Abiotic stabilization

The chemical half-life of DMSP in seawater is >8 years (Dacey and Blough 1987), which results in high abiotic stability under natural conditions (moderate temperatures and pH). Therefore, most of the DMSP removal is through enzymatic processes. In the microbial food web, dissolved DMSP has many fates and several recent reviews on the microbial pathways and involved mechanisms have been published (Bentley and Chasteen 2004 Kiene et al. 2000 Lomans et al. 2002 Yoch 2002). They all show that DMSP can be readily used in a complex network of enzymatic conversions. This versatility indicates that this single compound is of major importance for the nutrition of the bacterial community. Indeed, several studies have shown that DMSP alone can contribute 1 to 15% of the total bacterial carbon demand in surface waters. Moreover, DMSP assimilation can satisfy most, if not all the, sulphur demand of marine bacteria (Kiene and Linn 2000 Simo et al. 2002 Zubkov et al. 2001). Since the focal point of this section is the quantification of DMSP removal, only the overall effects of the main pathways originating from DMSP (Fig. 1) will be discussed here. 

Carotenoid oxidation products, as carotenoids, may exert protective or detrimental effects on human health. Efforts must be made to try to identify them in vivo where they may appear in lower quantities than carotenoids. Studies of abiotic systems can provide great support for their identification and the comprehension of their stability and reactivity. 

Morse JW, Bender ML (1990) Partition coefficients in calcite Examination of factors irrflnencing the validity of experimental resnlts and their application to natural systems. Chem Geol 82 265-277 Mucci A, Morse JW (1990) The chemistry of low temperature abiotic calcites Experimental studies on coprecipitation, stability and fractionation. Rev Aquatic Sci 3 217-254 Musgrove ML, Barmer JL, Mack LE, Combs DM, James EW, Cheng H, Edwards RL (2001) Geochronology of late Pleistocene to Holocene speleothems from central Texas Implications for regional paleoclimate. Geol Soc Am Bull 113 1532-1543... 

An increase in absorbance at 351 nm and a concomitant decrease in absorbance at 380 nm in the ultraviolet visible spectrum of methylcobalamin during the abiotic transfer of the methyl group to Hg2+ are characteristic for the loss of the methyl group and formation of aquocobalamin. In experiments monitored by both analytical techniques, gas chromatographic measurements of methylmercury formation were in good agreement with the spectropho-tometric measurement of aquocobalamin formation from methylcobalamin at 351 nm. Aerobic versus anaerobic reaction conditions had no measurable effect on either the methyl transfer rates, the stability of the reactants, or on the reaction products. 

Biotic and abiotic degradation of 1,2-dibromoethane in surface waters is slow relative to volatilization of the compound to the atmosphere (ERA 1987b). 1,2- Dibromoethane is resistant to hydrolysis (Jaber et al. 1984) the hydrolytic half-life of the compound has been reported to range from 2.5 years (Vogel and Reinhard 1982) to 13.2 years (HSDB 1989). As a result of its hydrolytic stability and the limited biological activity in subsurface soils, 1,2- dibromoethane leached to groundwater is expected to persist for years. 

Figure 25.1 indicates that a substantial amoimt of carbon is trapped in marine sediments as frozen methane. This gas is produced in the surfece sediments by methanogens from metabolism of detrital organic matter and abiotically in more deeply buried sediments. At sufficiently low temperatures, the methane becomes encased in a cage of water molecules, fitrming a solid gas hydrate. As shown in Figure 25.3, the stability... 

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... 

In the past the mineral matrix was considered as inert, only providing stabilization support for enzymes and humic substances however, due to the overwhelming amount of evidence at the molecular level, there is no doubt that minerals participate in abiotic catalysis of humification reactions in soils. Naidja et al. (2000) referred to mineral particles as the Hidden Half of enzyme-clay complexes, which not only prolong the activity of immobilized enzymes but also are readily able to participate in electron transfer reactions. Many environmental factors can negatively affect the... 

The term biochemical stabilization refers to the biotic or abiotic production of organic substances that are refractory to decomposition by microorganisms and contribute, through condensation and complex formation, to the stabilization of otherwise easily decomposable substrates such as enzymes. This stabilization process coincides with the process of humification. 

Stabilization of biomolecules can also result from abiotic humification catalyzed by mineral colloids (see Chapter 2). 

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