4:30 PM - 4:45 PM
[ACG44-05] Effects of marine nitrogen fixation on the formation of atmospheric water-soluble organic nitrogen revealed by a laboratory incubation experiment
Keywords:Organic nitrogen, Marine atmospheric aerosol, Nitrogen fixation, Trichodesmium, Biogeochemical linkage between the atmosphere and the ocean
Marine atmospheric aerosols formed in the sea surface play a key role in the climate system, as they act as cloud condensation nuclei (CCN) and ice nuclei (IN) and thus can control the atmospheric radiative budget through cloud formation. Water-soluble organic nitrogen (WSON) in marine aerosols affects the physicochemical properties such as water-solubility, acidity, and light-absorbing properties of aerosol particles. A previous ship-board measurement of atmospheric and marine biological paraments in the subtropical North Pacific suggests that nitrogen-fixation in the ocean surface significantly contributed to the formation of aerosol WSON in the atmosphere. Trichodesmium is a genus of filamentous cyanobacteria and is one of the most representative nitrogen-fixing organisms, which is widely distributed in the tropical and subtropical oceans. It has been recognized that Trichodesmium contributes up to approximately 50% of marine nitrogen fixation in the global ocean. However, effects of nitrogen fixation in the ocean surface on the formation of atmospheric reactive nitrogen have not been understood. This study aims to elucidate the contribution of nitrogen-fixing organisms to the formation of atmospheric reactive nitrogen including WSON by a laboratory incubation experiment.
In the laboratory experiment, Trichodesmium erythraeum IMS101 (hereafter referred to as Trichodesmium) was cultured in artificial seawater YBC-II medium (15-L) in a polycarbonate bottle installed in an incubator, the temperature of which was set at 25oC. Atmospheric aerosol (PM2.5) and gas samples were collected on quartz fiber filters set at three stages of a 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 analyzer with a TN unit (TOC/TN). The WSON concentration was defined as the difference between the concentrations of WSTN and inorganic nitrogen (IN) which was measured by ion chromatography. To investigate the temporal variation of Trichodesmium, in vivo chlorophyll a (Chl a) fluorescence intensity was measured using a Qubit 2.0 fluorometer and chlorophyll a concentration was measured using an ultra-high performance liquid chromatography (UHPLC). In addition, heterotrophic bacterial concentration in the seawater was measured using a flow cytometry.
A life cycle of Trichodesmium was confirmed by monitoring the Chl a concentration in seawater during the incubation period of about one month. In the growth phase, the increase in the Chl a concentrations corresponded well with the increase in the DN and DOC concentrations as well as the DN/DOC ratio in the seawater. The results suggest that the growth of Trichodesmium released DN and DOC in the seawater, where DN was preferentially released more than DOC. The temporal variation of the Chl a concentration showed a significant positive correlation with the WSON concentration in the atmospheric samples. The overall results demonstrate that the growth of Trichodesmium in seawater contributed to the formation of atmospheric WSON, mostly through the nitrogen fixation process.
In the laboratory experiment, Trichodesmium erythraeum IMS101 (hereafter referred to as Trichodesmium) was cultured in artificial seawater YBC-II medium (15-L) in a polycarbonate bottle installed in an incubator, the temperature of which was set at 25oC. Atmospheric aerosol (PM2.5) and gas samples were collected on quartz fiber filters set at three stages of a 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 analyzer with a TN unit (TOC/TN). The WSON concentration was defined as the difference between the concentrations of WSTN and inorganic nitrogen (IN) which was measured by ion chromatography. To investigate the temporal variation of Trichodesmium, in vivo chlorophyll a (Chl a) fluorescence intensity was measured using a Qubit 2.0 fluorometer and chlorophyll a concentration was measured using an ultra-high performance liquid chromatography (UHPLC). In addition, heterotrophic bacterial concentration in the seawater was measured using a flow cytometry.
A life cycle of Trichodesmium was confirmed by monitoring the Chl a concentration in seawater during the incubation period of about one month. In the growth phase, the increase in the Chl a concentrations corresponded well with the increase in the DN and DOC concentrations as well as the DN/DOC ratio in the seawater. The results suggest that the growth of Trichodesmium released DN and DOC in the seawater, where DN was preferentially released more than DOC. The temporal variation of the Chl a concentration showed a significant positive correlation with the WSON concentration in the atmospheric samples. The overall results demonstrate that the growth of Trichodesmium in seawater contributed to the formation of atmospheric WSON, mostly through the nitrogen fixation process.