Impact of Tourism on Bacterial Communities of Karst Underground River: A Case Study from Two Caves in Fengdu, Chongqing

Abstract

In this research, the bacterial community compositions of underground water in a tourist and pristine cave were studied. Xueyu Cave and Shuiming Cave are tourist and pristine caves, respectively, in the same karst cave system located in Chongqing, southwest China. To understand the impact of tourism on bacterial community compositions in underground water that flows through the caves, filtered materials from water were collected, and 16S rDNA gene sequences were obtained by high-throughput sequencing. The Shuiming Cave (the pristine cave) had less diversity than Xueyu Cave (the tourist cave) based on the Shannon's diversity index according to Illumina operational taxonomic units (OTUs). Proteobacteria, represented mostly by γ-Proteobacterium and Bacteroidetes, dominated both systems. OTUs from Shuiming Cave were dominated by 38% Proteobacteria, 24% Chlorobi, and 19% Bacteroidetes. In the Xueyu Cave, OTUs from upstream samples were comprised of 62% Proteobacteria but comprised 64% in the downstream samples. In the Xueyu Cave, Bacteroidetes accounted for 11% of the total OTUs in the upstream sample and 16% in the downstream. Among the γ-Proteobacterium and Bacteroidetes, Acinetobacter, Pseudomonas spp., and Flavobacteriaceae, which are related to potentially pathogenic species, were prevalent in the Xueyu Cave, while Methylococcaceae-uncultured, Methylomonas spp., and Methylobacter, all methane-oxidizing bacteria, had high relative abundances in the Shuiming Cave. These results revealed that potentially more pathogenic bacteria are present in the stream waters from the tourist cave, which has important implications for the protection of tourist caves. The RDA analysis of the environmental factor and bacteria community in groundwater showed that the distribution of pathogenic bacteria was positively correlated with the cave air CO2, and the Spearman correlation analysis of the two environmental factors indicated that the influence of the number of tourists on the structure of the bacterial community in the groundwater was more obvious and led to the disappearance of a large number of native bacteria. We proposed that tourist caves control the number of daily tourists and that they enter in batches and increase the import and export of closed devices to avoid the cave air exchange inside and outside. In addition, it was recommended that they increase the import and export of sterilization devices to reduce tourists with bacteria and organic matter, and avoid leaving garbage in the hole to avoid cave microbial exchange inside and outside. A reduction in the fixed lighting inside caves should be required to reduce long exposure, since the tourists can bring individual source lighting.