Karst landscapes in China are widely distributed, covering 22 million km² and accounting for 15% of the Earth's land area. The karst dynamic system is an open system characterized by a three-phase dynamic equilibrium among CaCO₃ , H₂O and CO₂. Karst carbon cycling is considered as an important means to address climate change, with microorganisms inhabiting the karst environment playing a crucial role in this carbon cycle. Karst regions in Southwest China comprise nearly one-third of the total country’s karst area. Within these regions, karst landforms are widely distributed in the Guangxi Zhuang Autonomous Region, particularly concentrated and contiguous in the southwest, northwest, central and northeast parts of Guangxi, covering about 37.8% of its total land area. There are various types of karst development, which are rare worldwide. Karst regions in Southwest China, exemplified by Guilin in Guangxi, represents typical karst landscapes in this country. Analyzing microorganisms that survive in this local karst environment provides highly representative insights. Some studies have shown that carbonic anhydrase-producing microorganisms play a significant role in promoting carbonate deposition. However, the mechanisms by which these microorganisms and their carbonic anhydrase participate in this process, as well as their influence on karst action, remain to be investigated. To clarify the characteristics of water and soil microbial communities in typical native karst areas, this study analyzed microbial diversity and community structures in the water and soil of the Guilin region using high-throughput sequencing technology. Additionally, we examined the effects of environmental factors on microbial communities and explored the potential functions of karst microorganisms in the carbon cycle.

In this study, we analyzed the environmental background of typical native karst regions, focusing on the physical and chemical properties as well as the elemental content of water and soil. High-throughput sequencing was then used to examine the diversity and community structures of bacteria in both water and soil within these karst regions. We utilized diversity indices, community composition analyses, correlation analyses between microbial communities and environmental factors, and potential function prediction analyses to enhance our understanding of bacterial species and community structures in typical karst regions. Additionally, we compared the bacteria from the Huanglong alpine karst area with the bacteria studied previously to analyze the differences of microorganisms across various karst geomorphological environments and to identify their unique microbial types.

The study results are as follows: (1) The typical karst region in Guilin provides an environment conducive to calcium carbonate deposition. The average water temperature reaches around 25 ℃, with a pH above seven. The contents of Ca²⁺ and \rmHCO_3^- are high, mainly in the \rmHCO_3^--Ca²⁺ type of water quality. The soil is alkaline, which favors calcium carbonate deposition, and there are no significant differences in basic physical and chemical properties among the sample sites. Si is the main element in the soil, with the highest content at Site S3 (62.31%), followed by Al and Fe. Ca content is very little and varies considerably within the group. (2) A total of 106 phyla, 1132 genera and 635 becteria species were identified. Proteobacteria is the dominant bacterial phylum across all communities. In the water samples collected from Guilin, the dominant genera are Acinetobacter, Limnohabitans, Exiguobacterium, Chryseobacterium, and Deinococcus. In the soil, the dominant genera are norank Vicinamibacteraceae and norank Vicinamibacterales. Aeromonas is a distinctive genus found in water sample from Huanglong. (3) In water samples from Guilin, Cl⁻ and \rmNO_3^- have the most significant impact on the bacterial community structure, whereas in the soil, the structure is primarily influenced by Soil Water Content (SWC) and Ca. (4) Microbial species such as Proteobacteria, Bacteroidota, Deinococcata, Acidobacteria and Firmicutes play significant roles in the karst environment through carbon cycle-related functions,including aerobic chemoheterotrophy, fermentation, and photosynthesis, as well as through various carbon-related metabolic pathways,such as carbon metabolism, glycolysis, amino acid synthesis and carbon fixation. Functional predictions reveal that most bacteria participate in the carbon cycle, with similar functional types present in both water and soil environments but in different proportions, dominated by chemoheterotrophy. Metabolic potential analysis highlighted glycolysis and the tricarboxylic acid (TCA) cycle as key pathways. (5) A comparison of microorganisms across different karst environments shows species differences at various taxonomic levels. The composition of bacterial communities is closely related to their specific karst microenvironments, and these bacteria significantly influence biogeochemical cycles, particularly the carbon cycle, within the karst systems. These findings enhance our understanding of microbial contributions to the carbon cycle in karst landscapes and offer insights into the biological mechanisms driving carbon sink formation.