Under anaerobic conditions, many bacteria reduce the strong electrophilic azo bond in the dye molecule through a non-specific enzymatic effect (Harbel et al., 1991; Solpan and Güven, 2002). The non-specific effect of anaerobic bacteria allows the biodegradation of a variety of textile dyes, making this process more suitable for commercial use. Anaerobic reduction of azo dyes by bacteria appears to be more appropriate for bleaching in wastewater treatment plants (Matthews and McEvoy, 1992). This method offers the following advantages: (1) reactions take place at neutral pH and are extremely non-specific when available with low molecular weight redo, (2) in static cultures, oxygen deficiency is easily achieved Mandatory and facultative anaerobic bacteria can reduce azo dyes, and (3) sewage systems often provide additional carbon sources that typically increase reduction. These carbon sources also facilitate the formation and regeneration of reducing equivalents through their oxidation. Under anaerobic conditions, dechlorination is the removal of one or more chlorine substitutions from the PCB molecule (dechlorination is a minor factor in systems that are primarily aerobic). In anaerobic dechlorination, chlorines are generally more easily removed in the meta and para positions, while orthosubstituted chlorines are the most resistant to anaerobic dechlorination (see figure 10.5). The order and preference of dechlorination depends on the presence or absence of adjacent chlorines and the position of these adjacent chlorines (e.g. in the ortho- or para-position for possible metaposition dechlorination; Bédard and Quensen, 1995). The progeny congener resulting from dechlorination could be a non-aroclor PCB congener.
Under anoxic conditions, DNRA is another reduction pathway that can be catalyzed by many bacterial genera (Cole and Brown, 1980; Cole, 1988; Stevens et al., 1998). Nitrous oxide can be formed by both DNRA and heterotrophic denitrification, while N2 is formed solely by heterotrophic denitrification. In the DNRA signaling pathway, one set of fermentative bacteria is responsible for the formation of NO2− (Cole and Brown, 1980), while another set of fermentative bacteria completes this pathway by reducing NO2− to NH4+, which also releases N2O. No N2O reductase was found in these bacteria (Smith, 1982). Tiedje et al. (1983, 1988) reported that compared to heterotrophic denitrification, which prefers a low C:N ratio medium, DNRA is more likely to occur in environments rich in C but containing little available nitrogen. In environments where NO3− is abundant, competition for C will determine which NO3− reduction process is preferred (Tiedje, 1988). Some studies suggest that DNRA may be an important source of N2O production in soils (Stanford et al., 1975; Caskey and Tiedje, 1979; Silver et al., 2001; Rütting et al., 2011). In tropical forest soils, Silver et al. (2001) that the DNRA accounted for approximately 75% of the turnover of the NO3− pool and that in these systems the rate of DNRA was mainly limited by the availability of NO3− and not by C or O2.
So far, no methodology has been developed to distinguish N2O production between DNRA and heterotrophic denitrification. Complementary file 3: Meiofaunal abundance and community structure in the L`Atalante basin and adjacent deep-sea oxygenated sediments. Comparison: (a) the total abundance of benthic metazoans (expressed as m-2 individuals) in the anoxic sediments of the L`Atalante deep hypersaline anoxic basin (DHAB) and deep-sea oxygenated sediments surrounding the anoxic basin; and (b) the contribution of different taxa present in the anoxic sediments of L`Atalante DHAB and in the deep-sea oxygenated sediments around the anoxic basin (expressed as a percentage). (PDF 24 KB) Metazoans of the deep anoxic hypersaline basin of L`Atalante. (a) Microscopic optical image (LM) of a copepod exuve (stained with Rose Bengal); (b) LM image of a dead nematode (colored with Rose Bengal); (c) LM image of the undescribed species of Spinoloricus (Loricifera; colored in pink Bengal); (d) LM image of the undescribed species of Spinoloricus stained in pink Bengal showing the presence of an egg; (e) LM image of the undescribed species of Rugiloricus (Loricifera, colored in pink Bengal) with an ovule; (f) LM image of the undescribed species of pliciloricus (loricifera, not coloured in pink Bengali); (g) LM image of Mauserexuvium of the undescribed species of Spinoloricus. Note the strong coloration of the internal structures in colored loricifera (c and d) compared to the pale coloration of the copepod and nematode (a, b). The loriciferan shown in Figure 1e was washed several times to highlight the presence of the inner egg. Scale beam, 50 μm.
