Deposits of manganese oxide were formed during the growing season on the surface of a cultivated acid soil (pH 5.0). A manganous ion oxidizing alga, Chlorococcum humicolum, was isolated from this soil. In liquid media, the alga deposited some oxide at pH 4.5, but oxidation was greater at pH 6.0. The pure alga formed a sparse deposit of oxide on the surface of soil agar at pH 5.0; however, the alga when mixed with certain bacteria deposited a dark brown oxide uniformly over the surface of this medium. The presence of bacteria caused the pH of the medium to increase to about 6.0. The pure alga also formed a dark brown deposit of oxide on soil agar adjusted to pH 6. It is suggested that the alga, in association with bacteria, formed the oxide deposit observed in the field and that the presence of manganese oxide on the surface of soils could indicate field situations which develop significant and perhaps toxic levels of soluble manganese.
