Jiyoung Lee

First Investigation of Microbial Community Composition in the Bridge (Gadeok Channel) between the Jinhae-Masan Bay and the South Sea of Korea

Microbial community composition varies based on seasonal dynamics (summer: strongly stratified water column; autumn: weakly stratified water column; winter: vertically homogeneous water column) and vertical distributions (surface, middle, and bottom depths) in the Gadeok Channel, which is the primary passage to exchange waters and materials between the Jinhae-Masan Bay and the South Sea waters. The microbial community composition was analyzed from June to December 2016 using 16S rRNA gene sequencing. The community was dominated by the phyla Proteobacteria (45%), Bacteroidetes (18%), Cyanobacteria (15%), Verrucomicrobia (6%), and Actinobacteria (6%). Alphaproteobacteria (29%) was the most abundant microbial class, followed by Flavobacteria (15%) and Gammaproteobacteria (15%) in all samples. The composition of the microbial communities was found to vary vertically and seasonally. The orders Flavobacteriales and Stramenopiles showed opposing seasonal patterns; Flavobacteriales was more abundant in August and December while Stramenopiles showed high abundance in June and October at all depths. The genus Synechococcus reached extremely high abundance (14%) in the June surface water column, but was much less abundant in December water columns. Clustering analysis showed that there was a difference in the microbial community composition pattern between the strongly stratified season and well-mixed season. These results indicate that the seasonal dynamics of physicochemical and hydrologic conditions throughout the water column are important parameters in shaping the microbial community composition in the Gadeok Channel.

Temporal and Vertical Variation in Microbial Community Composition in Response to Physicochemical Characteristics in a Water Column of Highly Eutrophied Jinhae Bay, South Korea

Microbial communities play an essential role in marine biogeochemical cycles. Physical and biogeochemical changes in Jinhae Bay, the most anthropogenically eutrophied bay on the coasts of South Korea, are well described, but less is known about the associated changes in microbial communities. Temporal and vertical variation in microbial communities at three depths (surface, middle, and bottom) at seven time points (June to December) at the J1 sampling site were investigated on the MiSeq platform based on the 16S rRNA gene. Overall, the microbial community was dominated by Proteobacteria, Cyanobacteria, and Bacteroidetes from June to November, whereas Firmicutes were dominant in December, especially in the middle and bottom layers. The results indicate that the microbial community composition strongly varied with temporal changes in the physicochemical water properties. Moreover, the community composition differed markedly between the surface and middle layers and the bottom layer in the summer, when the water column was strongly stratified and bottom water hypoxia developed. A redundancy analysis suggested a significant correlation between physicochemical variables (i.e., temperature, salinity, and oxygen concentration) and microbial community composition. This study indicates that temporal changes in water conditions and eutrophication-induced hypoxia effectively shape the structure of the microbial community.

Hypoxia in Korean Coastal Waters: A Case Study of the Natural Jinhae Bay and Artificial Shihwa Bay

Several coastal regions in Korea have suffered from hypoxia since the 1970s. We present the first review of Korean coastal hypoxia, focusing on its spatiotemporal variation, controlling factors, and effects on marine ecosystems. The review considers the two hotspots of the natural Jinhae Bay (JB) and artificial Shihwa Bay (SB), which are referred to as “Korean dead zones.” The hypoxia in the JB is attributed to eutrophication due to domestic and land-used waste input and thermal stratification based on the naturally sluggish water circulation, whereas the hypoxia in the SB is due to eutrophication resulting from domestic, land-used, and industrial waste input and haline stratification as a consequence of the artificially created water stagnation. The bottom-water hypoxia and stratification has resulted in excessive accumulation of phosphate in both bays, leading to an imbalance in nitrogen:phosphorus ratio between surface and bottom waters. Hypoxia has also created undesirable benthic community changes in the both bays: (1) mass mortality of large species and recolonization with elevated abundances of opportunists in JB, and (2) decrease of the number of species, abundance, and diversity of benthic communities in SB. Therefore, it behooves us to pay attention to these environmental changes. This review will be helpful in determining the direction of future studies of Korean coastal hypoxia.