Microbial transformations of plutonium

Echoing one of the conference’s up-and-coming areas—and a research topic that was more subtext than theme at the 2003 conference in Albuquerque—A.J. Francis of Brookhaven National Laboratory gave an enlightening talk on aspects of the metabolic processes by which soil microbes—bacteria and fungi— can become a key factor in environmental actinide chemistry. Sponsored by DOE’s Environmental Remediation Sciences Program, this research complemented studies by Annie Kersting, Mary Neu, and other investigators—some presented in poster format—who have begun to carve out a better understanding of what may well be the next frontier in actinide environmental remediation: actinide biogeochemistry.

Francis’ presentation springboarded from the issue of waste-repository remediation: the complex abiotic soil chemistry of various DOE waste sites further complicated by the presence of diverse colonies of microbes, most of which seem to thrive regardless of the presence of alpha- and gamma-emitting radionuclides. By sampling soil at Los Alamos’ transuranic waste burial site at TA-54, Francis and his coinvestigators studied both aerobic (employ molecular oxygen [O₂] as the primary electron-acceptor in their metabolism) and anaerobic (metabolize carbohydrates and fats in the absence of O₂) bacteria, to assess the effect of their metabolisms on the changes in actinide soil chemistry provoked by biotransformation.

The inclusion of both metabolic types is important because it tends to account for processes at different soil depths; aerobes more likely to be found in superficial soil layers where oxygen is more available, anaerobes somewhat more prevalent in deeper soil layers. When one considers that there are on the order of several million of each type of bacterial cell per gram of soil (plus abundant fungal cells per gram), the notion that “life matters” in actinide soil chemistry is difficult to ignore.

Given that such microbial numbers are also characteristic of sites such as the Waste Isolation Pilot Plant in New Mexico and Yucca Mountain in Nevada, the group’s experiments attempted to characterize the interactions of a number of bacterial species with a variety of plutonium (Pu) oxidation states, and their results suggest not only that bacteria should be counted but also that bacteria count.

Continued...