Anoxic respiration

Respiration, in which organic compounds undergo catabolism that requires molecular oxygen (aerobic or oxic respiration) or that occurs in the absence of molecular oxygen (anaerobic or anoxic respiration). Aerobic respiration uses the Krebs cycle to obtain energy from the reaction... 

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. 

The balance between relative rates of aerobic respiration and water movement were considered in Section 4.3.4. We saw that a subsurfece concentration minimum, the oxygen minimum zone (OMZ), is a common characteristic of vertical profiles of dissolved oxygen and is produced by in situ respiration. Waters with O2 concentrations less than 2.0 ppm are termed hypoxic The term anoxic is applied to conditions when O2 is absent. (Some oceanographers use the term suboxic to refer to conditions where O2 concentrations fall below 0.2 ppm but are still detectable.) As illustrated by Figure 4.21b, this water column is hypoxic in the OMZ. The dissolved oxygen concentrations are presented as % saturations in Figure 4.21c. With the exception of the mixed layer, the water column is undersaturated with respect to dissolved oxygen with the most intense undersaturations present in mid-depths. Surface supersaturations are the result of O2 input from photosynthesis and bubble injection. 

A particularly important consequence of bioirrigation and bioturbation is the introduction of relatively 02-rich bottom water into the sediments. This enhancement in O2 supply is analogous to the aeration of soil by earthworms. Bioturbation can occur as deeply as 1 m below the sediment surface, but is most intense in the top 10 cm. The depth of O2 penetration is also strongly influenced by the flux of sedimenting POM. High accumulation rates of organic-rich particles can fuel bacterially mediated aerobic respiration supporting rates of O2 removal that exceed the benthic animals abilities to reaerate the sediments. In this case, anoxic conditions result. Since animals require O2, bioturbation does not occur in anoxic sediments. Thus, the effects of bioturbation are limited to the oxic portion of the sediments. 

In the anoxic zone, heterotrophic respiration of particulate Mn02 and Fc203 or FeOOH causes manganese and iron to be reduced to Mn (aq) and Fe (aq). As dissolved ions, these trace metals diffuse through the pore waters. The ions that diffuse upwards will reenter the oxic zone, where they react with O2 to reform the oxyhydroxides. This produces a metal-enriched layer that lies just above the redox... 

Figure 6.3 Aerenchyma development and changes in respiration rate along the length of maize roots grown in anoxic media (adapted from Armstrong et al., 1991a). Reproduced by permission of Backhuys publishers...

To vahdate mineralization of toluene to CO under anoxic quinone and humus-respiring conditions, Cervantes et al. (2001) performed additional experiments using emiched phosphate-buffered basal sediments from Amsterdam petroleum harbor. After two weeks of incubation, 85% of added C-labeled toluene was observed as CO. Emiched sediment converted C-labeled toluene to in media supplemented with AQDS or with humic acid (Fig. 16.34A). There was negligible recovery of in the endogenous and sterile controls. The conversion of C-labeled toluene to was coupled to an increase in electrons recovered as AH QDS or as reduced humus (Fig. 16.34B). However, there was no toluene reduction in autoclaved sediments. These results indicate that humic substances... 

Sulfate ion is the major electron acceptor for respiration in anoxic marine sediments and may account for 25 % of the total sediment respiration in near shore sediments, 03 to 3 g C nr2 day 1. In salt marsh sediments, where total... 

Seventeen genera of facultative anaerobic bacteria (e.g., Pseudomonas and Bacillus) can perform denitrification under anaerobic or low-oxygen conditions, where they use NO3- as an electron acceptor during anaerobic respiration (Jaffe, 2000). In fact, in many estuaries, denitrification is limited by the availability of NC>3 (Koike and Sprensen, 1988 Cornwell et al., 1999). Sources of NC>3 and NC>2 for denitrification are from diffusive inputs from the overlying water column and nitrification in the sediments (Jenkins and Kemp, 1984). The activity of other bacterial processes under anoxic conditions has been shown to affect the activity of denitrifying bacteria. For example, SO42- reduction occurs in anoxic sediments whereby SC>42 is reduced to sulfide (Morse et al., 1992)—more... 

In oxic conditions nitrate is produced by respiration and reaches a maximum (3-10 iM) at ae = 15.30-15.50 kgm 3. Below this depth the concentrations of nitrate decrease sharply with vertical gradients 0.2-0.5 p,Mm4. After oxygen, nitrate is the second most abundant oxidizing agent in the oxic-anoxic interface. Nitrate disappears in the vicinity of ae = 15.90-16.00 kg m 3. 

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