Response of pulcherrimin-producing Metschnikowia yeast to different iron sources

Metschnikowia pulcherrima clade yeasts produce pulcherriminic acid, which binds with iron to form pulcherrimin, a red pigment. However, the functions of pulcherriminic acid in yeasts and bacteria that synthesize it remain incompletely understood. To gain deeper understanding of these functions the responses of pulcherriminic acid-secreting Metschnikowia yeast in the presence of various solid iron sources (metallic iron, magnetite, and lepidocrocite) containing iron in different valence states were examined. By applying the classical plate method, the formation of pulcherrimin, was observed both when iron source is in the close contact and at some distance. Mössbauer spectroscopy was used to determine type of iron compounds formed through the interaction of the iron materials and the growth medium and the yeast, electrochemistry was used to test the ability of Metschnikowia cells to reduce iron ions, and microscopy was used to examine the pulcherrimin accumulation sites in cells. The Mössbauer spectroscopy revealed the presence of non-soluble pulcherrimin, intermediate Fe³⁺ compounds and some Fe²⁺ containing substances. Electrochemical measurements suggested the presence of redox active centers, which are able to reduce Fe³⁺ to Fe²⁺, in the cell membranes. Microscopy showed that the red pigment mostly accumulated in the chlamydospores. Although inhibition of Metschnikowia yeast growth was observed in the initial phase, pulcherrimin-producing yeast showed good resistance to high concentrations of metals (both iron and copper).