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 identity. To elucidate the chemical nature of this unknown pool of individual ligands we develop and apply new approaches including high resolution liquid chromatography electrospray mass spectrometry in collaboration with the Proteomics & Mass Spectrometry Center at Princeton University.
To scan the myriad of signals acquired during the LC-MS runs for features that relate to metal complexing agents, we use specialized in-house written software (‘ChelomEx’) that searches for metal isotopic signatures associated with the complexes.
Our initial efforts involve cultures of soil diazotrophs (Azotobacter vinelandi, Azotobacter chroococcum) and a marine diazotroph (Trichodesmium erythreum) grown under differential metal availability. Analysis of the culture supernatants reveals the presence of many putative metal complexing agents in all media while MS/MS fractionation of the compounds provides structural information. For example, in the low Fe medium of A. vinelandi, we identify 15 putative Fe-chelators, including the known siderophores Azotobactin, Protochelin and Azotochelin as well as other related compounds yet unreported. Among these are putative derivatives of the known Fe ligands as illustrated in the figure below.