Microbial Diversity in Groundwater and Its Response to Seawater Intrusion in Beihai City, Southern China

Abstract

Seawater intrusion is a major concern commonly found in coastal aquifers worldwide. Because of the intense aquifer exploitation and land-based marine aquaculture in the coastal area of Beihai City, Guangxi Zhuang Autonomous Region, China, numerous underground aquifers in this area have been affected by seawater intrusion. However, the microbial communities in freshwater aquifers and their response to seawater intrusion are still unclear. In this study, groundwater from three aquifers was collected from three monitoring sites at different distances from the coastline in the coastal area of Beihai City, and the hydrochemical characteristics of these groundwater samples and the structure of the associated microbial communities were analyzed. The Cl^- concentration of the samples indicated that seawater intrusion had occurred in the research area up to 1.5 km away from the coastline, but the monitoring site 2 km away from the coastline had yet to be affected. Statistical analysis showed that the bacterial communities in different groundwater aquifers were significantly correlated with the Cl^- concentration, thereby suggesting that the extent of seawater intrusion might be one of the primary factors shaping bacterial composition in groundwater of this area, but the composition and distribution of archaea did not show a significant response to seawater intrusion and presented no apparent correlation with the Cl^- concentration. α-, γ-Proteobacteria and Bacteroidota were the dominant bacterial lineages, accounting for about 58-95% of the bacterial communities. Meanwhile, the predominant archaeal taxa were mainly composed of Crenarchaeota, Nanoarchaeota, and Thermoplasmatota, as accounting for 83-100%. Moreover, there was significant spatial heterogeneity of microbial communities in the aquifers affected by varying degrees of seawater intrusion. The microbial communities inhabiting the unconfined aquifer were influenced by the geochemical fluctuation caused by seawater infiltration from land-based marine aquaculture ponds and the diffusion of eutrophic surface water. In contrast, changes in microbial community structure in the confined aquifers were closely related to the environmental gradient caused by different degrees of seawater intrusion. In addition, we also found that the tidal cycle did not significantly affect the structure of microbial communities inhabiting confined aquifers that had been long affected by seawater intrusion.

Keywords: Beihai City; aquifers; groundwater; microbial diversity; seawater intrusion.

Figure 1

Schematic diagram of the study area and sampling location. (A) The black rectangle in the lower part of the plot indicates the location of Beihai City. (B) The southern coastal area of Beihai City and the location of the three monitoring sites involved in this study, the location of seawater (SW) collected from the Beibu Gulf and the location of groundwater discharge site (ZLV-CI) from the Zhuoluo beach. (C) The location of the three monitoring sites (G1, G2 and G3) illustrated in (B).

Figure 2

Variation of the Cl⁻ and SO₄²⁻ concentrations and tide level in different underground aquifers of the three monitoring sites. Blue bars in the plot represent the Cl⁻ concentration and the orange bars represent the SO₄²⁻ concentration. The black solid line above each graph indicates the tidal level of the Beihai Port of Beihai City. The green arrow indicates when microbial sampling was performed. The left y-axis represents the concentrations of Cl⁻ and SO₄²⁻, the right y-axis represents the tidal level, and the x-axis represents time. The tidal data in the figure was obtained from the China oceanic information network (http://www.nmdis.org.cn). G1, G2 and G3 indicate monitoring site. UC, unconfined aquifer; CI, confined aquifer I; CII, confined aquifer II.

Figure 3

Pearson correlation analysis between the Cl⁻ concentration and the α diversity indices of bacteria and archaea. The x-axis represents the Cl⁻ concentration (mg/L) and the y-axis represents the α diversity index value. The R and value of p in the plot were obtained by Pearson correlation analysis, the solid blue line is the linear regression fit curve and the shaded areas represents the 95% confidence intervals. The data of nine samples including the high tide samples of G1 monitoring sites and the high tide samples of the G2 and G3 were used for this analysis.

Figure 4

NMDS Analysis based on Weight-UniFrac distance matrix. (A) NMDS analysis of bacterial communities; (B) NMDS analysis of archaeal communities. The dashed lines divide the groundwater samples from the monitoring wells G1, G2 and G3 into different groups. Statistical analyses supported the groups with statistical significance (Supplementary Table 5). G1, G2 and G3 indicate monitoring site. ZLV, groundwater outlet at Zhuoluo; SW, seawater; UC, unconfined aquifer; CI, confined aquifer I; CII, confined aquifer II; H, high tide; L, low tide.

Figure 5

Alluvial bar plot of bacterial community composition and relative abundances. Only the top 10 classes/orders with relative abundance greater than 1% in all the samples are shown with the remaining taxa grouped together as “others” (Supplementary Figure 3A). G1, G2 and G3 indicate monitoring site. ZLV, groundwater outlet at Zhuoluo; SW, seawater; UC, unconfined aquifer; CI, confined aquifer I; CII, confined aquifer II; H, high tide; L, low tide.

Figure 6

Bubble chart of the relative abundance of bacterial lineages at the genus level. The graph shows bacterial lineages with relative abundance greater than 3% in all of the samples, bubbles of different sizes represent the relative abundance of each group, and the relative abundance ratio represented by the bubble size is listed on the right side of the graph. Aggregation was performed at the genus level, some groups are annotated only to family in the Silva (138) database, so their names at genus level are not indicated in the figure. G1, G2 and G3 indicate monitoring site. ZLV, groundwater outlet at Zhuoluo; SW, seawater; UC, unconfined aquifer; CI, confined aquifer I; CII, confined aquifer II; H, high tide; L, low tide. BCP: Burkholderia-Caballeronia-Paraburkholderia.

Figure 7

Alluvial bar plot of archaeal community composition and their relative abundance. Only the top 10 groups with a relative abundance greater than 0.1% in all the samples are shown in the figure, and the remaining taxa were grouped together as “others” (Supplementary Figure 3B). DSEG: Deep Sea Euryarchaeotic Group. G1, G2 and G3 indicate monitoring site. ZLV, groundwater outlet at Zhuoluo; SW, seawater; UC, unconfined aquifer; CI, confined aquifer I; CII, confined aquifer II; H, high tide; L, low tide.