Hi Rolf, I agree with the "Rayleigh decomposition" concept, but would also suggest that isotope dynamics during decomposition can also be governed by "compound discrimination". That is, decomposers (microorganisms, beetles, worms, etc.) will selectively go after whichever compound provides the greatest return in energy, effectively regardless of its isotopic composition. For example, for a decomposing plant, the energy gain from decomposing sugar, starch and cellulose will clearly outweigh the fact that they are isotopically "heavier" than lignin. For an animal, the Rayleigh model might work well (not sure for the right reasons though) because fats are not only isotopically "light" but are also high-energy, and may therefore be some of the first to go. ~Kevin __________________________________________ Kevin Tu, PhD Center for Stable Isotope Biogechemistry Department of Integrative Biology University of California, Berkeley Berkeley, CA 94720 ph: 510-642-1054 email: [log in to unmask] At 08:09 PM 7/26/2007, Mihali A. Felipe wrote: >On Fri, 27 Jul 2007, Siegwolf Rolf wrote: > >>Well the process of decomposition is not only performed by beetles >>and worms, It is mostly the breaking down of chemical bonds and the >>activities of microorganisms, and their digestion process is enzyme >>based, with a number of various biochemical processes. > >Sorry for the confusion, we informally use "bugs" here to refer to >any organism micro or macro. > >>And as it is, the lighter isotopes form weaker chemical bounds than >>heavier ones. Ergo the compounds with the heavier isotopes are more >>stable and it takes more energy to break them down and consequently >>the breaking down of the compounds with heavier isotopes occurs in >>a lower rate than for the compounds of with the light elements. >>Therefore you get "heavier" the more you decay. > >If the reaction is completely inorganic and abiogenic I might >immediately agree with you. But in a biochemical system things are >more complicated. You may be right that the end result is that the >lighter element get digested by enzymes more quickly. However, >processwise it would be interesting in itself for the very reason >that the steps are not as simple as you describe. > >Think of the breakdown of a protein molecule (a macromolecule). When >an organism digests it, will it really prefer to break up the amino >acids with the lighter carbon, oxygen or hydrogen isotopes? > > >>The term "Rayleigh decomposition" fits pretty well.... >> >>Actually a topic to include in a lecture, especially in the context >>of soil processes it could make it easier for the students to remember >> >>Rolf >> >>__________________________________ >>Dr. Rolf Siegwolf >>Lab for Atmospheric Chemistry >>Stable Isotopes and Ecosystem Fluxes >>Paul Scherrer Institut CH-5232 Villigen PSI >>Switzerland >> >>e-mail: [log in to unmask] >>Phone: +41 56 310 2786 >>Fax : +41 56 310 4525 >> >>Website: >>http://isotope.web.psi.ch >>__________________________________ >> > >- Mihali > >---- >. Mihali A. Felipe >. Gerstein Lab : Bioinformatics >. Yale University Molecular Biophysics and Biochemistry >. 203-432-6337