Manganese Oxidizing Bacteria in Guaymas Basin Hydrothermal Fluids, Sediments, and Plumes

The active seafloor hydrothermal system at Guaymas Basin in the Gulf of California is unique in that spreading centers are covered with thick sediments, and hydrothermal fluids are injected into a semi-enclosed basin. This hydrothermal activity is the source of a large input of dissolved manganese [Mn(II)] into Guaymas Basin, and the presence of a large standing stock of particulate manganese in this basin has been taken as evidence for a short residence time of dissolved Mn(II) with respect to oxidation, suggestive of bacterial catalysis. During a recent Atlantis/Alvin expedition (R/V Atlantis Cruise #7, Leg 11, Jim Cowen Chief Scientist), large amounts of particulate manganese oxides were again observed in Guaymas Basin hydrothermal plumes. The goal of the work presented here was to identify bacteria involved in the oxidation of Mn(II) in Guaymas Basin, and to determine what molecular mechanisms drive this process. Culture-based methods were employed to isolate Mn(II)-oxidizing bacteria from Guaymas Basin hydrothermal fluids, sediments, and plumes, and numerous Mn(II)-oxidizing bacteria were identified based on the formation of orange, brown, or black manganese oxides on bacterial colonies on agar plates. The Mn(II)-oxidizing bacteria were able to grow at temperatures from 12 to 50°C, and a selection of the isolates were chosen for phylogenetic (16S rRNA genes) and microscopic characterization. Endospore-forming Bacillus species accounted for many of the Mn(II)-oxidizing isolates obtained from both hydrothermal sediments and plumes, while members of the alpha- and gamma-proteobacteria were also found. Mn(II)-oxidizing enzymes from previously characterized Bacillus spores are known to be active at temperatures greater than 50°C. The presence of Mn(II)-oxidizing spores - some of which are capable of growing at elevated temperatures - in hydrothermal fluids and sediments at Guaymas Basin suggests that Mn(II) oxidation may be occurring immediately or very soon after hydrothermal fluids emerge from the seafloor.