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 volcano-shaped deposits as a Mn–Ca oxide. X-ray absorption spectroscopy (XAS) revealed a high-valent MnIII/IV oxide. The structural analysis involved XAS spectra collected for a single volcano at room temperature and for single cells at 20 K. On the basis of the XAS data, the oxides were identified as members of the birnessite family of layered manganese oxides containing di-μ-oxido-bridged MnIII/IVO6 octahedra as central building units. The deposits share structural motifs with synthetic water-oxidizing Mn–Ca oxides and with the Mn4Ca complex of photosystem II, the biological water-oxidation catalyst. Model reactions demonstrate low, but clearly detectable, activity of the manganese deposits for water-oxidation catalysis. The biogenic manganese oxides on the cell walls of Chara corallina thus represent an intriguing object to study how manganese-based catalysts for water oxidation are formed in a biological environment. The formation of BMOs in relation to cellular ion transport and the possibility of BMOs to fulfill a detoxification function in plants were also examined.