Microbial Reduction of Fe(III) in Acidic Sediments: Isolation of Acidiphilium cryptum JF-5 Capable of Coupling the Reduction of Fe(III) to the Oxidation of Glucose

Abstract

To evaluate the microbial populations involved in the reduction of Fe(III) in an acidic, iron-rich sediment, the anaerobic flow of supplemental carbon and reductant was evaluated in sediment microcosms at the in situ temperature of 12°C. Supplemental glucose and cellobiose stimulated the formation of Fe(II); 42 and 21% of the reducing equivalents that were theoretically obtained from glucose and cellobiose, respectively, were recovered in Fe(II). Likewise, supplemental H2 was consumed by acidic sediments and yielded additional amounts of Fe(II) in a ratio of approximately 1:2. In contrast, supplemental lactate did not stimulate the formation of Fe(II). Supplemental acetate was not consumed and inhibited the formation of Fe(II). Most-probable-number estimates demonstrated that glucose-utilizing acidophilic Fe(III)-reducing bacteria approximated to 1% of the total direct counts of 4′,6-diamidino-2-phenylindole-stained bacteria. From the highest growth-positive dilution of the most-probable-number series at pH 2.3 supplemented with glucose, an isolate, JF-5, that could dissimilate Fe(III) was obtained. JF-5 was an acidophilic, gram-negative, facultative anaerobe that completely oxidized the following substrates via the dissimilation of Fe(III): glucose, fructose, xylose, ethanol, glycerol, malate, glutamate, fumarate, citrate, succinate, and H2. Growth and the reduction of Fe(III) did not occur in the presence of acetate. Cells of JF-5 grown under Fe(III)-reducing conditions formed blebs, i.e., protrusions that were still in contact with the cytoplasmic membrane. Analysis of the 16S rRNA gene sequence of JF-5 demonstrated that it was closely related to an Australian isolate of Acidiphilium cryptum (99.6% sequence similarity), an organism not previously shown to couple the complete oxidation of sugars to the reduction of Fe(III). These collective results indicate that the in situ reduction of Fe(III) in acidic sediments can be mediated by heterotrophicAcidiphilium species that are capable of coupling the reduction of Fe(III) to the complete oxidation of a large variety of substrates including glucose and H2.

Microbial reduction of Fe(III) in the presence of oxygen under low pH conditions

“…However, A. cryptum JF-5 reduces Fe(III) also in liquid, shaken, Fe(III)-containing medium incubated under air. The rate of Fe(II) formation is approximately 2.6 times lower then the rate of O 2 consumption under oxic conditions (Küsel et al, 1999). Since the reduction of O 2 consumes four electrons, electron transport to O 2 is about 1 order of magnitude faster than electron transport to Fe(III) during simultaneous O 2 and Fe(III) respiration.…”

“…In the past, Acidiphilium species have been described as being obligate aerobes (Harrison, 1981 capacity to reduce Fe(III) is widespread among heterotrophic acidophiles inhabiting Fe(III)-rich habitats, although rates and the extent of Fe(III) reduction vary greatly between the isolates examined (Johnson and McGinness, 1991). Acidiphilium cryptum JF-5 is the first heterotrophic acidophile isolated under anoxic, Fe(III)reducing conditions (Küsel et al, 1999). The rate at which A. cryptum JF-5 reduces soluble Fe(III) is similar to that of the Acidiphilium strain SJH, believed to be the most efficient acidophilic heterotrophic Fe(III)-reducing isolate heretofore tested (Johnson and McGinnes, 1991).…”

“…Thus, these freshwater sediments are highly enriched with Fe(III)-(hydr)oxides that are apparently readily available for microbial reduction due to their poor crystallinity (Munch and Ottow, 1980). However, the acidic pH of these sediments appears to be inhibitory to the activity of most wellknown neutrophilic Fe(III)-reducing bacteria (Küsel et al, 1999;Küsel and Dorsch, 2000). The microbial reduction of Fe(III) under acidic conditions has received little attention (Johnson et al, 1993;Blodau et al, 1998;Friese et al, 1998), while the oxidation of Fe(II) in miningimpacted habitats has been more thoroughly examined (e.g.…”

“…In some acidic, coal mining sediments, rates of the microbial reduction of Fe(III) are highest close to the water-sediment interface, and the reduction of Fe(III) appears to be coupled to the oxidation of organic matter due to the absence of reduced sulphur compounds (Peine et al, 2000). Since acidophilic, heterotrophic, Fe(III)reducing Acidiphilium species can be easily isolated from acidic mining-impacted sediments or streams (Johnson et al, 1979;Küsel et al, 1999), these facultative anaerobes are likely involved in the reduction of Fe(III) under low pH conditions. Strictly anoxic conditions are not required for Fe(III) reduction by acidophilic heterotrophs, as the reduction of Fe(III) also occurs under oxic conditions on solid media containing Fe(III) or in oxic mixed liquid cultures containing Fe(II)-oxidizing and heterotrophic Fe(III)reducing acidophiles (Johnson and McGinness, 1991).…”

“…Strictly anoxic conditions are not required for Fe(III) reduction by acidophilic heterotrophs, as the reduction of Fe(III) also occurs under oxic conditions on solid media containing Fe(III) or in oxic mixed liquid cultures containing Fe(II)-oxidizing and heterotrophic Fe(III)reducing acidophiles (Johnson and McGinness, 1991). Acidiphilium cryptum JF-5, isolated from the highest growth-positive dilution of an acidic, coal mining sediment, simultaneously utilizes O 2 and soluble Fe(III) as electron acceptors coupled to the oxidation of glucose (Küsel et al, 1999). This co-utilization is in contrast to other facultatively anaerobic Fe(III)-reducing bacteria which do not significantly reduce Fe(III) until O 2 is consumed (Arnold et al, 1990).…”