What role do nitrifying bacteria play in determining the ¹⁸O signatures of NO₃^- in the ocean?

I have been involved in the development of a new method for coupled nitrogen and oxygen isotope analysis of nitrate in seawater (Sigman et al., 2001; Casciotti et al., 2002). This method utilizes denitrifying bacteria to convert nitrate and nitrite to N₂O for isotopic analysis at nanomole levels. This is the first method that is capable of measuring the ¹⁸O of nitrate in seawater and has many benefits over available methods for ¹⁸O analysis of nitrate in fresh water including smaller sample size requirements and fewer analytical interferences.

Application of this method has already yielded interesting and unexpected results for the ¹⁸O of seawater nitrate. We have observed a 1:1 fractionation of ¹⁵N and ¹⁸O in denitrification and nitrate assimilation in-situ. This is inexplicable using current models of isotope fractionation in biological systems, which would predict a ratio of 1:2 based on the mass differences of ¹⁵N and ¹⁸O. I have been developing a new mechanistic model for fractionation of ¹⁵N and ¹⁸O during denitrification. This model incorporates aspects of the reaction mechanism and standard bond-energy considerations and could be readily validated through future laboratory experiments. The same theory may also apply more generally to oxygen isotope fractionation in systems such as PO₄^- utilization or SO₄^- reduction, where multiple oxygen atoms are bound to a central atom. The use of oxygen isotopes has not been widely applied to the cycling of these elements and further development of measurements and theory in this area will undoubtedly advance our understanding of many important biogeochemical processes.

Another surprising finding is that NO₃^- in deep water has a ¹⁸O signature of 0-3 permil, much lower than would be expected from the expected incorporation of O₂ and H₂O into nitrate. The deviation from the biochemical ratio indicates that oxygen isotopic exchange occurs during nitrification, via a mechanism that is still unknown. Nitrifying bacteria are the primary source of nitrate in the sea and thus are likely to play an important role in determining the distribution of ¹⁸O in seawater nitrate. Work is currently underway to measure and model variations in ¹⁸O of nitrite and nitrate produced by nitrification in laboratory experiments and natural systems. Many additional culture experiments and field measurements are needed to learn what additional constraints the ¹⁸O of seawater NO₃^- can provide for the marine nitrogen cycle.

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