Nitrous oxide and nitrogen gas production in the Arabian Sea - a process and community based study

Lead Research Organisation: University of Warwick
Department Name: Biological Sciences


The element nitrogen (N) is key to life on Earth and it is continually being cycled between the atmosphere, biomass (animals, plants, microbes) and back to the atmosphere following death and decay. At the centre of this N cycling, on the land and in the sea, are a wide variety of microscopic organisms known as bacteria. In the atmosphere N exists largely as N2 gas but also in much smaller amounts as nitrous oxide (N2O) which is a potent greenhouse gas. Processes which remove N, as N2, can regulate the growth of plants and, indirectly, the balance of carbon dioxide (CO2) in the atmosphere and, hence, affect climate. Large areas of the global ocean are fully oxygenated or 'saturated' with oxygen (O2) but some parts are not. For example, the Black Sea completely lacks any O2 below 90 m and others such as the Benguela upwelling off south western Africa are also devoid of O2 / both these areas have oxygen minimum zones or OMZ. It is these O2 'starved' regions or OMZ that are significant for both N removal and N2O production in the global ocean. Our interest lies in that of the OMZ of the Arabian Sea which, due to its large size (that of France and Germany combined), plays a significant role in global N cycling / responsible for 20 % of N2O production and 30 % of N removal in the global ocean. While the significance of the Arabian Sea in the global N cycle is known, the metabolisms responsible for N2 and N2O production were, and are still in part, unclear. Recently, by looking a bit closer and in conjunction with N tracers (15N isotopes), we were the first to actually measure N2O production in the central Arabian Sea. Further, we demonstrated that most (>95 %) of the N2O produced could be explained simply by one pathway i.e. the metabolism of nitrite (NO2-) to N2O in the absence of O2. In addition, we measured N removal via two known paths of N2 production e.g. N2 from denitrification and N2 from anaerobic ammonium oxidation (anammox). However, a substantial portion of the N2 is coming from somewhere else and we have evidence that this extra new path of N2 production is directly coupled to the metabolism of decaying biomass. However, it is not as simple as this. One pathway of N2O formation requires some complexity to generate the high and low concentrations of N2O characteristic of the OMZ in the central Arabian Sea. Again, our 15N tracers uncovered some of this by showing that the ratio of N2 to N2O production during the metabolism of NO2- (NO2- to NO to N2O to N2) is not fixed and appears to be 'flexible'. For example, where water column N2O concentration is high, we measured a low ratio of N2 to N2O production from NO2- and vice versa where water column N2O concentration was low. Although this 'flexible' ratio explains the majority of N2O and helps redefine our understanding of N2O production in oxygen minimum zones / why this ratio should change is unknown. In this project we aim to characterise the water column at selected sites in the central Arabian Sea in terms of, for example, N2O, O2 and the bacteria driving the N-cycle. We will experimentally manipulate contrasting waters to test if the ratio of N2 to N2O production is 'fixed' or 'flexible', screen for N2 production coupled to organic matter and analyse the active bacteria involved in the metabolism of these gases by using molecular or 'genetic' techniques. A better understanding of the key processes and bacteria involved in these complex metabolisms in such an important area as the Arabian Sea should help the scientific community build better predictive climate models.


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Description Many will know that the Pacific is the largest ocean on Earth but few might appreciate that large parts of eastern Pacific contain practically no oxygen. As a consequence they are prone to producing large quantities of the three main green house gases. Our research has clearly identified both the fate and sinks of methane and novel sources of nitrous oxide.
Oceanic oxygen minimum zones are strong sources of the potent greenhouse gas N2O but its microbial source is unclear. We characterized an exponential response in N2O production to decreasing oxygen between 1 and 30 micromol of oxygen per liter within and below the oxycline (where oxygen drops rapidly) using 15N labelled nitrite, a relationship that held along a 550 km offshore transect in the North Pacific. Differences in the overall magnitude of N2O production were accounted for by archaeal functional gene abundance. A one-dimensional (1D) model, parameterized with our experimentally derived exponential terms, accurately reproduces N2O profiles in the top
350m of water column and, together with a strong 45N2O signature indicated neither canonical nor nitrifier-denitrification production while statistical modelling supported
production by archaea, possibly via hybrid N2O formation. Further, with just archaeal N2O production, we could balance high-resolution estimates of sea-to-air N2O exchange. Hence, a significant source of N2O, previously described as leakage from bacterial ammonium oxidation, is better described by low-oxygen archaeal production at the oxygen minimum zone's margins.

Oxygen minimum zones (OMZs) contain the largest pools of oceanic methane but its origin and fate are poorly understood. High-resolution (o15 m) water column profiles revealed a 300 m thick layer of elevated methane (20-105 nM) in the anoxic core of the largest OMZ, the Eastern Tropical North Pacific. Sediment core incubations identified a clear benthic methane source where the OMZ meets the continental shelf, between 350 and 650 m, with the flux reflecting the concentration of methane in the overlying anoxic water. Further incubations characterised a methanogenic potential in the presence of both porewater sulphate and nitrate of up to 88 nmol per g per day in the sediment surface layer. In these methane-producing sediments, the majority (85%) of methyl coenzyme M reductase alpha subunit (mcrA) gene sequences clustered with Methanosarcinaceae (? 96% similarity to
Methanococcoides sp.), a family capable of performing non-competitive methanogenesis. Incubations with 13C labelled methane (CH4) showed potential for both aerobic and anaerobic methane oxidation in the waters within and above the OMZ. Both aerobic and anaerobic methane oxidation is corroborated by the presence of particulate methane monooxygenase (pmoA) gene sequences, related to type I methanotrophs and the lineage of Candidatus Methylomirabilis oxyfera, known to perform nitrite dependent anaerobic methane oxidation (N-DAMO), respectively. Biological oceanic processes, principally the surface production, sinking and interior
remineralization of organic particles, keep atmospheric CO2 lower than if the ocean was abiotic. The remineralization length scale (RLS, the vertical distance over which organic
particle flux declines by 63%, affected by particle respiration, fragmentation and sinking rates) controls the size of this effect and is anomalously high in oxygen minimum
zones (OMZ). Here we show in the Eastern Tropical North Pacific OMZ 70% of POC remineralization is due to microbial respiration, indicating that the high RLS is the result of
lower particle fragmentation by zooplankton, likely due to the almost complete absence of zooplankton particle interactions in OMZ waters. Hence, the sensitivity of zooplankton to
ocean oxygen concentrations can have direct implications for atmospheric carbon sequestration. Future expansion of OMZs is likely to increase biological ocean carbon storage and
act as a negative feedback on climate change.
Exploitation Route Climate model predictions on the effects of expanding oxygen minimum zones.
Sectors Environment