Siderophores are soluble or membrane-embedded molecules that bind the oxidized form of iron, Fe(III), and play roles in iron acquisition by microorganisms. Fe(III)-bound siderophores bind to specific receptors that allow microbes to acquire iron. However, certain soil microbes release a compound (pulcherriminic acid, PA) that, upon binding to Fe(III), forms a precipitate (pulcherrimin) that apparently functions by reducing iron availability rather than contributing to iron acquisition. Here, we use Bacillus subtilis (PA producer) and Pseudomonas protegens as a competition model to show that PA is involved in a peculiar iron-managing system. The presence of the competitor induces PA production, leading to precipitation of Fe(III) as pulcherrimin, which prevents oxidative stress in B. subtilis by restricting the Fenton reaction and deleterious ROS formation. In addition, B. subtilis uses its known siderophore bacillibactin to retrieve Fe(III) from pulcherrimin. Our findings indicate that PA plays multiple roles by modulating iron availability and conferring protection against oxidative stress during inter-species competition.

The prediction and prevention of fugitive dust emissions from mine tailings surfaces depend largely on our ability to monitor and monitor and predict the evolution of tailings moisture content (TMC). Albedo measurements are demonstrated here to be valuable tools to quantify TMC in bauxite residue samples under controlled conditions in the laboratory. The difference in albedo between 1.30 and 1.55 µm obtained through the infrared integrating sphere method shows good correlations with those acquired with a field spectroradiometer while both are strongly correlated with TMC. Additionally, continuous spectroscopic characterization of evaporating residues is shown to reveal the evolution in their surface drying rates. These optical methods could help predict surface drying state, thereby improving the accuracy of dust emissions risk assessment protocols that support mining industries intervention and mitigation strategies.

Siderophores are soluble or membrane-embedded molecules that play a major role in Fe acquisition by microorganisms. Pulcherriminic acid (PA) is a compound produced by different microbes that sequesters Fe in the precipitated pulcherrimin, but which role in Fe homeostasis remains elusive. Using Bacillus subtilis (PA producer) and Pseudomonas protegens as a competition model, we demonstrated that PA is involved in a yet undescribed Fe-managing system. When challenged by a competitor, PA production creates a local Fe(III) source, which can be retrieved via the bacillibactin siderophore produced by B. subtilis. Furthermore, precipitation of Fe(III) as pulcherrimin prevents oxidative stress in bacterial competition by restricting the Fenton reaction and deleterious ROS formation. Together, our findings uncover that PA is at the core of a counterintuitive Fe management strategy that capitalizes on controlled Fe precipitation when challenged by a competitor. This makes PA a unique and multifunction tool in the iron war.