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Forty strains of Gram-positive, aerobic, heterotrophic bacteria isolated from saturated subsurface lacustrine, paleosol and fluvial sediments at the US Department of Energy's Hanford Site in south central Washington State were characterized by phylogenetic analysis of 16S rRNA gene sequences and by determination of selected morphological, physiological and biochemical traits. Phylogenetic analyses of 16S rDNA sequences from subsurface isolates in the context of similar sequences from previously described bacterial species indicated that 38 of the subsurface strains were most closely related to Arthrobacter.
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These analyses implied that roughly 25% of the subsurface isolates were most closely related to the genus Arthrobacter, making them the most numerically predominant component of the culturable aerobic microbial community. In addition, Arthrobacter-like bacteria were the only types of aerobic heterotrophs to be isolated from all five lithologies that were examined at this site.
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Because arthrobacters utilize a wide variety of organic compounds, including aromatic hydrocarbons (Hagedorn & Holt, 1975;
Keddie et al., 1986; Stevenson, 1967), they are widely believed to play a significant role in the transformation of organic matter in natural environments.
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The results of this study and others (Amy et al., 1992; Balkwill & Boone, 1997; Boivin-Jahns et al., 1995; Reeves et al., 1995; Rusterholz & Mallory, 1994; Zheng & Kellog, 1994) indicate that Arthrobacter strains are often among the predominant members of the culturable microbial communities in terrestrial subsurface environments. It is not yet known whether they are also predominant members of the broader microbial communities that would be detected by direct molecular analyses of subsurface sediments. Nevertheless, this study has indicated that most of the culturable arthrobacters in the Hanford sediments are likely to be members of novel species with distinctive physiological characteristics. The ecological significance and specific functional roles of these previously uncharacterized organisms should be examined in future studies. For example, it would be interesting to know whether the disparity of organic carbon concentrations between the upper regions of the lacustrine sediments and the unoxidized zone that lies immediately beneath it at the Hanford Site is in any way related to oxidation of organic matter by subsurface arthrobacters.
(K. Okazaki, T. Kawabata, M. Nakano, S. Hayakawa - 1999)
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