Nitrous oxide (N 2 O), also known as laughing gas, is a powerful greenhouse gas substantially contributing to global warming. As a crucial ozone-depleting substance, it plays an important role in regulating our climate. The oceans are a major natural source of N 2 O. However, our understanding of the processes involved in the turnover of N 2 O is still limited. Now a study led by researchers from the Max Planck Institute for Marine Microbiology in Bremen, Germany, and published in Limnology and Oceanography takes a leap forward in improving this understanding by unravelling the “Black Sea nitrous oxide conundrum”.
Little N 2 O escapes the Black Sea
In the ocean, large amounts of nitrous oxide are mainly produced in areas that lack oxygen. In these so-called deoxygenated waters, microorganisms capable of producing this gas feel at home. The Black Sea is the world’s largest anoxic basin, with a reservoir of oxygen-depleted water stretching from a depth of 150 meters down to more than 2000 meters. However, the Black Sea appears to emit only little N 2 O.
“There were two possible reasons for this”, explains first-author Jan von Arx. “Either there is little production of N 2 O, or the produced N 2 O is removed before it reaches the surface. We wanted to explore this by looking at the biological processes involved in nitrous oxide production and consumption, as well as identify the microorganisms responsible for the turnover of this important greenhouse gas.”
N 2 O never reaches the surface
Thus, the scientists boarded RV Poseidon and sailed to the western Black Sea to take water samples, assess the environmental conditions and set up a large series of experiments. Indeed, they discovered that active nitrous oxide turnover takes place in the Black Sea’s suboxic zone – the zone of water with little oxygen, which separates the anoxic bottom waters from the well-oxygenated surface. “Various microorganisms produced lots of nitrous oxide through different processes. However, this production was outpaced by N 2 O reduction – i.e. the conversion of N 2 O to N 2 by a different type of microorganisms”, von Arx explains. “Therefore, the small emissions of nitrous oxide from the Black Sea are likely the result of little but persistent production in the oxygenated waters, where it manages to escape consumption.”
An understudied biological filter for a dangerous gas
The microorganisms reducing N 2 O act as an efficient filter, keeping this potent greenhouse gas from reaching the atmosphere. The Max Planck scientists also managed to identify the main microorganisms involved in the process.
“On a global perspective, we unfortunately know very little about the N 2 O reduction rates in the world’s oceans”, says von Arx. “Thus, our picture of environmental cycling of this important greenhouse gas remains incomplete and more research is needed. This is especially true in the light of climate change.” Due to global warming, oxygen loss from the ocean is accelerating and volumes of oxygen-depleted waters are predicted to expand in the future. Under these conditions, nitrous oxide emissions can be expected to increase.
The scientists from Bremen are currently exploring similar questions in different oxygen-limited environments. Collecting and analysing data from many contrasting environments will allow them to obtain a fuller picture of nitrous oxide dynamics in the marine environment.
Uncertain times for the changing ocean
“Nitrous oxide is the third most abundant greenhouse gas and a strong ozone depleting substance that persists in the atmosphere for about 120 years. The ocean is an important natural source of N 2 O. Hence, we should aim to understand the dynamics of its sources and sinks there. We hope that our work can help to assess the response of N 2 O production from marine environments to ongoing climate change”, von Arx concludes.
TO THE POINT:
Limnology and Oceanography
Nitrous oxide turnover in the suboxic zone of the Black Sea
15-Aug-2025