[A-CG41] Biogeochemical linkages between the surface ocean and atmosphere

[ACG41-P03] Dynamics of microorganisms and organic matter at the air-sea interface explored through the analysis of seafoam “Nami no Hana”

*Koji Hamasaki¹, Tenga Kanetoshi¹, Can Huang¹, Yoko Makabe-Kobayashi¹, Yutaka Okamoto¹, Yoko Iwamoto² (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Graduate School of Integrated Science for Life, Hiroshima University)

Keywords:aerosol, microbe, seafoam

Aerosols observed over the ocean mostly contain sea salt particles but sometimes do significant amount of organic matter and microbial particles, and these differences can greatly change the cloud-forming characteristics of aerosols. This attracts strong interest in the dynamics of marine organic aerosols. Both theoretical models and experiments have shown that cloud nucleating activity changes depending on the structure of the organic substance, such as the type of functional groups and the number of carbon skeletons. Changes in the microbial flora and their decomposition of organic matter in the ocean surface layer and in aerosols should cause changes in aerosol properties of cloud formation. However, the relationship between ocean microbial activity and physicochemical properties of marine aerosols is not fully clarified yet. Previous research has shown that microbial accumulation and selection occur during the formation of sea spray aerosols (SSA), which are generated by the bursting and atomization of bubbles at the sea surface, and that these phenomena also include microbial attachment to organic particles. It has been hypothesized that organic particles in seawater are captured (scavenged) by bubbles formed under the water, rise up subsequently, and are released into the atmosphere along with droplets from the bubbles that burst on the sea surface. At this time, it is speculated that microorganisms attached to the organic particles are also released. Therefore, the purpose of this study was to clarify what kind of microbial selection occurs during the process of SSA generation such as bubble scavenging and the subsequent bursting of the bubbles. In order to verify the scavenging effect of bubbles, we conducted analyzes and experiments using seafoam so-called ”Nami no Hana” in Japanese, an aggregate of bubbles that occurred on the Maura Coast of the Noto Peninsula during winter (December 2022). By analyzing the microbial community structure of seawater and seafoam, we identified what types of marine microorganisms are susceptible to scavenging by bubbles. Seafoams were sampled three times on 12/20, 12/22, and 12/24, and seawater was sampled four times, including 12/21. By filtering seawater and seafoam with two types of filters, we separated them into particle-attached bacteria (>3.0 μm) and free-living bacteria (0.22-3.0 μm) and recovered them. DNA was extracted from these filter samples, a partial region of the 16S rRNA gene was PCR amplified, and the relative abundance of each taxonomic group was determined by referring to a database from the sequence information. In addition, bubbling experiments using bacterial strains isolated from seafoam (Vibrio atlanticus, Microbacterium algeriense, Verrucomicrobia sp.) and artificially created particles (alginate gel) were performed to reveal whether the presence of particles facilitates the concentration of bacteria into aerosols. The number of bacterial cells trapped on the filter as SSA was counted over time (1, 3, 6, 12, and 24 hours) using a fluorescence microscope, and differences in the presence or absence of particles were compared. Based on the results of community structure analysis of seawater and seafoam, we compared the proportions of highly dominated bacterial taxa (top 12 classes) for each sample collected on the same day. It was found that Alphaproteobacteria well known as a free-living taxon was more abundant in seawater than in seafoam. On the other hand, Bacteroidia, Gammaproteobacteria, and Planctomycetes, which are more commonly distributed as particle-attached bacteria, were shown to be more abundant in seafoam samples. These results suggest that scavenging of the particles concentrates particle-attached bacteria. Also, the bubbling experiments showed that even bacterial cells alone can be released into the atmosphere as SSA. Furthermore, when sodium alginate, which forms gel-like particles, was added, the number of released bacteria increased significantly. It increased by about 4 times in V. atlanticus, about 2.8 times in M. algeriense, and more than 10 times in Verrucomicrobia sp. It was suggested that gel-like organic particles in natural seawater, such as alginate gel, are one factor that helps selection and concentration of bacteria in SSA.