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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 GD1T, 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° and 11.7 ± 1.1°, 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 δ18ONO3 to δ15NNO3 did not follow the 1:1 relationship characteristic of heterotrophic, marine denitrification. Instead, δ18ONO3 increased slower than δ15NNO3, with a conserved ratio of 0.5:1. This result suggests that the periplasmic nitrate reductase (Nap) of S. gotlandica strain GD1T fractionates the N and O in nitrate differently than the membrane-bound nitrate reductase (Nar), which is generally prevalent among heterotrophic denitrifiers and is considered as the dominant driver for the observed isotope fractionation. Hence in the Baltic Sea redoxcline, other, as yet-unidentified factors likely explain the low apparent fractionation. [ABSTRACT FROM AUTHOR]