섬진강 하구역에서 염분구배에 따른 식물플랑크톤과 박테리아의 군집특성

Abstract

We estimated the phytoplankton and bacterial community dynamics along the salinity gradients in the Seomjin River estuary, Korea, by investigating the seasonal abiotic and biotic factors at 14 sites at high and low tide during the spring tide season. We performed two types bioassays to identify the effect of additional nutrients (+N, +P, +NP) on phytoplankton growth in spring and autumn. We also performed high-resolution 16S rRNA tag pyrosequencing to obtain snapshots of the bacterial diversity and community structure along the salinity gradients in spring and autumn. The nitrate+nitrite and silicate were mainly introduced from the Seomjin River, while phosphate and ammonia were supplied weakly from the sediments of the Seomjin River mouth. In summer and winter, phytoplankton consisted mainly of diatoms(Skeletonema spp.) in saltwater. Aulacoseira sp. predominated in seawater compared to freshwater. In spring, Leptocylindrus danicus and Navicula sp. had a higher relative distribution in saltwater and freshwater, respectively. Cryptophytes(Cryptomonas spp.) predominated in most sampling sites in autumn. Bioassays confirmed that the saltwater zone near Gwangyang Bay had higher relative phytoplankton growth for +N and +NP than for +P, while growth was higher for +P and +NP than for +N in the freshwater zone. The algal bioassays and the field surveys indicated that nitrogen and phosphates could be limiting factors in the saltwater and freshwater zones in the Seomjin River estuary. The bacterial community analysis showed that bacterial diversity differed at each sampling site and season. In spring, the most widely distributed classes were Betaproteobacteria (Avg. 30.4%) and Actinobacteria (Avg. 16.2%) near the freshwater area, and Alphaproteobacteria (Avg. 12.8%), Bacilli (Avg. 10.0%), and Gammaproteobacteria (Avg. 8.5%) near the saltwater area. In autumn, Alphaproteobacteria (Avg. 40.1%) predominated, followed by Actinobacteria (Avg. 17.1%), Betaproteobacteria (Avg. 13.5%), Flavobacteria (Avg. 10.6%), Gammaproteobacteria (Avg. 8.8%), and Sphingobacteria (Avg. 2.4%). Diversity was very high at Station 9, where freshwater and saltwater species were mixed. Our study demonstrated dynamic changes in the phytoplankton and bacterial communities along tidal levels and salinity gradients.