Keywords: Bioinorganic chemistry / Manganese / Oxygen / X-ray absorption spectroscopy / X-ray microfocus experiments Chara corallina freshwater algae produce brown deposits of manganese oxides on their cell wall surfaces when growing in manganese-rich media. We report on the formation, topology, composition, atomic structure, and catalytic activities of these biogenic manganese oxides (BMOs). The deposits are volcano shaped and exhibit 3-5 μm craters in their centers. Microfocus X-ray irradiation and detection of characteristic X-ray fluorescence lines allowed elemental mapping at 5 μm spatial resolution and the identification of the volcanoshaped deposits as a Mn-Ca oxide. X-ray absorption spectroscopy (XAS) revealed a high-valent Mn III/IV oxide. The structural analysis involved XAS spectra collected for a single volcano at room temperature and for single cells at 20 K.

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