RELATIONSHIP OF MICROBIAL PROCESSES 319 



the soil system by using techniques specifically tailored to metabolites identified from the 

 simpler in vitro systems. 



Application of enrichment techniques to the isolation of plutonium-resistant fungi, 

 which have been demonstrated (previous section) to be the most resistant class of 

 microorganisms, and actinomycetes from soil with the use of starch as a carbon source 

 (Schneiderman et al., 1974) resulted in the isolation of 14 fungal cultures and 13 cultures 

 of actinomycetes distinct in colonial morphology. Of these, 7 of the actinomycetes and 5 

 of the fungal isolates were capable of growth at 100|Ug/ml plutonium as the soluble 

 DTPA complex. There appeared to be a succession of actinomycete types in the soil 

 during incubation, as indicated by the different colony morphologies obtained from 

 enrichments after 4 and 25 days of incubation. Although this phenomenon may have 

 resulted from changes in the soil arising from the production of metabolites or chemical 

 degradation products, it may also have resulted from a response to the presence of 

 plutonium. Only one actinomycete isolate was found which was common to enrichments 

 from both incubation periods, and this organism was present at all plutonium 

 concentrations in the media. In contrast, the fungal isolates exhibited six common 

 morphological types regardless of incubation period. 



Subsequent enrichment studies by R. A. Pelroy, Battelle-Northwest (unpublished 

 data. 1976), have resulted in the isolation of 30 distinct cultures of bacteria from soil. Of 

 these. 11 were resistant to plutonium at concentrations as high as lOO^L/g/ml. These 

 studies also indicate that carbon source as well as soil plutonium concentration will play a 

 role in determining the types and numbers of plutonium-resistant microorganisms present 

 in soil, which provides presumptive evidence that microbial metabolites, which will differ 

 with carbon source, may play a role in plutonium resistance. This subject is discussed in 

 the next section. The presence of plutonium-resistant organisms is apparently related to 

 factors that may be expected to vary with soil type and environmental conditions. Again 

 similar studies have not been conducted with other transuranic elements. 



Microbial Transformations 



Several means exist whereby microorganisms can transform trace metals in soil. These 

 may be generalized to (1) direct mechanisms, such as alteration in valence state or 

 alkylation; (2) indirect mechanisms, such as interactions with normal metabolites or 

 microbial alterations of the physicochemical environment: and (3) cycling mechanisms, 

 such as uptake during cell growth and release on cell decomposition. In the last case, any 

 combination of indirect and direct methods of alteration may be operational. Although 

 there have been no studies conducted to date that would allow the unequivocal 

 separation of these mechanisms, studies have been conducted that demonstrate the 

 alteration of plutonium form in vitro by soil microorganisms and provide evidence for 

 transformation of plutonium. 



Direct Transformations. The potential for direct transformation of the transuranic 

 elements through alteration of valence state or alkylation is ditTicult to assess. Although 

 the transuranic elements have the potential for existing in aqueous solution in several 

 valence states, information is not available to assess the role of soil microflora in direct 

 alteration of valence. More information is available regarding the mechanism of metal 

 alkylation. 



Alkylation of metals involving the alkyl donor methyl cobalamine and other alkyl 

 cobalamines has been clearly demonstrated for mercury, arsenic, and platinum (Wood, 



