Chapter 5
Biogeochemistry of Organic Nitrogen Compounds Jeffrey L. Bada
Downloaded by UNIV OF ROCHESTER on April 29, 2013 | http://pubs.acs.org Publication Date: June 29, 1998 | doi: 10.1021/bk-1998-0707.ch005
Scripps Institution of Oceanography, University of California at San Diego, L a Jolla, C A 92093-0212
Nitrogen containing organic compounds represent the second most abundant reservoir o f nitrogen on the surface o f the Earth. However, the organic compounds that make up this global nitrogen pool are not well characterized. Although amino acids and the nitrogenous bases o f nucleic acids make up only a few percent o f the total organic nitrogen reservoir, the geochemical reactions o f these compounds have been extensively studied. Because hydrolysis reactions are rapid on the geologic time scale, both proteins and nucleic acids ( D N A and R N A ) are not preserved for more than 10 to 10 years i n most environments. The racemization reaction o f amino acids converts the L-amino acids present in the biosphere into a racemic mixture (D/L amino acid ratio = 1.0) i n the geosphere i n less than 10 years. Anhydrous conditions, such as those that may be associated with amber entombed insects, may retard both biopolymer hydrolysis and racemization. Condensation reactions between amino acids and sugars, including sugars at apurinic sites i n nucleic acid fragments, likely result i n the incorporation o f these compounds into geopolymers such as humic acids. Although rearrangement reactions in geopolymers may scramble the original molecular structures, part o f the global organic nitrogen inventory was originally derived from amino acids and nucleic acid bases. 3
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The largest surface reservoir o f nitrogen on the Earth after atmospheric N is nitrogencontaining organic compounds (1). Although this organically bound nitrogen was originally produced by biological processes, in the geochemical environment a complex series o f reactions have altered the biogenic organic nitrogen molecules and scrambled their original molecular architecture. A s a result, the organic nitrogen i n surflcial reservoirs contains molecules with complex geochemical histories, ages and sources. In living organisms the bulk o f the organic nitrogen is i n the form o f protein bound amino acids (2). For example, over 50 % o f the dry weight o f an average E. Coli bacterial cell consists o f protein. The ratio o f protein to R N A is about 5, while the protein/DNA ratio is around 20 (2). The ratio o f protein to other forms o f organic nitrogen such as porpyhrins is generally less than that o f nucleic acids, with the 2
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© 1 9 9 8 American Chemical Society
In Nitrogen-Containing Macromolecules in the Bio- and Geosphere; Stankiewicz, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
65 exception o f photosynthetic organisms i n which the porpyhrin chlorophyl may be a significant organic nitrogen component. A n inventory o f organic nitrogen containing molecules on the Earth's surface is given i n Table I. A s expected, amino acids do indeed constitute the bulk o f the organic nitrogen i n the biosphere, yet they make up only a few percent o f the total organic nitrogen i n the various geologic reservoirs. What molecules then account for Table I. The inventory of organic nitrogen and amino acids on the surface of the Earth [based on (1,3,4)].
Downloaded by UNIV OF ROCHESTER on April 29, 2013 | http://pubs.acs.org Publication Date: June 29, 1998 | doi: 10.1021/bk-1998-0707.ch005
Reservoir biosphere sediments soils ocean polar ice
organic Ν (10 gN) 15
10-14 4-6x10
5
60-300 200-500 (0.5-l)
+
amino acids (10 g of AA) 15
