Anoxic conditions

Estuaries exhibit physical and chemical characteristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1 to 10 years for lakes and 10 years for oceans), redox and salinity gradients are often transient, and diurnal variations in nutrient concentrations can be significant. The biological productivity of estuaries is high and this, coupled with accumulation of organic debris within estuary boundaries, often produces anoxic conditions at the sediment-water interface. Thus, in contrast to the relatively constant chemical composition of the... 

Recently, some of the specific faecal bacteria involved in the metabolism of dietary isoflavonoids were isolated (Hur et al., 2000). They have been shown to selectively convert genistin and daidzin to their respective aglycones. One of the isolated bacteria, under anoxic conditions, was further shown to metabolise genistein and daidzein to their respective dihydroxy-genistein and dihydroxy-daidzein. In the case of lignans, enterodiol and enterolactone were shown to be excreted in vivo only in rats harbouring a gut microflora (Rowland et al, 1999). 

Sulfides and disulfides can be produced by bacterial reactions in the marine environment. 2-Dimeth-ylthiopropionic acid is produced by algae and by the marsh grass Spartina alternifolia, and may then be metabolized in sediment slurries under anoxic conditions to dimethyl sulfide (Kiene and Taylor 1988), and by aerobic bacteria to methyl sulfide (Taylor and Gilchrist 1991). Further details are given in Chapter 11, Part 2. Methyl sulfide can also be produced by biological methylation of sulfide itself (HS ). Carbon radicals are not the initial atmospheric products from organic sulfides and disulfides, and the reactions also provide an example in which the rates of reaction with nitrate... 

Aeckersberg F, FA Rainey, F Widdel (1998) Growth, natural relationships, cellular fatty acids and metabolic adaptation of sulfate-reducing bacteria that utilize long-chain alkanes under anoxic conditions. Arch Microbiol 170 361-369. 

Rabus R, R Nordhaus, W Ludwig, F Widdel (1993) Complete oxidation of toluene under strictly anoxic conditions by a new sulfate-reducing bacterium. Appl Environ Microbiol 59 1444-1451. 

Under anoxic conditions, TNT can serve as a terminal electron acceptor (Esteve-Nunez et al. 2000), with utilization of the compound as a source of nitrogen. A number of products were formed by oxidation of the methyl group and loss of nitrite to 4-hydroxybenzoate (Esteve-Nunez and Ramos 1998). 

The application of intermittent aeration, i.e., operation with alternate aerobic and anoxic conditions... 

Different effects of formaldehyde on the hydrolysis of urea are reported. On the one hand, Garrido and colleagues,3 applying anoxic conditions, observed that an inhibitory effect started at 50 mg/L formaldehyde and the levels of inhibition were 50% and 90% for concentrations of formaldehyde of 100 mg/L and 300 mg/L, respectively. Similar effects were found by Campos and colleagues,33 working with an anoxic USB, who observed that formaldehyde concentrations in the reactor of 250 to 300 mg/L caused an inhibition of around 53%. This inhibition on the ureolytic activity was also reported by Walker.36 On the other hand, Eiroa and colleagues37 carried out batch assays at different initial urea concentrations from 90 to 370mg/L N-urea in the presence of 430 mg/L formaldehyde. They observed that a complete hydrolysis was achieved and initial urea hydrolysis rates remained constant. 

Nitrification-denitrification reactions can be carried out in suspended growth by separating the reactor into different cells. A typical arrangement would control the first cell under anoxic conditions to carry out the denitrification reactions. These reactions require organic carbon and this... 

It could destroy or poison them. Of course, many anaerobic organisms could and did hide in primitive anoxic conditions till today. 

FIGURE 8.9 Mitochondrial 02 and H2S consumption from non-limiting 02 to anoxic conditions, (a) Isolated mitochondria were exposed to repeated bouts of 12.5 pM H2S until anoxia was achieved, (b) At higher 02 levels, both 02 and H2S consumption events are coincident, but as the 02 levels decline the events become uncoupled and 02 consumption is limited first. The multiphasic kinetics of 02 consumption may result from transient inhibition of cytochrome c oxidase by H2S. Under anoxia, H2S consumption continues at a low level (after [36] reproduced with permission of the Company of Biologists). 

Processes reported in Table 1 are typically anaerobic (AN). In agreement with the observations reported by Wuhrmann et al. [49], azo-dye bioconversion occurs with the standard organism and other facultative or obligatory aerobic bacteria in exclusively anoxic conditions. Different methods can be used to establish the required anaerobic conditions. A common procedure is simply sparging oxygen-free gas... 

The role of cell respiration has been taken into account to interpret the azo-dye conversion by particle-supported biofilm under aerobic conditions [5, 24]. The rapid depletion of oxygen expected/measured as one moves inside the biofilm promotes the establishment of the anoxic conditions needed for azo-dye conversion. 

Azo dye molecules have color due to their azo bond, auxochromes, and system of conjugated double bonds. The azo bond, while resistant to aerobic degradation, can be cleaved under anaerobic or anoxic condition, resulting in decolorization and the production of aromatic amines. Anaerobic reduction of the azo dyes is relatively easy to achieve, but the products have been found to be biorecalcitrant... 

Nitrate and oxygen also may play an important role in determining the rate of azo dyes reduction. Wuhrmann et al. demonstrated that obligate aerobes might actually decolorize azo dye compounds under temporary anoxic conditions. However, high nitrite concentrations in the mixed liquor of activated sludge plants could significantly inhibit dye removal. 

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