Source-dependent variations in organic carbon degradation rates in lake sediments

Lake sediments are globally important organic carbon (OC) sinks. Biomolecule chemical reactivity, adsorption and physical shielding have been suggested as important factors in controlling OC degradation rates in sediments. Yet, few studies have investigated the relative importance of these variables, or traced how OC from different organismal sources changes over time due to source-dependent variations in degradation rates.

We investigate the factors that control organic biomolecule degradation based on analyses of eukaryotic DNA, biomarkers, and (macro)molecule compositions (using pyrolysis-GC/MS) in sediments of five lakes in central Switzerland that differ in trophic state. We specifically target biomolecules of dominant phytoplankton groups (diatoms, green algae), and terrestrial vascular plants. We show that the decay rates of diatom DNA are significantly higher than those of diatom lipid biomarkers and (macro)molecules, consistent with the higher chemical reactivity of DNA. However, the decay rates of green algal DNA and vascular plant DNA are much slower than those of diatom DNA and similar in magnitude to their corresponding membrane lipids and (macro)molecules. In the case of vascular plant biomolecules (DNA, lignin, polyaromatic compounds), no significant biomolecule degradation was detected over the time scales studied (1-5 centuries).

Our results suggest that chemical reactivity and physical shielding, but not adsorption, are key variables controlling organic biomolecule decay in the lakes studied. In the case of green algae and vascular plants, greater chemical resistance of cell wall structural components to microbial attack appears to facilitate long-term preservation of even highly reactive, intramolecular compounds, such as DNA. These findings have important implications for the use of sedimentary eukaryotic DNA records to reconstruct past environmental changes.