RECYCLING VS. LOSS OF FIXED NITROGEN IN THE BAY OF BENGAL

The Bay of Bengal (BoB) harbors one of four major oceanic oxygen minimum zones (OMZs). While other OMZs act as sinks for marine fixed nitrogen (N) via N₂ production, N loss in BoB is minimal. A previous study in the northern BoB found that N loss was limited by nitrite (NO₂⁻) availability due to efficient NO₂⁻ oxidation at nanomolar O₂ levels. Here, we expand this analysis to the southern half of BoB. In April/May 2024, aboard the R/V Sonne (SO305 BIOCAT), we sampled 8 stations, spanning 7°N to 15°N and 84°E to 89°E, revealing a south-to-north decline in O₂ levels that reached <1 µM north of 13°N. Microbial nitrogen turnover was evaluated using ¹⁵N-labeled substrates to assess rates of anammox, denitrification, nitrate (NO₃⁻) reduction, and NO₂⁻ oxidation. Our findings indicate that BoB contains "hotspots" with significant N loss, primarily driven by anammox. Peak anammox rates reached 8 nM N d⁻¹ in incubations with ¹⁵NO₂⁻, coinciding with the detection of a secondary NO₂⁻ maximum (0.35 µM). At most stations, however, nitrogen recycling processes outpaced N loss. While NO₃⁻ reduction was detected throughout the transect, the resulting NO₂⁻ was rapidly consumed by nitrite oxidation, which hence outcompeted N₂-producing anammox bacteria. Ongoing meta-genomics analyses will further complement these results. In conclusion, this study offers one of the most comprehensive assessments of nitrogen cycling in the BoB, confirming the potential for N loss at nanomolar O₂ yet highlighting NO₂^- accumulation as critical for changing the balance between recycling and loss of fixed N.

Presentation Preference: Oral

Primary Presenter: Victor Fernandez Juarez, University of Southern Denmark (SDU) (victor.fj@biology.sdu.dk)

Authors:

Víctor Fernández-Juárez, University of Southern Denmark (SDU)  (victor.fj@biology.sdu.dk)

Lubrina Levin, University of Southern Denmark (SDU)  (lubrina@biology.sdu.dk)

Peihang Xu, University of Southern Denmark (SDU)  (peihang@biology.sdu.dk)

Isabell Schlangen, University of Southern Denmark (SDU)  (schlangen@biology.sdu.dk)

Mats Jacobsen, University of Southern Denmark (SDU)  (matsjacobsen@biology.sdu.dk)

Hermann W. Bange, GEOMAR Helmholtz Centre for Ocean Research Kiel  (hbange@geomar.de)

Carolin Löscher, University of Southern Denmark (SDU)  (cloescher@biology.sdu.dk)

Laura Bristow, University of Gothenburg (GU)  (laura.bristow@gu.se)

Bo Thamdrup, University of Southern Denmark (SDU)  (bot@biology.sdu.dk)