including any incorporated plutonium become available to other organisms. We do not know in what complexed form or forms plutonium is
stored in the microbial cells, but whatever form(s) these may be, it
is very probable that this complexed plutonium when released from the
microbial cells is more soluble than the relatively insoluble plutonium compounds thus far found deposited in soil; consequently, it
will be more biologically available.
This suggests that an increase
of microbial population during the growing season could result in a
sudden increase in the amount of plutonium available to other trophic
levels.
Besides making plutonium more available to other organisms, soil
microorganisms can also play a role in plutonium transport in soil.
Krasilnikov, who found that naturally occurring actinides are absorbed
by soil microorganisms
(1958),
claimed that the movement of soil
microbial cells containing natural radioactive elements determined
the migration of these substances (1967).
Thus, the soil fungus
Aspergillus niger, which absorbs and translocates plutonium, and
other plutonium-absorbing soil microorganisms, could be moved to
other locations within the soil system.
Upon death of the microbial
cells, any cellular plutonium not assimilated by another organism
would become available for translocation by water, possibly in a
chemical form which would facilitate this translocation.
The activities of mobile predators such as nematodes and protozoa
further complicate the picture.
These predators and other animals
feed on soil microorganisms and thus spread any plutonium that is
incorporated in their food.
Obviously,
the number of these predators
will be approximately proportional to the number of microorganisms
present in the soil.
It was pointed out earlier that the typical desert of western United
States is not a barren wasteland, but supports some vegetation.
The
above results and discussion show that increased plant activity
results in an increase in the soil microbial population.
This in
turn, in plutonium-contaminated soils, could increase the amount of
biologically available plutonium during the growing season as well as
cause a seasonal increase in plutonium translocation in the soil.
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