Diversity of microbial plankton across the Three Gorges Dam of the Yangtze River,China

The Three Gorges Dam （TGD） of the Yangtze River, China, is one of the largest irrigation and hydroelectric engineering projects in the world. The effects of huge man-made projects like TGD on fauna and macrophyte are obvious, mainly through changes of water dynamics and flow pattern; however, it is less clear how microorganisms respond to such changes. This research was aimed to examine differences in microbial diversity at different seasons and locations （in front of and behind the TGD）. In addition, differences between particle-attached and free-living communities were also examined. The community structures of total and potentially active microorganisms in the water columns behind and in front of the TGD were analyzed with the DNA- and RNA-based 16S rRNA gene phylogenetic approaches over three different seasons. Clone libraries of 16S rRNA genes were prepared after ampli- fication from extracted DNA and, for some samples, after preparing cDNA from extracted rRNA. Differ- ences were observed between sites at different seasons and between free-living and particle-attached communities. Both bacterial and archaeal communities were more diverse in summer than in winter, due to higher nutrient levels and warmer temperature in summer than in winter. Particle-attached micro- organisms were more diverse than free-living communities, possibly because of higher nutrient levels and heterogeneous geochemical micro-environments in particles. Spatial variations in bacterial community structure were observed, i.e., the water reservoir behind the TGD （upstream） hosted more diverse bacte- rial populations than in front of the dam （downstream）, because of diverse sources of sediments and waters from upstream to the reservoir. These results have important implications for our understanding of responses of microbial communities to environmental changes in river ecosystems affected by dam construction.

Seasonal hydrological dynamics govern lifestyle preference of aquatic antibiotic resistome

Antibiotic resistance genes(ARGs)are a well-known environmental concern.Yet,limited knowledge exists on the fate and transport of ARGs in deep freshwater reservoirs experiencing seasonal hydrological changes,especially in the context of particle-attached(PA)and free-living(FL)lifestyles.Here,the ARG profiles were examined using high-throughput quantitative PCR in PA and FL lifestyles during four seasons representing two hydrological phenomena(vertical mixing and thermal stratification)in the Shuikou Reservoir(SR),Southern China.The results indicated that seasonal hydrological dynamics were critical for influencing the ARGs in PA and FL and the transition of ARGs between the two lifestyles.ARG profiles both in PA and FL were likely to be shaped by horizontal gene transfer.However,they exhibited distinct responses to the physicochemical(e.g.,nutrients and dissolved oxygen)changes under seasonal hydrological dynamics.The particle-association niche(PAN)index revealed 94 non-conservative ARGs(i.e.,no preferences for PA and FL)and 23 and 16 conservative ARGs preferring PA and FL lifestyles,respectively.A sharp decline in conservative ARGs under stratified hydrologic suggested seasonal influence on the ARGs transition between PA and FL lifestyles.Remarkably,the conservative ARGs(in PA or FL lifestyle)were more closely related to bacterial OTUs in their preferred lifestyle than their counterparts,indicating lifestyle-dependent ARG enrichment.Altogether,these findings enhanced our understanding of the ARG lifestyles and the role of seasonal hydrological changes in governing the ARG transition between the lifestyles in a typical deep freshwater ecosystem.