Community differentiation of bacterioplankton in the epipelagic layer in the South China Sea

Abstract

The South China Sea (SCS) is the largest marginal sea in the western tropical Pacific Ocean and is characterized by complex physicochemical environments. To date, the biogeographic patterns of the microbial communities have rarely been reported at a basin scale in the SCS. In this study, the bacterial assemblages inhabiting the epipelagic zone across 110°E to 119°E along 14°N latitude were uncovered. The vertical stratification of both bacterial taxa and their potential functions were revealed. These results suggest that the water depth-specific environment is a driver of the vertical bacterioplankton distribution. Moreover, the bacterial communities were different between the eastern stations and the western stations, where the environmental conditions were distinct. However, the mesoscale eddy did not show an obvious effect on the bacterial community due to the large distance between the sampling site and the center of the eddy. In addition to the water depth and longitudinal location of the samples, the heterogeneity of the phosphate and salinity concentrations also significantly contributed to the variance in the epipelagic bacterial community in the SCS. To the best of our knowledge, this study is the first to report that the variability in epipelagic bacterioplankton is driven by the physicochemical environment at the basin scale in the SCS. Our results emphasize that the ecological significance of bacterioplankton can be better understood by considering the relationship between the biogeographic distribution of bacteria and the oceanic dynamics processes.

Keywords: bacterioplankton community; epipelagic layer; functional traits; longitudinal distribution; pyrosequencing; stratification distribution.

Figure 1

Sea surface map of the 11 sampling stations where seawater samples were collected during the cruise in the northern basin of the South China Sea. The points are profile locations labeled with code numbers

Figure 2

Richness and diversity of the bacterioplankton increase with depth. Each panel shows the depth distribution of each variable (mean ± one standard deviation) across all 11 sampling profiles. The different color circles indicate the samples at different sites

Figure 3

Mean relative abundances of the bacterial groups at each depth (5 m, n = 11; 25 m, n = 11; 75 m, n = 11; 200 m, n = 10), (a) on the phylum level, and (b) on the class level within Proteobacteria

Figure 4

Distributions of the 22 selected dominant operational taxonomic units (OTUs) across depths. Each panel shows the vertical distribution of the relative abundances of each OTU (mean ± one standard deviation) across all 11 sampling stations. The open circles indicate the samples

Figure 5

Distributions of the nine selected dominant operational taxonomic units (OTUs), which differed significantly in relative abundances between stations 5–11 and stations 1–4 at different depths. Each panel includes the depth distribution of the OTU relative abundance (mean ± one standard deviation) across the western (blue; open circles) and eastern (red; closed circles) sampling stations. An asterisk indicates that the relative abundances of the OTU were significantly different between the western and the eastern stations

Figure 6

Redundancy analysis ordination of community compositions and environmental variables. The triplot shows the relationship among the spatial and environmental variables, bacterial communities, and the specific bacterial genus components. Only specific bacteria genera with fit values >80% were shown in the triplot. NA, nitrate; NI, nitrite; P, phosphate; SI, silicate. (A) Represents 5 m depth; (B) represents 25 m depth; (C) represents 75 m depth; (D) represents 200 m depth; the numbers 1–11 represent the stations

Figure 7

Distributions of the four functional gene categories in bacterioplankton at different depths. Each panel shows the depth distribution of each functional gene category (relative abundance, mean ± one standard deviation) across all 11 sampling stations. The open circles indicate the samples

Figure 8

Predicted functions of the epipelagic bacterial communities. The depth distributions of each class of the gene function (relative abundance, mean ± one standard deviation) across all 11 sampling stations are included. An asterisk indicates that the relative abundances of the functional categories are significantly different (Kruskal–Wallis test, p < .05) among the four depths