Rock and the role of nitrogen in the iron cycle

Background/Question/Methods

Iron is a nutritional requirement for life.  However for many microorganisms iron is not only a nutritional necessity, but also serves as a source of energy or as a terminal electron acceptor in respiratory microbial metabolisms.  Microbially-catalyzed iron redox reactions between the Fe(III) and Fe(II) valence states play a fundamental role influencing modern environmental biogeochemistry in oxic, suboxic, and anoxic zones of aquatic, terrestrial, and subsurface environmental systems.  In the last decade, the couple between nitrogen and iron biogeochemical cycles has been recognized in anaerobic environments.  To date, a diversity of microorganisms in the domain Bacteria and Archaea have been identified from various surface and subsurface environments that are capable of Fe(II) oxidation coupled to denitrification or nitrate reduction to NO₂^- or NH₄⁺.  Some of these microorganisms are capable of oxidizing solid-phase Fe(II) bearing minerals including the products of microbial Fe(III) reduction.  However, N-mediated Fe(II) oxidation processes are restricted to areas where oxidized nitrogen species, nitrate and nitrite, are present.  Recent studies indicate that a new pathway coupling ammonium oxidation with Fe(III) reduction, Feammox, results in the production of dinitrogen gas, nitrate, or nitrite.

Results/Conclusions

Together the production of oxidized nitrogen species (nitrate) under anoxic conditions allows for the production of an electron acceptor that could be utilized by Fe(II) oxidizing bacteria.  The oxidation of Fe(II) in suboxic and anoxic environments closes a gap in the iron redox cycle providing for iron redox cycling in environments devoid of oxygen.  Feammox and N-dependent Fe(II) oxidation are also mechanisms that play a role in controlling the loss of N as N₂ or the retention of N as NH₄⁺ in environmental systems.  Together these microbially-mediated processes would not only play an important role in modern biogeochemical cycling, but likely played a significant role throughout geologic time.