Characterisation and environmental implications of Pseudomonas putida -mediated biogenic iron-manganese mixed-oxide nanoparticles

Abstract

Biogenic iron-manganese (Fe-Mn) mixed oxides are thought to be of significant environmental importance because of their strong reactivity and frequent occurrence in natural settings. Despite their critical role in elemental biogeochemical cycling, their formation pathways in different environmental conditions and detailed characterisation remain underexplored. This research presents an in-depth investigation into the synthesis of biogenic Fe-Mn mixed oxides, mediated by the aerobic Mn(II)-oxidising bacterium Pseudomonas putida MnB1, under conditions mimicking the oxic–anoxic interface of a water column, a potential pathway described here for the first time. Synthesis of biogenic Fe-Mn mixed oxides was performed in laboratory experiments consisting of two successive steps: (1) bio-oxidation of Mn(II) to produce biogenic Mn oxide; and (2) oxidation of Fe(II) by the biogenic Mn oxides. A series of analytical techniques, including X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, transmission electron microscopy selected area electron diffraction, and inductively coupled plasma mass spectrometry, revealed the formation of heterogeneously distributed distinct phases of Fe and Mn oxide minerals, subsequently identified as poorly crystalline birnessite and Fe(III) oxyhydroxide nanocrystals closely associated with bacterial cells and extracellular polymeric substances. The synthesised biogenic Fe-Mn mixed oxides showed diverse morphologies, including large granules, long chains and ultrathin sheets, produced extracellularly on the bacterial cell surfaces. The synthesis process described in this study provides a better understanding of one of the possible pathways of Mn and Fe cycling at the oxic-anoxic interface of the water column. Potential environmental implications of the formation of these phases are also discussed, including trace metals transport dynamics in aquatic environments.