A marshy cold spring occurs in late Pleistocene glacial outwash exposed on the floor of the Squalicum Creek valley near Bellingham, Washington. A 5-m by 25-m zone of black soil with an average depth of 30 cm surrounds the spring. This soil contains about 43% MnO2 and 20% to 30% iron oxide calculated as Fe2O3; the oxidized material appears amorphous when analyzed by X-ray diffraction. Field observations and laboratory studies indicate Mn and Fe are accumulated due to bacterial oxidation of trace amounts of these metals supplied by ground water. Two strains of pseudomonad bacteria isolated from the black soil rapidly oxidize Fe and Mn when grown on a culture medium containing soil organic matter as a nutrient. Both bacteria can be cultured on tryptone-glycerol agar but are unable to oxidize Mn or Fe added to this synthetic medium. Although the composition of the culture medium is important in controlling whether oxidation occurs, the reaction is not merely due to the catalytic effect of hydroxy acids contained within the nutrient mixture. Optical microscopy and X-ray fluorescence analysis using a scanning electron microscope equipped with an energy-dispersive detector reveal that Fe and Mn oxides precipitated by the microbes occur as extra-cellular deposits, and these metals are not accumulated within the bacterial cells.
Although microbiologists have shown that several genera of bacteria are able to oxidize Mn in laboratory cultures, these studies are of uncertain value when used to explain the role microbes play in sedimentary Mn-oxide deposits, because most studies have involved microbes isolated from ordinary soil rather than from Mn-rich environments. This locality demonstrates the ability of soil bacteria to accumulate high concentrations of Fe and Mn under natural conditions.