Simultaneous photoreduction and microbial oxidation of iron in a stream in the New Jersey Pinelands’

Photoreduction of ferric to ferrous iron was determined in water held in bottles placed in a New Jersey Pinelands stream, although concentrations of ferrous iron in the stream itself remained below detection (15 ng mlI). The low ambient levels of ferrous iron apparently resulted from the action of iron-oxidizing microorganisms in water and soil, as shown by a temperature optimum for, and by the inhibitory effect of sterilization upon, the disappearance of ferrous iron. Measurements of 14C0, incorporation into soil organic matter indicated that the iron-oxidizing microorganisms were not chemolithoautotrophs, but might have been iron-depositing bacteria or chemolithoheterotrophs. The consequences of iron oxidation by microorganisms include deposition of iron oxide compounds (Hanert 198 1 a), the production of organic carbon (Ehrlich 198 l), and the acidification of surface waters (Gorham et al. 1984). Ferrous iron oxidizes spontaneously in oxic waters, although the rate of oxidation is extremely slow below pH 5 (Crerar et al. 1979; Stumm and Morgan 198 1). It is in naturally acidic waters, therefore, that microbial catalysis of iron oxidation is most easily demonstrated (Braddock et al. 1984; Brock et al. 1976; Ehrlich 198 1). The reduced iron which serves as substrate for microbial iron oxidation may be derived from the weathering of minerals (Garrels and McKenzie 197 1) and may also result from chemical reduction of ferric iron (Wetzel 1983). Because photoreductive reactions are another potential source of ferrous iron (Waite and Morel 1984), iron-ox1 Supported by the New Jersey Department of Environmental Protection and Rutgers Center for Coastal and Environmental Studies, Division of Pinelands Research. 2 Present address: Laboratory of Soil Microbiology, Department of Agronomy, The Pennsylvania State University, University Park 16802. idizing microorganisms might be expected to colonize iron-rich surface waters. Using lake water in which iron photoreduction was measured, McMahon (1969) sought but did not find an effect of microorganisms on the redox equilibrium of iron. Collienne ( 19 8 3), while demonstrating iron photoreduction in two acidic manmade lakes, suggested that iron oxidation in these waters was catalyzed by organic compounds. Thus, photoreduction of iron has never been shown to support iron-oxidizing microorganisms. Waters which flow in the Pine Barrens (Pinelands) of New Jersey are acidic (pH 3.4-4.6) and high in iron (Johnson 1979; Morgan 1984). The delivery of ferrous iron from ground to surface waters leads to the accumulation of iron oxide deposits; Crerar et al. (1979) used kinetic arguments, microscopic observations, and the seasonal occurrence of iron oxide precipitates to conclude that microorganisms are responsible for iron oxidation in the Pinelands. According to Ghiorse (1984) however, physiological data are essential to proving the geochemical activity of microorganisms. Our present study was undertaken to provide physiological evidence which complements the work of Crerar et al. (1979). Additional objectives included investigating