Abstract
Sorption of organic molecules to mineral surfaces is an important control upon the aquatic carbon (C) cycle. Organo-mineral interactions are known to regulate the transport and burial of C within inland waters, yet the mechanisms that underlie these processes are poorly constrained. Streamwater contains a complex and dynamic mix of dissolved organic compounds that coexists with a range of organic and inorganic particles and microorganisms. To test how microbial metabolism and organo-mineral complexation alter amino acid and organic carbon fluxes we experimented with 13C-labelled amino acids and two common clay minerals (kaolinite and montmorillonite). The addition of 13C-labelled amino acids stimulated increased microbial activity. Amino acids were preferentially mineralized by the microbial community, concomitant with the leaching of other (non-labelled) dissolved organic molecules that were removed from solution by clay-mediated processes. We propose that microbial processes mediate the formation of organo-mineral particles in streamwater, with potential implications for the biochemical composition of organic matter transported through and buried within fluvial environments.
Highlights
Upon addition of the clay particles we observed a significant increase in the mineralisation of both the 13C-labelled amino acids (p < 0.001, F = 38.63; df = 4) and non-labelled dissolved organic matter (DOM) within streamwater (p = 0.001, F = 10.96; df = 5) (Fig. 3A)
The study suggests that microbial metabolism was the dominant process determining the fate of dissolved amino acids, with 55.7 ± 2.3% of the amino acids mineralised in the clay treatments and 45.7 ± 0.9% mineralised in the amino acid control (Fig. 3A)
The addition of 13C-labelled amino acids appears to stimulate microbial activity, with a concomitant increase in the production of non-labelled DOM. This is observable in the amino acid control, there was no increase in non-labelled DOM concentrations
Summary
In this study we test how the competing processes of microbial metabolism and organo-mineral complexation affect the fate of free amino acids and other dissolved organic molecules in streamwater. By tracing the 13C label in the organic and inorganic carbon pools allowed us to assess the mineralization and retention of the amino acids. Loss of amino acids and DOM was estimated via mass balance, from which we infer the relative importance of organo-mineral interactions for carbon fluxes in our experimental systems. Organo-mineral sorption is a surface-area limited reaction and we hypothesize that clay type and concentration influence the sorption dynamics of free amino acids and their mineralization by microorganisms
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