ENVIRONMENTAL CHEMISTRY AND
MICROBIOLOGY OF TRACE METALS:
Metal Complexing Agents
In natural waters and in culture media, the bioavailability of trace metals to microorganisms depends on their binding to a variety of weak and strong complexing agents. See also Metal Uptake by Phytoplankton and
The Role of Metals in Nitrogen Cycling in Soils. In nature, these organic compounds include humic substances, intracellular molecules released by cell lysis, and ligands released by microorganisms for the purpose of metal detoxification or uptake. Most available data on these complexing agents come from electrochemical measurements that provide information on their concentrations and binding strengths but not on their chemical structures. We have undertaken a collaboration with David H. Perlman, Director of our Proteomics & Mass Spectrometry Center to identify these organic complexing agents, both in natural samples and in culture media.
Our initial effort involves the LC-MS/MS analysis of culture media from the soil diazotroph Azotobacter vinelandi, the marine diazotroph Trichodesmium erythreum, and the marine coccolithophore Emiliana huxleyi grown under various metal conditions. Initial analysis of the data, reveals the presence of many putative metal complexing agents in all media. Identification of these compounds is obtained through the characteristic isotopic mass distribution of the bound metals while MS/MS fractionation of the compounds provides structural information. For example, in low Fe medium of A. vinelandi, we identified 15 putative Fe-chelators, including the known siderophores Azotobactin, Protochelin and Azotochelin as well as other related compounds yet unreported. Among these were putative derivatives of the known Fe ligands as illustrated in the figure below.