The bacterial diversity and community composition altered in the oxygen minimum zone of the Tropical Western Pacific Ocean

The oxygen minimum zones(OMZs) are globally expanding,yet the variation pattern of microbial communities related to dissolved oxygen levels remain unclear.Spatial variability of bacterial diversity and community composition(repre sented by 16 S rRNA) of six stations was investigated within the water column in the seamount area of Tropical Western Pacific Ocean(TWPO) in May 2019.The seawater has dissolved oxygen(DO) concentration of 3.01-6.68 mg/L and the core of the oxygen minimum zones was located between the depths of 650 m and 1 750 m.The bacterial alpha-diversity showed unimodal pattern with the decreasing DO with depths and peaked in the upper oxycline(UO) of OMZs.The bacterial community structure of the mixed layer(ML) and the bottom layer clustered and separated from each other,while those of UO and the OMZ core(OM) clustered and overlapped.Overall,bacterial community composition transitioned from being Alphaproteobacteria and Gammaproteobacteria-dominant in ML to being Gammaproteobacteria and Nitrososphaeria/Deltaproteobacteria-dominant in UO and OM,and then changed to being Clostridia and unidentified Actinobacteria-dominant in the bottom layer.Moreover,both bacterial alpha-diversity and the abundant classes fitted varying sectioned functions with DO.The DO solely explained 40.37% of the variation of bacterial community composition among layers(P<0.001).The predicted function profiling showed that the water column was predominant by chemoheterotrophy,cyanobacteria,and photoautotrophy in ML,by chemoheterotrophy and nitrate/sulfide cycling in UO and OM,and by chemoheterotrophy and ferme ntation in the bottom layer.Our findings revealed the DO-associated variation in bacterial diversity and community composition,and help to clarify the potential responses of microbes and their involved biogeochemical processes to the expansion and intensification of OMZs.