Identification of groundwater microbial communities and their connection to the hydrochemical environment in southern Laizhou Bay, China

The microbial community plays an important role in the biogeochemical cycle in coastal groundwater ecosystems. However, the composition and controlling factors of the microbial community in coastal closed groundwater systems (CCGSs) with high salinity have rarely been studied. Here, we investigated and analyzed the hydrochemical characteristics and microbial community composition of seven brine samples with high total dissolved solid (TDS) values ranging from 74.5 to 132.3 g/L within and across three coastal saltworks (Yangkou, Hanting, and Changyi) in southern Laizhou Bay (SLB). The bacterial diversity was independent of salinity. Compared with those of low-salinity groundwater, the diversity of the microbial community in brine was lower, but the richness was slightly higher. There was a significant correlation between the microbial community diversity and groundwater sources, which indicated that the microbial communities were affected by groundwater sources. A comparison of the microbial community compositions of the three saltworks showed that the Hanting and Changyi saltworks had similar microbial communities due to their similar sampling depths. In addition, the main force shaping the differences in the microbial communities in both coastal open groundwater systems (COGSs) and CCGSs was identified as the hydraulic connection with the seawater controlled by hydrogeological conditions formed throughout geological history. This study can help to elucidate the biogeochemical processes in coastal aquifers.