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Prior to 1988, both environmental data and the redox properties of oxidized manganese (Mn) and iron (Fe) had led many investigators to speculate about the existence of organisms that could couple anaerobic respiration-linked metal reduction to organic carbon oxidation or growth (8, 9, 11, 29); although pure cultures of such organisms had been isolated (34, 35) their ability to couple metal reduction to growth or carbon oxidation had not been reported. In 1988, three reports appeared describing organisms that could obtain energy for anaerobic growth by using oxidized Mn or Fe as their electron acceptors (7, 18, 23). Two of these organisms, MR-1 (23) and sp. 200 (7), were identified as Shewanella (formerly Alteromonas ) putrefaciens (20), a facultative anaerobe and obligate respirer. The third organism, GS-15 (18), remains an unidentified obligate anaerobe capable of utilizing a variety of different carbon sources. In this minireview, we discuss the properties of S. putrefaciens MR-1 and other recently characterized bacteria in this group and describe their metabolism in an environmental perspective. We suggest that the ability to reduce Mn and Fe oxides under anoxic conditions may be much more widespread than previously suspected"Isolation and characterization of Mn(IV)-reducing bacteria: (almost materials and methods)
We have used enrichment cultures, in which it is possible to specify which electron acceptors and carbon sources are present, to isolate Mn(IV) reducers (24, 30). when MnO2 was used as the electron acceptor and nonfermentable carbon sources such as succinate and acetate were used as sources of energy, enrichments were obtained that yielded S. putrefaciens MR-1 and several other active Mn(IV) reducers. In this enrichment culture procedure, primary enrichments are used to establish secondary enrichments, and pure cultures are isolated by plating these secondary enrichments onto plates with soft agar overlays containing MnO2; isolates are obtained from the zones of clearing (i.e. Mn reduction) within these agar overlays. One note concerning these agar overlays is that the bacteria form clearing zones, but they do not form visible colonies. This is because the bacteria require contact with the solid Mn(IV) oxides and are found only on the oxides at the edges of the clearing zones (23, 27). S. putrefaciens sp. 200 has also been shown to require contact with Fe oxides to mediate their reduction (2).The Summary:
The type strain of S. putrefaciens (ATCC 8071) has since been shown to be very similar to MR-1 in its anaerobic metabolism (19, 23, 27), including the ability to couple its growth to the reduction of Mn(IV) and Fe(III). Thus, MR-1 is not a new organism or an organism with a unique property; it is simply a member of a previously characterized species whose metal-reducing properties were not known. One wonders how many other such examples exist in our culture collections. It is also now clear that the organism isolated by Westlake and colleagues (34, 35, 44) and further studied by Hoffman and colleagues (1-3, 7) is also a member of S. putrefaciens ; this group has a wide distribution, and it is found in such diverse environments as butter (5), fish (13-15, 40, 45), feces (21), cutting oil (37), oil fields (36, 43, 44), and various freshwater and seawater environments (13, 23, 27, 28, 32, 42). Laboratory studies with MR-1 revealed that it couples its anaerobic growth to the reduction of Mn(IV), Fe(III), and many other electron acceptors, including oxygen (23-25, 30) (Table 1) All S. putrefaciens isolates studied to date are facultative anaerobes: this may be a decided advantage in those stratified environments in which the oxic-anoxic interface moves up and down as a result of seasonal changes, bioturbation, or human activities. Clearly, if an organism is attached to a metal oxide particle and the zone becomes oxic, there is little advantage in being a strict anaerobe!
In this brief review, we have tried to introduce the reader to the importance and conceptual aspects of microbial metal reduction by focusing on a single group of Mn(IV)- and Fe(III)-reducing organisms in the group S. putrefaciens . While this group is abundant and of worldwide distribution, it is only the tip of a very large "iceberg" of metal reducers. A good example of another metal reducer is the organism GS-15 (16-18), an obligate anaerobe with substantial carbon versatility and an ability to tolerate very high concentrations of metals. In addition, we have now isolated over 200 strains of manganese reducers (MR-203 is our latest organism [Editor's question: bioengineered S. putrefaciens into 200+ strains?] ), consisting of a wide variety of different taxa, including Pseudomonas spp. Bacillus spp., and many others. These isolates are from very diverse environments, including Lake Oneida, N.Y.; Lake Michigan; Green Bay, Wis.; the Black Sea; and Lake Baikal, USSR (22, 27, 30-32). A careful coupling of field and laboratory studies will be needed before the importance of these metal-reducing microbes, and their associated activities, are adequately understood."So, again, from the 2016 paper [Link] "... a number of Shewanella strains were tested for manganese-oxidizing capacity under aerobic conditions. All were able to oxidize Mn(II) and to produce solid dark brown manganese oxides[(Mn(IV)?]." Further on they state, "The current study extends the versatility of Shewanella species and describes manganese oxidation as a trait within the genus."
R.C.