Amino group in Leptothrix sheath skeleton is responsible for direct deposition of Fe(III) minerals onto the sheaths

Leptothrix species produce microtubular organic-inorganic materials that encase the bacterial cells. The skeleton of an immature sheath, consisting of organic exopolymer fibrils of bacterial origin, is formed first, then the sheath becomes encrusted with inorganic material. Functional carboxyl groups of polysaccharides in these fibrils are considered to attract and bind metal cations, including Fe(III) and Fe(III)-mineral phases onto the fibrils, but the detailed mechanism remains elusive. Here we show that NH₂ of the amino-sugar-enriched exopolymer fibrils is involved in interactions with abiotically generated Fe(III) minerals. NH₂-specific staining of L. cholodnii OUMS1 detected a terminal NH₂ on its sheath skeleton. Masking NH₂ with specific reagents abrogated deposition of Fe(III) minerals onto fibrils. Fe(III) minerals were adsorbed on chitosan and NH₂-coated polystyrene beads but not on cellulose and beads coated with an acetamide group. X-ray photoelectron spectroscopy at the N1s edge revealed that the terminal NH₂ of OUMS1 sheaths, chitosan and NH₂-coated beads binds to Fe(III)-mineral phases, indicating interaction between the Fe(III) minerals and terminal NH₂. Thus, the terminal NH₂ in the exopolymer fibrils seems critical for Fe encrustation of Leptothrix sheaths. These insights should inform artificial synthesis of highly reactive NH₂-rich polymers for use as absorbents, catalysts and so on.