Hydroxypyridinones in nitrogen-fixing bacterial cultures: a metal buffer for molybdenum and simulation of natural conditions

Abstract

Abstract Organic matter regulates the availability of important trace elements in aquatic and terrestrial ecosystems by acting as a source and container for microbes. To overcome the limitation of trace elements, nitrogen-fixing bacteria, e.g. release low-molecular-weight chelators (metallophores), which scavenge the essential cofactors of the nitrogenase, iron, and molybdenum (Mo), via complexation and subsequent uptake. The formation of metallophores is triggered by limiting conditions, which must be replicated in the laboratory in order to study metallophores as a mediator in metal cycling. While ethylenediaminetetraacetic acid (EDTA)-based buffer systems for metal cations are well established, there is limited knowledge regarding the buffering of oxoanions such as molybdate in a bacterial growth medium. To mimic the availability of molybdenum in nature under laboratory conditions, this study created a Mo-buffer system for bacterial growth media of the model organisms Azotobacter vinelandii and Frankia sp. CH37. We investigated selected hydroxypyridinones (HPs) as potential molybdenum-chelating agents, determining the amount required for efficient molybdenum complexation by calculating speciation plots of the various candidate complexes in artificial growth media at various pH values. The Mo–maltol system was identified as an ideal, nontoxic molybdenum-buffer system. In the presence of the Mo–maltol system, the growth of Frankia sp. was limited under diazotrophic conditions, whereas A. vinelandii could acquire molybdenum through the release of protochelin and subsequent molybdenum uptake. The study paves the way for unravelling molybdenum recruitment and homeostasis under limiting conditions in bacteria.