Stress tolerance in bacterial strains of the genus Rhodanobacter isolated from a mixed waste contaminated subsurface

Oak Ridge Integrated Field Research Challenge (ORIFRC) site is characterized by low pH and consists of high nitrate, organics and heavy metals. ORIFRC field laboratory comprises variety of contaminants (uranium, technetium, nitrate, volatile organic carbon species etc.) which are of interest to US Department of Energy. Rhodanobacter is a dominant bacterial species found at this site and ideal for remediation of such mixed contaminated sites.

A collaborative study was conducted by researchers at Florida State University, NCMR-NCCS Pune, University of Illinois, Georgia Institute of Technology and Symbiosis School of Biological Sciences to understand the physiologic basis of stress tolerance in members of the genus Rhodanobacter. The study was conducted in order to understand how bacterial strains of the genus Rhodanobacter survive and dominate in the mixed waste contaminated habitats of the ORIFRC site. To address this, a systematic analysis of relevant phenotypic properties of strains of the genus Rhodanobacter was studied.

Eight strains of Rhodanobacter were isolated from high and low contaminated zones and used for pH and nitrate utilization studies. NaCl, nitrate, nitrite and heavy metal tolerance capacity was studied for the two selected strains of R. denitrificans. Based on metals known to be present at the ORIFRC site, Rhodanobacter strains were tested for tolerance to zinc, cadmium, cobalt, nickel, copper and uranium. To determine the effect of incubation time on growth of R. denitrificans at high metal concentrations, studies were carried out with nickel and uranium since these two metals were found to be present at very high concentrations at the test site.

The results supported the growth potential of Rhodanobacter in acidic subsurface groundwater conditions and confirmed that under suitable cultivation conditions, isolated R. denitrificans strains can tolerate acidic pH consistent with ORIFRC site pH values. It was also observed that organisms adapted to stress better under conditions of high organic content. The ability of Rhodanobacter strains to grow at extremely low pH and under high nitrate and heavy metals concentrations is responsible for their dominance at the contaminated subsurface of the ORIFRC site. The data indicated that both the strains are well adapted to the eco physiological conditions of the contaminated ORIFRC site.

As bacteria from the genus Rhodanobacter are denitrifiers, their activity in the ORIFRC site subsurface is also linked to carbon and nitrogen cycling and may play a critical role in the bioremediation of uranium. Based on prior findings and the results of the current study, researchers postulated that low pH tolerance and high level of stress tolerance for a range of metals along with denitrification potential gives a selective advantage to members of the genus Rhodanobacter. Bacteria from the genus Rhodanobacter are facultative anaerobes, and this physiologic capability makes them ideal candidates for robust growth in the contaminated subsurface of the ORIFRC site. Due to their enhanced stress tolerance abilities, Rhodanobacter spp. survives at low pH and in the presence of elevated concentrations of heavy metals, nitrate and nitrite.