N and O Isotope Fractionation in Nitrate during Chemolithoautotrophic Denitrification by <i>Sulfurimonas gotlandica</i>

Chemolithoautotrophic denitrification is an important mechanism of nitrogen loss in the water column of euxinic basins, but its isotope fractionation factor is not known. <i>Sulfurimonas gotlandica</i> GD1^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‰ 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 δ¹⁸O_NO3 to δ¹⁵N_NO3 did not follow the 1:1 relationship characteristic of heterotrophic, marine denitrification. Instead, δ¹⁸O_NO3 increased slower than δ¹⁵N_NO3, with a conserved ratio of 0.5:1. This result suggests that the periplasmic nitrate reductase (Nap) of <i>S. gotlandica</i> strain GD1^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 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

Appendix D. Disturbance-induced changes in the macrofaunal community.

Disturbance-induced changes in the macrofaunal community

Appendix B. Description of sediment properties at the experiment site.

Description of sediment properties at the experiment site

Appendix C. Disturbance-induced differences in ecosystem functions.

Disturbance-induced differences in ecosystem functions

Appendix A. Schematic presentation of the hypoxic disturbance.

Schematic presentation of the hypoxic disturbance