Response of Hanford Site Soil Arthrobacter Isolates to Uranium Contamination

Uranium is one of the most prevalent radiological groundwater and soil contaminants at Hanford, the U.S. Department of Energy (DOE) site, Washington State, USA. Bioremediation strategies, such as injections of a soluble sodium tripolyphosphate amendment into the contaminated groundwater in order to sequester uranium through the formation of insoluble uranyl phosphate minerals, may have resulted in providing a readily available nutrient for various microorganisms that thrive under oligotrophic conditions in the form of aqueous orthophosphate; this may lead to an increase in their growth. Increased microbial activity that influences meta-autunite stability is an important geochemical factor affecting the uranium dissolution and transport in the specific environmental conditions present at the Hanford Site’s subsurface. However, the role of bacteria in phosphate remediation technology and the interactions between uranyl phosphates and the microbes are unknown. The lack of knowledge of the long-term stability of the sequestered uranium in the subsurface that may undergo subsequent remobilization severely limit the design of remediation strategies for uranium contaminated sites. Arthrobacter sp., a genus of grampositive aerobic bacteria are known for their ability to survive for elongated periods under adverse environmental conditions, account for about 25% of the microbial population in Hanford soil. Five Arthrobacter strains, previously isolated from the Hanford subsurface, were obtained from the Subsurface Microbial Culture Collection (SMCC) (Florida State University). The interaction studies consisted of acclimating different microbial strains in various substrates; subsequent pre-screening tests using direct cell count and electrical cell substrate impedance sensing (ECIS) methods helped identify the most rapidly growing and uranium resistant strain of Arthrobacter sp. to be used in the following autunite mineral leaching experiments. The results obtained at these specific uranium concentrations, Arthrobacter sp. exhibit uranium resistance for bacterial cell growth in the various substrates have shown that G954 is the fastest growing strain while no significant difference in cell growth is observed between the G975 and G968 strains. The uranium tolerance assessment experiments have helped identify G975 as the most resistant Arthrobacter strain. These experiments have indeed provided a first insight into the essential defining parameters, which are needed in support of the ensuing microbial meta-autunite dissolution experiments mimicking the oligotrophic Hanford environment. WM2010 Conference, March 7-11, 2010, Phoenix, AZ. Topic 7.1 Environmental Remediation Progress Toward Closure of Contaminated Sites.