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. 76