▪ Abstract  Manganese(IV) oxides produced through microbial activity, i.e., biogenic Mn oxides or Mn biooxides, are believed to be the most abundant and highly reactive Mn oxide phases in the environment. They mediate redox reactions with organic and inorganic compounds and sequester a variety of metals. The major pathway for bacterial Mn(II) oxidation is enzymatic, and although bacteria that oxidize Mn(II) are phylogenetically diverse, they require a multicopper oxidase-like enzyme to oxidize Mn(II). The oxidation of Mn(II) to Mn(IV) occurs via a soluble or enzyme-complexed Mn(III) intermediate. The primary Mn(IV) biooxide formed is a phyllomanganate most similar to δ-MnO2 or acid birnessite. Metal sequestration by the Mn biooxides occurs predominantly at vacant layer octahedral sites.

Saline and warm Mediterranean water flowing through the Bosporus Strait maintains a permanent pycnocline with vertical separation of oxic (O 2 ), suboxic (absence of O 2 and H 2 S), and anoxic (H 2 S) zones in the Black Sea. The stable suboxic zone implies restricted vertical mixing of the upper oxic and lower anoxic layers and limited vertical flux of oxygen that cannot balance the upward flux of sulfide. We report data that directly confirm massive lateral injections (Ͼ200 km from the Bosporus) of oxygen-enriched waters of the Bosporus plume, created by the mixing of shallow, cold, intermediate-layer Black Sea water with Mediterranean water. These plume waters are laterally injected into the oxic layer and, more importantly, into the suboxic and anoxic layers over several small vertical scales (''fingers'' of ϳ5 m) at water densities ( t ) from 15.0 to 16.4. O 2 injection oxidizes Mn(II) to Mn(III,IV), which then oxidizes H 2 S. The onset of H 2 S detection occurs in deeper waters in the southwest (Ͼ170 m; t ഠ 16.4) relative to the west central Black Sea (110 m; t ഠ 16.2) and coincides with increased MnO 2 and S 8 formation in the southwest.

Manganese oxides are the only known oxidants of Cr(III) in the environment, and predictions of the fate of Cr(III) have been based on Cr(III) oxidation rates with well-characterized Mn(III,IV) oxide minerals. Our research, however, indicates that the presence of Mn(II)-oxidizing bacteria, may accelerate these rates through the production of very reactive Mn oxides or intermediates formed in the oxidation process. Experiments with the Mn(II)-oxidizing Bacillus sp. strain SG-1 show that this bacterium can accelerate Cr(III) oxidation compared to both abiotic and biologically produced Mn oxides. Initial rates of Cr(III) oxidation by biogenic oxides were approximately 7 times faster than Cr(III) oxidation rates by equivalent amounts of synthetic δ-MnO 2 and 25 times faster by SG-1 spores with Mn(II). Cr(III) oxidation by SG-1 is not direct; Mn is required, but only in small amounts, indicating that it is recycled. Cr(III) oxidation is inhibited above 5 μM dissolved Mn(II), while Mn(II) oxidation is not, suggesting that the processes are controlled by different mechanisms. These results illustrate the need to consider bacterial activity and the concentration of Mn when predicting the potential for Cr(III) oxidation.