Low-molecular weight organic matter

It is important to realise that a large portion of the potentially soluble organic matter is extractable only after demineralisation. This observation proves that low molecular-weight organic matter is trapped in mineral matrices probably in interlayer sites or at grain boundaries. The hydration of silicates and the synthesis of non-maeromoleeular organic matter were probably two strongly interrelated processes. 

According to Chian and deWalle (1977) as well as Albaiges et al. (1986) the acidic fraction has been observed previously to represent a major contribution to the low molecular-weight organic matter in seepage water samples (see Fig. 2c-d). Especially the abundant occurence of C2-C7 alkanoic acids has formerly been described as a result of the anaerobic fermentation (Yan and Jen, 1992 Harmsen, 1993) In addition to the short chain alkanoic acids the corresponding hydroxylated compounds as well as branched C4-C9 alkanoic acids and C4-C9 dicarboxylic acids were detected. 

The percentage of low-molecular weight organic matter in meteorites like these is rather small, but accumulation of matter from space over bilhons of years could have resulted in significant amounts. It is assumed that extraterrestrial sources might have provided half or even more of all organic matter during the formation of the Earth (18,19). 

Any low-molecular-weight organic compounds are normally a gas at standard temperature and pressure (STP) may also be found in the soil solution. These compounds will be produced as a result of the decomposition of organic matter in soil. Many of these will be readily taken up and used by microorganisms and thus their life spans in the soil solution are short. 

Natural waters also contain a wide range of low-molecular-weight organic molecules (LMWOM), which are regarded as products of the microbial decomposition of, primarily, plant matter some have even been used as identifiers for certain genera of decomposers.123124... 

Contaminant species Particulate matter and suspensions Colloidal impurities High-molecular-weight organics Middle-molecular-weight organics Low-molecular-weight organics Multivalent ionic solutes Acids... 

Brinkmann, T., Horsch, P., Sartorius, D., and Frimmel, F. H., Photoformation of low-molecular-weight organic acids from brown water dissolved organic matter. Environ. Sci. Technol, 37, 4190-4198, 2003. 

DOC is involved primarily in the absorption of the UV component of solar radiation in seawater [119]. As a result, it seems hkely that UV radiation is responsible for most of the photochemical transformations of this carbon [120]. The depth to which UV can penetrate in seawater is about 20 m in the case of UV-B (280-320 nm) radiation and about 60 m in the case of UV-A (320-400 nm) radiation [121]. As a result, the photochemical effects of UV are restricted to the upper regions of a photic zone extending to a depth of about 100 m in the open ocean. Photochemical transformations of the organic matter measured as DOC can be caused by the direct absorption of hght or indirectly by the production of peroxides and free radicals [122]. The major identifiable photoproduct of DOC is CO2 [122,123], but a number of low molecular weight organic products are also produced [100,101,120,122]. 

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