Molybdenum Availability Is Key to Nitrate Removal in Contaminated Groundwater Environments

Abstract

The concentrations of molybdenum (Mo) and 25 other metals were measured in groundwater samples from 80 wells on the Oak Ridge Reservation (ORR) (Oak Ridge, TN), many of which are contaminated with nitrate, as well as uranium and various other metals. The concentrations of nitrate and uranium were in the ranges of 0.1 M to 230 mM and <0.2 nM to 580 M, respectively. Almost all metals examined had significantly greater median concentrations in a subset of wells that were highly contaminated with uranium (>126 nM). They included cadmium, manganese, and cobalt, which were 1,300-to 2,700-fold higher. A notable exception, however, was Mo, which had a lower median concentration in the uranium-contaminated wells. This is significant, because Mo is essential in the dissimilatory nitrate reduction branch of the global nitrogen cycle. It is required at the catalytic site of nitrate reductase, the enzyme that reduces nitrate to nitrite. Moreover, more than 85% of the groundwater samples contained less than 10 nM Mo, whereas concentrations of 10 to 100 nM Mo were required for efficient growth by nitrate reduction for two Pseudomonas strains isolated from ORR wells and by a model denitrifier, Pseudomonas stutzeri RCH2. Higher concentrations of Mo tended to inhibit the growth of these strains due to the accumulation of toxic concentrations of nitrite, and this effect was exacerbated at high nitrate concentrations. The relevance of these results to a Mo-based nitrate removal strategy and the potential community-driving role that Mo plays in contaminated environments are discussed.

Determining Roles of Accessory Genes in Denitrification by Mutant Fitness Analyses

“…While physically almost identical, they are different chemically, and WO 4 2Ϫ is a very effective antagonist of Mo uptake and metabolism. Mo limitation is of interest because soil and freshwater environments often contain Mo at very low concentrations that naturally limit denitrification and can lead to persistence of nitrate contamination (27,28). Hence, we report here a comprehensive analysis of gene phenotypes related to denitrification in RCH2 by using genome-wide fitness profiling.…”

Global Isotope Metabolomics Reveals Adaptive Strategies for Nitrogen Assimilation

“…46 The strain N2A2 is similar to the RCH2 strain in that it contains the full denitrification pathway where nitrate is converted to nitrite and eventually becomes nitrogen gas. This study found that when N2A2 undergoes denitrification, nitrite tends to build up in the presence of groundwater contamination.…”

Iron‐ and aluminium‐induced depletion of molybdenum in acidic environments impedes the nitrogen cycle

“…This Mo depletion process caused by iron or aluminium minerals is pH-dependent where molybdate concentration decreases as the pH increases from pH 3.1 to 6.3 for Fe precipitates, or from pH 5.5 to 6.7 for Al precipitates (Allada et al, 2002;Gustafsson, 2003;Smith et al, 2005;Goldberg, 2010;Paikaray and Hendry, 2013). However, it is not clear if these mechanisms are effective at depleting molybdate from solution in a regime that limits Mo bioavailability in natural environments (Thorgersen et al, 2015). At Oak Ridge Reservation (ORR) in Tennessee, USA between 1951 and 1983, the U.S. government disposed of millions of litres of waste containing nitric acid and toxic metals.…”

“…In 1997, an analysis of the groundwater in 46 wells surrounding the parking lot revealed that there was a plume of nitrate that extended approximately 1 km to a depth of over 100 m (Kornegay et al, 1994;Jones, 1998). In the most recent survey at ORR in 2015, groundwater from wells in the contamination plume were still at low pH range (pH 3-5) and contained high concentrations of nitrate (up to 230 mM) and various metals, including uranium (up to 0.6 mM) (Smith et al, 2015;Thorgersen et al, 2015). Interestingly, of 26 metals analyzed, molybdenum (Mo) was the only metal that had a lower median concentration (<< 1 nM) in the highly contaminated wells compared with pristine wells (up to 330 nM) (Thorgersen et al, 2015).…”

“…In the most recent survey at ORR in 2015, groundwater from wells in the contamination plume were still at low pH range (pH 3-5) and contained high concentrations of nitrate (up to 230 mM) and various metals, including uranium (up to 0.6 mM) (Smith et al, 2015;Thorgersen et al, 2015). Interestingly, of 26 metals analyzed, molybdenum (Mo) was the only metal that had a lower median concentration (<< 1 nM) in the highly contaminated wells compared with pristine wells (up to 330 nM) (Thorgersen et al, 2015). In essence, the highly contaminated wells were depleted in Mo and the reason for Mo depletion and its possible biological effects are the focus of the present study.…”