Metallophores as Mediators for Metal Cycling: Development of Libraries for Metal Ion Buffering and as Redox Carriers as well as Profiling of Metallophores
7/2018 The project C05 is completed
Within the line of ChemBioSys projects dealing with the mediation of metal availability to communities by means of chemical signals, this work aims to explore multiple effects of metallophores. Although siderophores are generally recognized as biological iron uptake agents, recent evidence suggests that they play multiple significant roles in biogeochemical cycles. Indeed for some candidate metabolites the term “metallophores” is more appropriate since they e.g. promote the bacterial acquisition of molybdenum (Mo) and vanadium (V) in addition to iron (Fe). This aspect is of particular importance in nitrogen (N2) fixation, the biological conversion of dinitrogen to plant-available ammonium (NH4+). This fixation process is the primary natural input of nitrogen to ecosystems, and influences plant growth and carbon exchange. It serves us as an excellent biological system studying the multiple functions of metallophores: Apart from iron, bacteria need to acquire the essential nitrogenase metal cofactors Mo and V in a soil environment where metal speciation, and thus metal bioavailability, is dependent on a variety of factors such as organic matter content, mineralogical composition, and pH value. Unlike iron, molybdenum and vanadium are usually present in the highly soluble form of oxoanions, such as e.g. molybdate (MoO42-) and vanadate (VO43-), making them susceptible to leaching. We argue that binding of these soluble forms to natural organic matter helps to prevent leaching and is thus a crucial step that serves Mo and V to nitrogen fixing organisms.
We will take advantage of the broad variety of biological systems studied within the frame of the CRC to determine novel metallophores and to discover their multiple functions. In particular, our project will address the role of bacterial and plant exudates on the acquisition and mediation of Mo and Fe in two consortia consisting of (i) Alder and nitrogen-fixing actinomycete Frankia in its rhizosphere and of (ii) Fe(II) oxidizers and Fe(III) reducers at redox interfaces within dynamic flow-through systems in collaboration with project C04. We aim to develop libraries of synthetic chelating agents for oxoanions and bivalent metals to mimic natural metal sources in artificial media. In terms of Mo this library is intended to provide non-toxic cis-dioxometallate complexes, which are stable under various physiological conditions and not taken up by bacteria functioning as a molybdenum buffer. Those complexes would serve as the only source of metal in the culture medium. In terms of Fe the library is intended to provide redox carriers with an adjustable electrochemical potential to facilitate the modeling of electron shuttling and/or metallophore behavior within a community of microorganisms of Fe(II) oxidizers and Fe(III) reducers. In both cases we will identify metallophores along with other compounds released by symbiotic (and non-symbiotic) bacterial associations in the standardized media using a novel metallophore profiling approach. This project will reveal how communities can be structured by metal chelating reagents. Analytical methods and molecule libraries constructed within this project will serve all partners within ChemBioSys that deal with the mediation of metal availability by natural products and their analogs.