobic conditions suggest that each known pathway is rapid enough to resupply the sulfate required for sulfate reduction if sulfate is the major end product of the oxidation (Table IV). Clearly, different pathways will be important in different lakes, depending on the depth of the anoxic zone and the availability of light. All measurements of sulfate reduction in intact cores point to the importance of anaerobic reoxidation of sulfide. Little is known about anaerobic oxidation of sulfide in fresh waters. There are no measurements of rates of different pathways, and it is not yet clear whether iron or manganese oxides are the primary electron acceptors. 

Substances in the bulk solution diffuse into the biofilm, where they are consumed (such as oxygen, point 1 in Figure 6) or recycled (such as sulfate through stepwise reoxidation of H2S from sulfate reduction, point 5). Within the biofilm, very steep gradients exist for oxygen or hydrogen sulfide and also for ferrous iron from reductive dissolution of ferric oxides. These gradients result from the coexistence of anaerobic and aerobic metabolisms such as aerobic respiration (point 1), reduction of ferric oxides (point 3), and sulfate... 

Prior to the onset of sulfate reduction extensive organic matter degradation can occur by bacterially-mediated oxygen reduction (see Chapter 4). The influence of benthic bacterial activity under aerobic conditions on carbonate mineral dissolution has been nicely demonstrated by Moulin et al. (1985) for pore waters from sediments of the Gulf of Calvi, Corsica. Under aerobic conditions, the oxidation of organic matter may be written ... 

Parkes, R.J., and Buckingham, WJ. (1986) The flow of organic carbon through aerobic respiration and sulfate reduction. In Proceedings of 4th International Symposium on Microbial Ecology (Megusar, F., and Gantar, M., eds.), pp. 617-624, Ljubljana, Slovenia. 

There are two fermentative processes that at first appear to be quite similar to oxygen and nitrate-dependent respirations the reduction of C02 to methane and of sulfate to sulfide. However, on closer examination, it is clear that they bear little resemblance to the process of denitrification. In the first place, the reduction of C02 and of sulfate is carried out by strict anaerobes, whereas nitrate reduction is carried out by aerobes only if oxygen is unavailable. Equally important, nitrate respirers contain a true respiratory chain sulfate and C02 reducers do not. Furthermore, the energetics of these processes are very different. Whereas the free energy changes of 02 and nitrate reduction are about the same, the values are much lower for C02 and sulfate reduction. In fact, the values are so low that the formation of one ATP per H2 or NADH oxidized cannot be expected. Consequently, not all the reduction steps in methane and sulfide formation can be coupled to ATP synthesis. Only the reduction of one or two intermediates may yield ATP by electron transport phosphorylation, and the ATP gain is therefore small, as is typical of fermentative reactions. 

In contrast to the specialized dissimilatory sulfate reducers, many organisms (humans as well) are capable of assimilatory sulfate reduction. This process, which requires chemical energy in the form of ATP and a series of transfer reactions, can occur anaerobically and aerobically. It produces low concentrations of hydrogen sulfide that are immediately incorporated into organic compounds. Many microbes, plants, and animals have such a metabolic ability. 

In recent years various workers f1-7J have successfully developed models based on the mathematics of diffusion (8) to describe vertical profiles of selected chemical parameters in marine sediments dominated by sulfate reduction. Several papers 9, 10) have also proposed models for nitrogen diagenesis in the upper aerobic zone of such sediments. Most of these models, however, deal with only one or two relatively well behaved parameters, such as SO5" or CO2, which do not interact strongly with other components of the sediment besides organic matter. A truly comprehensive model for such sediment should deal simultaneously with all of the major chemical parameters of the system and ideally should be formulated as an initial value prob-... 

Figure 1. Simulations of sediment chemistry showing effects of differences in pH at the top of the zone of sulfate reduction. Values for model constants are listed in Table II unless otherwise on figure. P and ANP are parameters of the aerobic model that control the depth and pH at...

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