MANY bacteria oxidize thermodynamically unstable manganese(II) to Mn oxides and deposit the oxides on their surfaces1,2, a process that appears to account for most Mn oxidation in natural waters3-5 and sediments6. Among the reasons that have been proposed for the evolutionary selection of this process are protection from damage by toxic metals and oxygen species, protection from ultraviolet light, and strengthening of the bacterial sheath or capsule1,7. Mn oxides may promote harmful free radical reactions, however, and marine Mn-oxidizing bacteria are particularly susceptible to photoinhibition8. Here we report that Mn oxides lyse complex humic substances, which in general cannot be used by microorganisms directly9-11, to form low-molecular-weight organic compounds that can be used as substrates for microbial growth. Mn-oxidizing bacteria may thus be able to use the carbon pool in humic substances, which represent one of the largest organic reservoirs in natural waters, sediments and soils.