N and O Isotope Fractionation in Nitrate during Chemolithoautotrophic Denitrification by Sulfurimonas gotlandica

Chemolithoautotrophic denitrification is an important mechanism of nitrogen loss in the water column of euxinic basins, . but its isotope fractionation factor is not known. Sulfurimonas gotlandica GDI(T), a recently isolated bacterial key player in Baltic Sea pelagic redoxcline processes, was used to determine the isotope fractionation of nitrogen and oxygen in nitrate during denitrification. Under anoxic conditions, nitrate reduction was accompanied by nitrogen and oxygen isotope fractionation of 23.8 +/- 2.5%0 and 11.7 +/- 1.1%o, respectively. The isotope effect for nitrogen was in the range determined for heterotrophic denitrification, with only the absence of stirring resulting in a significant decrease of the fractionation factor. The relative increase in delta O-80(NO3) to delta N-15(NO3) did not follow the 1:1 relationship characteristic of heterotrophic, marine denitrification. Instead, delta O-18(NO3) in creased slower than delta N-15(NO3), with a conserved ratio of 0.5:1. This result suggests that the periplasmic nitrate reductase (Nap) of S. gotlandica strain GDI(T) fractionates the N and O in nitrate differently,than the ' membrane-bound nitrate reductase (Nar), which is generally prevalent among heterotrophic denitrifiers and is consideered as the dominant driver for the observed isotope fractionation. Hence in the Baltic Sea redoxcline, other, as yet-unidentified factoi likely explain the low apparent fractionation.