Several other classes of xenobiotics are also effectively reduced by the P450 monooxygenase system under anaerobic conditions. These include tertiary amine N-oxides (converted to tertiary amines), hydroxylamines (primary amines) and hydrazo derivatives (primary amines). Several single-celled organisms (prokaryotes and protozoa) can live in permanent anoxic conditions. Although some metazoans may temporarily survive in the absence of oxygen, it is thought that multicellular organisms cannot go their entire life cycle without free oxygen. The deep sea includes some of the most extreme ecosystems on the planet, such as the deep hypersaline anoxic basins of the Mediterranean. These are permanent anoxic systems that are inhabited by enormous and partly unexplored microbial biodiversity. In all sediments collected from the inner part of the anoxic basin, we found specimens of three animal phyla: Nematoda, Arthropoda (only Copepoda) and Loricifera. The presence of metazoan meiofauna under permanent anoxic conditions has also been reported earlier in deep seabed sediments of the Black Sea, although these records have been interpreted as the result of a shower of corpses that sank into the anoxic zone from adjacent oxygen-rich areas [20].
Our specimens collected in the L`Atalante basin were first stained with a protein binding spot (bengal pink) and examined under a microscope; here, all copepods were empty exuviae, and the nematodes were only faintly colored (suggesting that they had been dead for some time, Figure 1a, b), while all loricifera, when colored, were strongly colored (Figure 1c, d). Differences in colour intensity between living and dead metazoans were confirmed by additional experiments on deep-sea ematodes and copepods (supplementary dossier 2). Taxonomic analysis revealed that loricifera collected from anoxic sediments belong to three scientifically new species and belong to the genera Spinoloricus (Figure 1c, similar to the new species of Spinoloricus turbatio recently discovered in the deep seabed hydrothermal vents of the Galapagos sprawl) [21], Rugiloricus (belonging to the Cauliculus group; Figure 1e) and Pliciloricus (Figure 1f) [22]. Anoxic waters are areas of seawater, fresh water or groundwater that are depleted of dissolved oxygen. The U.S. Geological Survey defines anoxic groundwater as having a dissolved oxygen concentration of less than 0.5 milligrams per liter. [1] Anoxic water can be compared to hypoxic water, which contains little dissolved oxygen (but is not absent). This condition is usually found in areas where water exchange is limited.
Depending on the environmental conditions, composition and genetic capabilities of the denitrifying microbial community, denitrification can be used both as a source (e.g. under adverse conditions for N2O reduction or in the presence of denitrifiers without the NoZ gene) and as a sink (e.g. by complete denitrification of «conventional» denitrifying agents and by the potential contribution of atypical N2O reducers carrying only one nosZ gene) of N2O (Braker & Conrad, 2011). Under anoxic conditions, hypolimnetic mineralization includes other reduced substances such as S(-II), CH4 and NH4+ (Matzinger et al., 2010). Among them, the production of methane (CH4) under the ice is attracting the attention of researchers because of the high global warming potential of CH4. Methane is produced anaerobically by archaea by methanogenesis and consumed by microbial methanotrophs. The accumulation of CH4 in winter can result in a massive release of CH4 into the atmosphere when ice is degraded (Phelps et al., 1998). If a lake does not become completely oxygen-free under the ice, much of the methane is oxidized, reducing its potential contribution to atmospheric CH4 (Schmid et al., 2007). In addition to diffusion, methane is transported to the surface by boiling, which often leads to a buildup of frozen gas bubbles in the ice sheet.
An exotic type of methane`s interaction with the ice sheet has been reported in Lake Baikal, the only lake where methane has been found in the form of solid methane hydrates that form under high pressure.