Pathways of Organic Matter Degradation

In comparison to all other heterotrophs, the microorganisms oxidizing methane and other Cj compounds such as methanol, have a unique metabolic pathway which involves oxygenase enzymes and thus requires O. Only aerobic methane-oxidizing bacteria have been isolated and studied in laboratory culture, yet methane oxidation in marine sediments is known to take place mostly anaerobically at the transition to the sulfate zone. Microbial consortia that oxidize methane with sulfate have in particular been studied at methane seeps on the sea floor and the communities can now also be grown in the laboratory (Boetius et al. 2000 Orphan et al. 2001 Nauhaus et al. 2002) Anaerobic methane oxidation is catalyzed by archaea that use a key enzyme related to the coenzyme-M reductase of methanogens, to attack the methane molecule (Kruger et al. 2003 see Sect. 5.1). The best studied of these ANME (ANaerobic MEthane 

Within the sediment, most organic material remains associated with the particles or is sorbed 

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Anoxic conditions require the absence of DO and the presence of nitrates. Such conditions are typically only found when artificially implemented. The aerobic and anoxic pathways of organic matter degradation are identical. The addition of nitrate to wastewater is widely used as a control measure to avoid anaerobic conditions in sewers (Section 6.2.7). 

In long-term evolutionary scales, humans now have the abilities to intervene rapidly in this interdependent relationship and alter the stability of the rates of metabolism of organic matter. For example, reduction of ozone in the stratosphere and associated increased UV irradiance could lead to accelerated photolytic degradation of macromolecules of DOM by both abiotic and biotic pathways to C02. In addition, the photolytic enhancement of substrates for bacterial metabolism by UV photolysis can result in accelerated rates of biogeochemical cycling of nutrients and stimulated... 

This activity is particularly useful for degradation of strongly hazardous substances or recalcitrant pollutants that are difficult to remove in chemical or biochemical processes. In this respect any pathway leading to abatement of chromate(VI) pollution arouses interest. One such pathway seems to be created by cooperation between iron and chromium photocatalytic cycles, which were reported as effective in conversion of chromate(VI) into Crm species [20-23,97]. A synergistic photoreduction of CrVI and Cu11 mediated by Ti02 [98], or photocatalytic reduction of Crvl and oxidation of organic matter by environmental polyoxometallates as photocatalysts [99], may constitute alternative possibilities. 

A seven year smdy on the groundwater in the Paris region of France, revealed DEA was present at a concentration above that of its parent compound. The atrazine degradation pathway and the higher solubility of DEA in water may explain this finding. Recent work showed that photolysis of triazines and acetanilides followed pseudo first order kinetics, and the photodegradation in soils was accelerated as the content of organic matter increased. Another study showed that humic substances enhanced terbutylazine photolysis. ... 

The conceptual model for the degradation of organic matter in marine sediments was first proposed by Froelich et al. (1979). Although many more details, variations and specific pathways of these redox-reactions have become known in the meantime, this Froelich-modeF of the primary redox-reactions in marine sediments is still valid... 

The degradation of organic matter via fermentative pathways to small organic molecules such as lactate, butyrate, propionate, acetate, formate, Hj and CO is very important in marine sediments, since these compounds are the main substrates for sulfate reduction and partly for methane formation. 

Fig. 6.6 Pathways of nitrogen in marine surface sediments. Arrows black, organic matter degradation gray, particulate organic nitrogen dotted, diffusion of solutes.

Exposure pathways were estimated using the modified soil module of the MSCE-POP model (http //www.msceast.org). At present the scheme is complemented with the fraction of dissolved organic matter (/doc) and with the fraction of the chemical non-equilibrium adsorbed by solid phase (/n0n-equii) or low available with individual degradation rate. The scheme of a pollutant distribution between different soil components is shown in Figure 16. 

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