Molybdenum Limits Microbes’ Ability to Remove Harmful Nitrate from Soil

The Science

The Oak Ridge Reservation (ORR) in Tennessee was created as part of the Manhattan Project in the 1940s. This legacy left the site uniquely contaminated with acidic, high-nitrate-and high-metal substances. Microbes living naturally in this environment can use molybdenum to remove nitrate. However, the small concentration of molybdenum in the ORR limits how much nitrate the microbes can remove. Two studies report on why molybdenum is found in limited concentrations at ORR. The studies also revealed the mechanisms that some nitrate reducing microbes use to survive in these extreme conditions.

The Impact

Nitrate contamination is a serious concern. Excessive nitrate in groundwater or soil causes both environmental and health problems. Microorganisms need the metal molybdenum to remove nitrate contamination from soils. However, the ORR environment contains very low concentrations of molybdenum. This research explains how iron and aluminum-containing minerals reduce the available amount of molybdenum from ORR groundwater. The study also found a strain of microbes at ORR that is less sensitive than usual to limited molybdenum. This microbe strain can therefore reduce nitrate at ORR better than other microbes.

Summary

Molybdenum availability is crucial for nitrate removal by the denitrification pathway in microorganisms. The soluble from of molybdenum is incorporated into the cell by a protein called Mod transporter. However, when molybdenum occurs at very low concentration as observed in the highly acidic and nitrate contaminated groundwater at the ORR, it inhibits microbial denitrification. These studies demonstrated that molybdenum limitation in the ORR groundwater is a result of the incorporation of soluble molybdenum into insoluble iron- and aluminum-containing minerals. This mineralization process, called precipitation, occurs as the pH increases during the dilution of acidic and contaminated groundwater into the environment. These results show that molybdenum depletion by iron and aluminum precipitation can dramatically inhibit nitrate reduction by ORR microorganisms. By analyzing contaminated sediment cores from ORR, further evidence was found for direct co-precipitation of molybdenum, iron, and aluminum.

Researchers searched for nitrate-reducing microbes in the molybdenum-limited area of the ORR and identified a metal-resistant strain of Bacillus sp. named XG196. This strain was much less sensitive to molybdenum-limitation than other microbes from the same site. XG196 can reduce nitrate under low molybdenum concentration because it contained a novel variant of the molybdate binding protein (ModA), which is part of the Mod transporter. XG196 ModA had a much higher affinity for molybdate than other ModA proteins previously characterized in other microbes. This protein variant not only has the highest molybdenum affinity reported so far, but it is also the first ModA to be characterized from a Bacillus strain.

Funding

This work by ENIGMA (Ecosystems and Networks Integrated with Genes and Molecular Assemblies), a Science Focus Area Program at Lawrence Berkeley National Laboratory, is based on work supported by the Department of Energy Office of Science, Office of Biological and Environmental Research.