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Microorganisms and microbial activity affect the biogeochemical cycling and overall environmental behavior of major elements such as C, N, P, S, and transition metals such as Mn and Fe. By extension, microorganisms are expected to affect the environmental behavior of minor and contaminant metals. Recent research has demonstrated that this is indeed true for less common elements, including the actinides. Naturally occurring bacteria can influence the speciation and solubility of actinides through several mechanisms, including the following: (1) intra- or extracellular accumulation, (2) direct or indirect redox, (3) direct or indirect pH changes, (4) production of mineral acids, organic acids, and chelators, (5) biomineralization, and (6) biocolloid formation. In turn, actinides can induce physiological changes to bacteria, such as the excretion of specific agents and inhibiting growth. We are studying these processes both to increase our knowledge of actinide chemistry and to improve our ability to predict the fate of actinides in the environment. A primary goal is to provide the underlying science needed to develop biostabilization and bioremediation technologies to restore actinide-contaminated sites. In this paper we present recent research results on several specific actinide-bacteria systems and conclusions on how those interactions can stabilize or destabilize actinides in subsurface and soil environments. [ABSTRACT FROM AUTHOR]