Dual Isotopes of Nitrite in the Amundsen Sea in Summer

Abstract

Nitrite (NO 2 − ) is a key intermediate in the nitrogen (N) cycle, and its transformation is mainly accomplished by various microbial communities. However, due to few studies on the nitrite cycle, a clear assessment of its role in marine ecosystems has so far been lacking. Here, we present the first data on nitrogen and oxygen isotopic composition of NO 2 − in the Amundsen Sea in summer and explore the biogeochemical processes that influence the nitrite cycle. Extremely low δ 15 N NO 2  and abnormally high δ 18 O NO 2  were found in the upper waters of the Amundsen Sea, with δ 15 N NO 2  as low as -58.4‰ and δ 18 O NO 2  as high as 44.4‰. Enzymatic isotopic exchange reactions between nitrate and nitrite have been proposed to be responsible for these isotopic anomalies. The mirror-symmetrical variation between δ 15 N NO 2  and δ 18 O NO 2   suggests that the isotopic fractionation effects of nitrogen and oxygen are opposite in isotope exchange reactions. Dual isotopes of nitrite indicate that ammonia oxidation is the main source of nitrite, thus nitrification plays an important role in the formation of primary nitrite maximum in the upper Amundsen Sea. The nitrogen and oxygen isotopic compositions of nitrite provide technical support for clarifying multiple processes of marine nitrogen cycle. It is necessary to further understand nitrite cycle in the Southern Ocean in the future.

Keywords: isotope, nitrite, ammonia oxidation, isotope exchange, nitrogen cycle, Southern Ocean