The Changjiang River Estuary (CRE) in the East China Sea suffers from seasonal hypoxia in summer. The vertical distributions and seasonal changes of microbial communities in the CRE were well documented. However, little is known about the diurnal changes of bacterial communities in the hypoxic zone of the CRE. Here, 16S rRNA gene analysis was used to explore the changes of bacterial communities in the oxic surface and hypoxic middle seawater layers during 24 h in the CRE. Significant differences between the hypoxic and oxic layers were observed: the phyla Cyanobacteria, Bacteroidetes and Acidimicrobiia were enriched in the oxic layer, whereas the phylum SAR406 and the class Deltaproteobacteria were more abundant in the hypoxic layer. In addition, some subtle diurnal variations of the bacterial relative abundance were found in both two layers. The relative abundance of Synechococcus increased at night, and this change was more obvious in the hypoxic layer. The similar trend was also found in some phototrophic and several heterotrophic bacteria, such as Rhodobacteraceae, OM60 and Flavobacteriaceae. Their relative abundances peaked at 16:00 in the oxic layer, while the relative abundances peaked at around 7:00 and decreased until 13:00 in the hypoxic layer. Together, the results of the present study suggest that some photosynthetic bacteria and several heterotrophic bacteria have similar diurnal variations implying the light and physicochemical heterogeneity in the course of a day are important for bacterial diurnal changes in the CRE.

东海长江口（CRE）在夏季遭受季节性缺氧。CRE 中微生物群落的垂直分布和季节性变化有据可查。然而，对 CRE 缺氧区细菌群落的昼夜变化知之甚少。在这里，16S rRNA 基因分析用于探索在 CRE 中 24 小时内含氧表面和缺氧中间海水层中细菌群落的变化。观察到缺氧层和好氧层之间的显着差异：蓝藻门、拟杆菌门和酸性微生物门在好氧层中富集，而 SAR406 门和 Deltaproteobacteria 类在低氧层中更丰富。此外，在两层中都发现了细菌相对丰度的一些细微的昼夜变化。聚球藻在夜间增多，这种变化在缺氧层更为明显。在一些光养细菌和几种异养细菌中也发现了类似的趋势，如红杆菌科、OM60 和黄杆菌科。它们的相对丰度在好氧层的 16:00 达到峰值，而相对丰度在 7:00 左右达到峰值，并在缺氧层下降到 13:00。总之，本研究的结果表明，一些光合细菌和几种异养细菌具有相似的昼夜变化，这意味着一天中的光和物理化学异质性对于 CRE 中细菌的昼夜变化很重要。