Influence of the cold bottom water on taxonomic and functional composition and complexity of microbial communities in the southern Yellow Sea during the summer

The formation and presence of the cold bottom water (Yellow Sea Cold Water Mass, YSCWM) is a striking hydrological phenomenon in the southern Yellow Sea during the summer and has important effects on the marine ecosystem. To better understand its influence on microbial community structure and function, we compared the bacterial, archaeal and microeukaryotic communities in the cold water mass area (CWMA) and the southern area (SA) during the summer using amplicon and metagenomic sequencings. The habitat environment in the deep waters of the CWMA was characterized by higher salinity/DO/PO4-P, greater depth/distance to the coast, and lower levels of temperature/chlorophyll a/DIN/SiO3-Si/N:P ratio compared to that of the SA. Pure depth or distance to the coast explained a small portion of the microbial community variance, while environment explained a significant fraction of the variance when partialling the effects of depth and distance to the coast. Oligotrophic taxa (e.g. SAR11 clade Ia, Nitrosopumilus, Chloropicophyceae) dominated the deep water communities in the CWMA, while the common coastal taxa (e.g. Roseobacter strain HIMB11, Bacillariophyta, Noctilucophyceae) were more dominant in the deep waters of the SA, suggesting the great impact of the oligotrophic condition in the YSCWM on microbial communities. The microbial co-occurrence networks in the CWMA were less complex but contained a higher proportion of mutual exclusion relationship among prokaryotes; the prokaryotic α-diversity in the CWMA was significantly lower than in the SA while the microeukaryotic α-diversity was significantly higher in the CWMA, implying that prokaryotes and microeukaryotes respond to the cold water mass differently and the competition among prokaryotes was intensified under the impact of the YSCWM. Genes that relate to replication and repair accounted for a significantly lower proportion in the CWMA, which was likely an adaptation to the low carbon environment.