Understanding the Variation of Bacteria in Response to Summertime Oxygen Depletion in Water Column of Bohai Sea

Abstract

Aiming to reveal the variation in bacteria community under oxygen depletion formed every summer in water column of central Bohai Sea, a time-scenario sampling from June to August in 2018 at a 20-day interval along one inshore-offshore transect was settled. Water samples were collected at the surface, middle, and bottom layer and then analyzed by high-throughput sequencing targeting both 16S rRNA and nosZ genes. Compared to the surface and middle water, oxygen depletion occurred at bottom layer in August. In top two layers, Cyanobacteria dominated the bacterial community, whereas heterotrophic bacteria became dominant in bottom water of Bohai Sea. Based on the time scenario, distinct community separation was observed before (June and July) and after (August) oxygen depletion (p = 0.003). Vertically, strict stratification of nosZ gene was stably formed along 3 sampling layers. As a response to oxygen depletion, the diversity indices of both total bacteria (16S rRNA) and nosZ gene-encoded denitrification bacteria all increased, which indicated the intense potential of nitrogen lose when oxygen depleted. Dissolved oxygen (DO) was the key impacting factor on the community composition of total bacteria in June, whereas nutrients together with DO play the important roles in August for both total and denitrifying bacteria. The biotic impact was revealed further by strong correlations which showed between Cyanobacteria and heterotrophic bacteria in June from co-occurrence network analysis, which became weak in August when DO was depleted. This study discovered the variation in bacteria community in oxygen-depleted water with further effort to understand the potential role of denitrifying bacteria under oxygen depletion in Bohai Sea for the first time, which provided insights into the microbial response to the world-wide expanding oxygen depletion and their contributions in the ocean nitrogen cycling.

Keywords: 16S rRNA; Bohai Sea; bacteria; community variation; nosZ gene; oxygen depletion.

FIGURE 1

Map showing the sampling sites in Bohai Sea along the inshore–offshore transect. The reported hypoxic area was shown in the blue dash line oval.

FIGURE 2

The vertical profile of biogeochemical parameters (temperature, salinity, Chla, DO, NH₄⁺, NO₃^–, and NO₂^–) of each water sample.

FIGURE 3

Alpha diversity of 16S rRNA and nosZ gene-encoded bacteria at each sampling time. Thick bar represents median, black point represents average, boxes represent the middle 50% of the data, and whiskers represent the upper and lower quartiles.

FIGURE 4

The community composition of 16S rRNA (phylum level) and nosZ gene denitrifiers (genus level) was revealed by pyrosequencing.

FIGURE 5

Non-metric multidimensional scaling (NMDS) plot depicting distribution pattern of 16S rRNA (A) and nosZ gene (B) bacteria. Axis defines 2D space that allows the best spatial representation of sample distance, based on binary Jaccard distance with stress = 0.154 (A), stress = 0.218 (B). The points in the figure represent different sampling times, symbols represent different water layers, and the distance between points represents the degree of difference.

FIGURE 6

CCA ordination plot showing the relationship of environmental parameters with total bacterial (16S rRNA gene) (A) and denitrifying bacteria (nosZ gene) (B). Correlations between environmental variables and CCA axes were represented by the length and angle of arrows.

FIGURE 7

The network co-occurrence shows correlations between phototrophic and heterotrophic bacteria along the succession of sampling time. Genus of Cyanobacteria was presented in red front, and genus of Bacteroides was presented in blue front.

FIGURE 8

Heatmap profile showing the nitrogen metabolism-related KO based on predicted gene copy in each sample (columns represent the 25 KEGG orthology (KO) functions, rows represent the samples, and the color intensity in the heatmap represents the abundance of the functional genes).

FIGURE 9

Neighbor-joining phylogenetic tree of the dominant (Top 20 OTUs) OTUs and the reference sequences from typical OMZs. Bootstrap values were 1000 replicates.