1:45 PM - 3:15 PM
[ACG45-P02] Role of marine nitrogen fixation on the formation of marine atmospheric reactive nitrogen
Keywords:Atmospheric reactive nitrogen, Organic nitrogen, Marine atmospheric aerosol, Nitrogen fixation, Biogeochemical linkage between the atmosphere and the ocean
Marine atmospheric reactive nitrogen emitted from the sea surface is an important factor that affects air quality and climate over the ocean via influencing atmospheric photochemical field, particle formation, and cloud formation. Among atmospheric reactive nitrogen, water-soluble organic nitrogen (WSON) and ammonia/ammonium affect the particle formation, water-solubility, acidity, and light-absorbing properties of aerosol particles. Our previous ship-board measurement in the subtropical North Pacific suggested that reactive nitrogen, mainly dissolved organic nitrogen and ammonium, produced and exuded by nitrogen-fixing microorganisms in the ocean surface significantly contributed to the formation of aerosol WSON (Dobashi et al., 2023). Trichodesmium is a genus of filamentous cyanobacteria and is one of the most representative nitrogen-fixing microorganisms. The nitrogen-fixing cyanobacterium is widely distributed in the tropical and subtropical oceans, contributing up to approximately 50% of marine nitrogen fixation in the global ocean. However, the effects of nitrogen fixation at the ocean surface on the formation of atmospheric reactive nitrogen have not been studied by a laboratory incubation experiment so far. This study aims to elucidate the contribution of nitrogen-fixing Trichodesmium to the formation of atmospheric reactive nitrogen through a laboratory incubation experiment.
In the laboratory experiment, Trichodesmium erythraeum IMS101 (hereafter referred to as Trichodesmium) was cultured in an artificial seawater YBC-II medium in an acrylic tank installed in an incubator, whose temperature was set at 25oC. Atmospheric aerosol (PM2.5) and gas samples were collected by a three-stage NILU impactor. Atmospheric and seawater samples were collected every 24 hours. Water-soluble total nitrogen (WSTN) and water-soluble organic carbon (WSOC) concentrations in the atmospheric sample as well as dissolved nitrogen (DN) and dissolved organic carbon (DOC) concentrations in the seawater samples were measured by a total organic carbon and nitrogen analyzer. The WSON concentration was defined as the difference between the concentrations of WSTN and inorganic nitrogen (IN), measured by ion chromatography. In vivo chlorophyll a (Chl. a) fluorescence intensity was measured using a Qubit 2.0 fluorometer and Chl. a concentration was measured using ultra-high performance liquid chromatography. In addition, heterotrophic bacterial concentration in the seawater was measured using flow cytometry.
During the incubation period of approximately two months, DN and DOC concentrations in seawater increased with increasing the Chl.a concentration during the exponential phase, suggesting that DN and DOC were exuded from Trichodesmium in that phase. Throughout the experiment, ammonium and ammonia were the most abundant reactive nitrogen (avg. 70%) in both particle and gas phases, followed by WSON (avg. 20%). During the decline and death phases, the concentrations of atmospheric ammonia and gas-phase basic WSON significantly increased, suggesting the decomposition of DN and dead Trichodesmium cells and/or photochemical reactions in seawater, followed by the production of low-molecular-weight, more volatile, basic DN that was more easily transferred from seawater to the atmosphere. Overall results demonstrated that the atmospheric emission of acidic WSON was more evident during the exponential and stationary growth phases, while the emissions of basic reactive nitrogen were more significant during the decline and death phases. This study, for the first time, demonstrated the sea-to-air emissions of WSON and ammonia associated with the growth and decline of Trichodesmium. The sea-to-air flux of the reactive nitrogen relevant to nitrogen fixation was also estimated and will be discussed in the session.
In the laboratory experiment, Trichodesmium erythraeum IMS101 (hereafter referred to as Trichodesmium) was cultured in an artificial seawater YBC-II medium in an acrylic tank installed in an incubator, whose temperature was set at 25oC. Atmospheric aerosol (PM2.5) and gas samples were collected by a three-stage NILU impactor. Atmospheric and seawater samples were collected every 24 hours. Water-soluble total nitrogen (WSTN) and water-soluble organic carbon (WSOC) concentrations in the atmospheric sample as well as dissolved nitrogen (DN) and dissolved organic carbon (DOC) concentrations in the seawater samples were measured by a total organic carbon and nitrogen analyzer. The WSON concentration was defined as the difference between the concentrations of WSTN and inorganic nitrogen (IN), measured by ion chromatography. In vivo chlorophyll a (Chl. a) fluorescence intensity was measured using a Qubit 2.0 fluorometer and Chl. a concentration was measured using ultra-high performance liquid chromatography. In addition, heterotrophic bacterial concentration in the seawater was measured using flow cytometry.
During the incubation period of approximately two months, DN and DOC concentrations in seawater increased with increasing the Chl.a concentration during the exponential phase, suggesting that DN and DOC were exuded from Trichodesmium in that phase. Throughout the experiment, ammonium and ammonia were the most abundant reactive nitrogen (avg. 70%) in both particle and gas phases, followed by WSON (avg. 20%). During the decline and death phases, the concentrations of atmospheric ammonia and gas-phase basic WSON significantly increased, suggesting the decomposition of DN and dead Trichodesmium cells and/or photochemical reactions in seawater, followed by the production of low-molecular-weight, more volatile, basic DN that was more easily transferred from seawater to the atmosphere. Overall results demonstrated that the atmospheric emission of acidic WSON was more evident during the exponential and stationary growth phases, while the emissions of basic reactive nitrogen were more significant during the decline and death phases. This study, for the first time, demonstrated the sea-to-air emissions of WSON and ammonia associated with the growth and decline of Trichodesmium. The sea-to-air flux of the reactive nitrogen relevant to nitrogen fixation was also estimated and will be discussed in the session.