levels, The rate cf CO, evolution and cumulative CO) over the incubation
period were significantly reduced only at the 144 ug/g level of Pu amendment, although numbers of all classes of organisms except the fungi were
depressed below this level (Table 3). This is in marked contrast to the
results of studies with a number of other heavy metals (Drucker ct al.,
1973) such as Ag and Hg, in which respiration rate was a sensitive measure
of metal effect at levels as low as 1 g/g in soil.
Differences in the
effects of the metala may be related to differences in soil solubility as
well as toxicity, as discussed below.
It should also be noted, however,
that the effect on respiration rate was dependent upon the magnitude of
the soil respiration rate in Pu treated soil relative to untreated controls,
which, in turn, was dependent upon the initial level of microorganisms in
soil.
In soils exhibiting a higher CO. evolution rate, the reduction of
respiration rate due to Pu amendment was more pronounced.
Studies of the toxicity of other transuranic elements to soil microflora
have not been conducted.
Mechanism of Effect.
To understand the long-term effects of microorganisms
on transuranic form, it is important to distinguish, where possible,
chemical and radiation effects of the transuranics on soil microorganisms.
Pronounced initial chemical toxicity, as noted above, may result in the
development of special pathways of detoxification leading to alteration of
transuranic form. The lack of chemical toxicity may imply chemfcal
modifications of the transuranic elements through interaction with ceil
metabolitea.
In contrast, radiation resistance is aseociated with an
enhanced ability to repair radiation damage to key macromolecules without
development of new biochemical pathways leading to alteration of transuranic
form.
However, the possibilities for indirect alteration of transuranic
form would be higher for a radiation resistant organism than for an
organism which did not exhibit either radiation or chemical resistance
since, due to competitive advantage, these organisms may be expected to be
present in larger numbers in the vicinity of transuranic colloids than
less resistant organisms.
5
© CONTROL
‘
A 23950 DTPA, 10.0uCilg
A 2385u DTPA, 10:0 uCilg
295, NOs), 10.0 uCil
O 285u WNo,)4, 10.0 4uCirg
2
x
5 |— %,
wy
oO
=
=
ih
ui
Oo
oc
2a
=
2
=>
=
1
The effects of Pu on soil microorganisms may be due largely to radiation
_damage,
Schneiderman «? al, (1975) measured the effects of Pu form and
solubility on soil metabolic activity and on the types, numbers, and
resistance of soil fungi and actinomycetes in soil separately amended with
233buy (1 to 1465 ug/g) and 73%pu (0.6 ug/g) in soluble nitrate and DTPA
complex forms, and with C, N, and water to provide optimal microbial
activity.
Subsamples of soil were removed over a 95-day aerobic incubation
period to determine changes in numbers of fungi and actinomycetes and
relative water solubilities (< 0,01 ») of the Pu forms.
Comparisons of
soil fungal numbers in the presence of 23%py and 7°3pu at common radioactivity levels, but at different mass concentrations, indicated that Pu
toxictty was due to radiation rather than chemical effects (Fig. 8).
Solubility of Fu in soil influenced Pu toxicity to microorganisms with the
more soluble Pu-DPTPA forms resulting in greatest reductions in numbers.
Similar studies have not been conducted with other transuranic elements.
150
rn)
Lt
0
0
4
8
12
16
20
24
28
|
32
36
INCUBATION TIME, DAYS
Fig.
8.
um on survival of soil
Effect of different {isotopes of plutoni
fungi (Schneiderman et al., 1975).
151