RELATIONSHIP OF MICROBIAL PROCESSES 325 



section; also Alexander, 1971). In either case the plutonium was not in the form initially 

 added. Thus applications of gel-permeation chromatography, thin-layer chromatography, 

 and thin-layer electrophoresis indicate that soil microorganisms are capable, through 

 simple expressions of metabolic potential, of changing the chemical form of Pu— DTPA 

 with the resulting formation of plutonium complexes exhibiting a range in chemical 

 properties. Differences in plutonium distribution in microbial systems and in plutonium 

 form resulted from both simple interaction with metabolites and perhaps more specific 

 processes. These differences were dependent on organism type, metabolism, and 

 plutonium resistance. Investigations are presently under way with pure cultures of these 

 soil microorganisms to define complexation mechanisms. Detailed study is being directed 

 toward those organisms > producing exocellular metabolites which form plutonium 

 complexes that are soluble on elution through soil. 



Although published information on the transformation of transuranic elements other 

 than plutonium is limited, it is likely that transformations similar to that of plutonium 

 will occur. The extent of these transformations will be dependent on the solubility of the 

 element, its availability to microorganisms, its toxicity to microorganisms, and its 

 potential for complexation. Investigations are currently under way with pure cultures of 

 soil microorganisms isolated in the same manner as the mixed cultures described above. 

 Exocellular complexes mobile in soil columns are being chemically characterized for 

 detailed study. Although microbial interactions remain to be elucidated, the solubility 

 and potential for complexation may be preliminarily assessed from known chemistry 

 (Table 7). It is evident that the transuranic elements form DTPA complexes with 

 stabihties similar in magnitude to Pu— DTPA over environmental pH ranges. It can be 

 concluded that complexation with organic ligands produced by soil microflora is highly 

 probable, and investigations to identify and characterize the indirect processes and the 

 ligands responsible for complexation of plutonium in soil are equally applicable to those 

 of other transuranic elements. 



Cycling During Decomposition. A final process whereby the soil microflora may play a 

 role in transformation of the transuranic elements involves the biological uptake (plants 

 and microorganisms) of the elements and subsequent release on decomposition. Several 

 studies have demonstrated plant uptake of plutonium and americium and incorporation 

 into aboveground tissue. These tissues, deposited on soil either through Utter fall or 

 agricultural incorporation of crop residues, will be subject to microbial decomposition. 

 Furthermore, recent studies (Wildung and Garland, 1974) have indicated that barley roots 

 (uncontaminated with soil particles) contained three to eight times as much plutonium as 

 the shoots. The roots of plants are in intimate contact with the soil and can be expected 

 to decompose rapidly (weeks) under appropriate conditions of temperature and moisture 

 even in arid regions (Wildung, Garland, and Buschbom, 1975). Relatedly, microorganisms, 

 owing to their distribution in soil and large absorptive surface, compete efficiently with 

 plants for ions in soil (Alexander, 1961). Studies described in a previous section 

 demonstrated the association of plutonium with microbial cells. Growth of microbial 

 cells, a significant portion of the soil biomass, may therefore represent an important 

 mechanism for biological incorporation of the transuranic elements. Decomposition of 

 microbial cells generally proceeds at a more rapid rate than that of plant tissues. 



Little is known of the form of the transuranic elements in plant or microbial tissues, 

 of the form, rate, and extent of the transuranics released on decomposition of these 

 tissues, or of the chemical reactions governing transuranic solubility after decomposition. 



