Chemical and biological interactions during nitrate and goethite reduction by Shewanella putrefaciens 200

Although previous research has demonstrated that NO3- inhibits microbial Fe(III) reduction in laboratory cultures and natural sediments, the mechanisms of this inhibition have not been fully studied in an environmentally relevant medium that utilizes solid-phase, iron oxide minerals as a Fe(111) source. To study the dynamics of Fe and NO3- biogeochemistry when ferric (hydr)oxides are used as the Fe(111) source, Shewanella putrefaciens 200 was incubated under anoxic conditions in a low-ionic-strength, artificial groundwater medium with various amounts of NO3- and synthetic, high-surface-area goethite. Results showed that the presence of NO3- inhibited microbial goethite reduction more severely than it inhibited microbial reduction of the aqueous or microcrystalline sources of Fe(III) used in other studies. More interestingly, the presence of goethite also resulted in a twofold decrease in the rate of NO3- reduction, a 10-fold decrease in the rate of NO2- reduction, and a 20-fold increase in the amounts of N2O produced. Nitrogen stable isotope experiments that utilized delta(15)N values of N2O to distinguish between chemical and biological reduction of NO2- revealed that the N2O produced during NO2- or NO3- reduction in the presence of goethite was primarily of abiotic origin. These results indicate that concomitant microbial Fe(III) and NO3- reduction produces NO2- and Fe(II), which then abiotically react to reduce NO2- to N2O with the subsequent oxidation of Fe(II) to Fe(III).