Bacterial contribution to manganese oxidation in a deep coastal sediment

The characteristics of Mn(II) removal from sediment porewater and the potential role of manganese-oxidizing bacteria in this process were examined in sediments from a 335-m deep station in the Laurentian Trough of the St. Lawrence estuary. Manganese-oxidizing bacteria were most abundant in the thin layer of oxidized surface sediment, where Mn(II) removal rates were also fastest. The first-order rate constants for Mn(II) removal decreased from 1·2 × 10³ day⁻¹ to 6·6 day⁻¹ over the first 30-mm depth. In experimental slurries, sediments removed Mn(II) from reduced zone porewater by a two-step process: a rapid saturation of Mn(II) binding sites was followed by a slower O₂-enhanced removal rate which paralleled the apparent rate of Mn(II) oxidation. Sodium azide and mercuric chloride were tested specifically for their usefulness as bacterial poisons in sediment slurry systems. Sodium azide interfered with Mn(II) removal at low concentrations and was not an effective poison. Mercuric chloride inhibited bacterial activity at concentrations far lower than those at which significant interference of Mn(II) removal occurred. The response of sediment slurries treated with mercuric chloride indicated that the initial oxidation of sorbed Mn(II) was not bacterially-mediated under the experimental conditions tested.