| The northern coast of the Yellow Sea is one of the main marine ranching areas in northern China.Most of the artificial reefs(ARs)deposited in this area were stones,mainly for stock enhancement of sea cucumber(Apostichopus japonicus).However,the bottom water has extremely low hypoxic during the period of high temperature and rainy at the end of August since 2013,which affects the distribution of nitrogen(N)and phosphorus(P)and further controls or "limits" aquatic plant primary production and biomass formation.Therefore,exploring the biogeochemical cycling of N and P nutrients in marine ranching under seasonal hypoxia is of great scientific and practical importance for regional material cycling and environmental evolution and conservation.Marine microorganisms are often at the forefront of responding to and recovering from environmental stress,and they are widely used as important indicators for evaluating environment changes induced by both anthropogenic and natural factors.When hypoxia occurs,changes in the structure of microbial communities in seawater and sediments can reflect changes in environmental conditions.However,the data on seasonal changes of microorganisms in the marine ranching areas are limited.In this study,the effects of seasonal hypoxia on N and P nutrient fractions were assessed through a case study of a hypoxic zone in a marine ranching in the northern Yellow Sea,China,and the microbial community and its influencing factors were explored using 16S rDNA high-throughput sequencing.The main findings were as follows:1.Four hydrological features-temperature(T),dissolved oxygen(DO),pH and salinity(Sal)were affected by the month,region and water layer.Specifically,month had a significant effect on T,DO,pH and Sal,region had a significant effect on T and Sal,and water layer had a significant effect on DO,T and Sal.2.Seasonal hypoxia has a strong controlling effect on N distribution and a weak effect on P distribution.Pearson correlation analysis showed that DO in seawater had a high correlation with dissolved inorganic nitrogen(DIN),bottom dissolved oxygen had a weak correlation with N in sediment,and month had a significant effect on DIN,dissolved inorganic phosphorus(DIP),total nitrogen(TN)and total phosphorus(TP)in seawater and sediment.3.The stratification in community composition and dissolved oxygen content appeared in August.Specifically,strong temperature-induced and water salinity-induced column stratification occurred in August,and the microbial diversity of the bottom water was significantly higher than that of the surface and middle layers in August.4.The results showed that environmental effects explained more of the variation in bacterial community composition in seawater as compared with spatial effects did,whereas,sedimentary bacterial communities were more closely related to spatial effects.The Redundancy analysis(RDA)showed that environmental variables such as T,Sal,and DO in seawater drove the changes in microbial community,and the spatial differences in the sediment samples were more significant.Co-occurrence network analysis showed that the complexity of the seawater networks was higher than that of the sediment networks and the sediment networks showed strong stability to resist external environmental disturbances,and the seawater microbial network in August was particularly sensitive to environmental changes.5.The metagenome function prediction showed functional groups related to carbon and nitrogen cycling were detected in the marine ranching,and there were some the regional and seasonal differences among these groups.Functions were predicted based on FAPROTAX and the abundances of functional groups chemoheterotrophy,aerobic chemoheterotrophy,chloroplasts、phototrophy,photoautotrophy,cyanobacteria,oxygenic photoautotrophy and nitrification were higher. |
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