Magnetite (Fe3O4) and greigite (Fe3S4) crystals in multicellular magnetotactic prokaryotes

Magnetotactic bacteria produce iron oxides, iron sulfides or both in organelles called magnetosomes. Most of these bacteria are unicellular and biomineralize magnetite (Fe3O4). In contrast, multicellular magnetotactic prokaryotes (MMPs) consisting of several gram-negative cells have only been known to crystallize the magnetic iron sulfide greigite (Fe3S4). In this work, we describe MMPs that mineralize magnetite in bullet-shaped crystals. Another unusual aspect is that magnetite occurs either as the only crystals or together with greigite crystals. MMPs containing only greigite in the magnetosomes occur in the same environment. These findings show that morphology, ultrastructure, and behavior are the main characteristics of the MMPs, not the type of magnetic crystal biomineralized in the magnetosomes.