Date Published: June 6, 2019
Publisher: Public Library of Science
Author(s): Dong-Mei Wu, Qiu-Ping Dai, Xue-Zhu Liu, Ying-Ping Fan, Jian-Xin Wang, Raffaella Casotti.
Bacterioplankton play a key role in the global cycling of elements. To characterize the effects of hypoxia on bacterioplankton, bacterial community structure and function were investigated in the Changjiang Estuary. Water samples were collected from three layers (surface, middle, and bottom) at ten sampling sites in the Changjiang Estuary hypoxic and non-hypoxic zones. The community structure was analyzed using high-throughput sequencing of 16S rDNA genes, and the predictive metagenomic approach was used to investigate the functions of the bacterial community. Co-occurrence networks are constructed to investigate the relationship between different bacterioplankton. The results showed that community composition in hypoxic and non-hypoxic zones were markedly different. The diversity and richness of bacterial communities in the bottom layer (hypoxic zone) were remarkably higher than that of the surface layer (non-hypoxic). In the non-hypoxic zone, it was found that Proteobacteria, Bacteroidetes, and Flavobacteriia were the dominant groups while Alphaproteobacteria, SAR406 and Deltaproteobacteria were the dominant groups in the hypoxic zone. From the RDA analysis, it was shown that dissolved oxygen (DO) explained most of the bacterial community variation in the redundancy analysis targeting only hypoxia zones, whereas nutrients and salinity explained most of the variation across all samples in the Changjiang Estuary. To understand the genes involved in nitrogen metabolism, an analysis of the oxidation state of nitrogen was performed. The results showed that the bacterial community in the surface layer (non-hypoxic) had more genes involved in dissimilatory nitrate reduction, assimilatory nitrate reduction, denitrification, and anammox, while that in the middle and bottom layers (hypoxic zone) had more abundant genes associated with nitrogen fixation and nitrification. Co-occurrence networks revealed that microbial assemblages in the middle and bottom layers shared more niche spaces than in the surface layer (non-hypoxic zone). The environmental heterogeneity in the hypoxic and non-hypoxic zones might be important environmental factors that determine the bacterial composition in these two zones.
Bacterioplankton are important microorganisms in marine ecosystems, that play important roles in biogeochemical cycling, energy flow, and the ocean food web. Bacterioplankton in aquatic ecosystems are very sensitive to changes in environmental conditions, and thus bacterial community composition can act as an environmental indicator. Studies of the relationships between physicochemical factors and bacterial community structures can enhance our understanding of microbial ecology. Additionally, investigations of functional potential of bacteria will assist in understanding their roles in biogeochemical cycling.
Bacterial community structure was significantly different between hypoxic and non-hypoxic areas. Meanwhile, the bacterial richness and diversity were decreased with water depth. We detected 37 phylum, including Proteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes and unclassified, as well as others with low abundance. Alphaproteobacteria and Gammaproteobacteria were the predominant classes in all water samples. In the hypoxic area, SAR406 and Deltaproteobacteria were the dominant groups, while Flavobacteriia, Acidimicrobiia, and Beltaproteobacteria were dominant groups in the non-hypoxic area. RDA results showed that DO had the greatest impact on the bacterial community in the hypoxic zone. These findings expand our current understanding on bacterial structure and function in hypoxic and non-hypoxic areas of the Changjiang Estuary.