Search

Epikarst springs are commonly used for drinking water in karst mountainous areas, but they tend to bring health risks to residents because of their vulnerability. In this work, a modified slow sand filtration system (M-SSF) was established as a case study to purify and conserve the epikarst spring water. The outcomes indicate that the purification of M-SSF relies mainly on the adsorption and ion exchange of the filter medium (mixtures of heat-treated red clay and crushed limestone, MHRCCL) during the schmutzdecke juvenility, and on the schmutzdecke-formed food chain of pollutants → bacteria → protozoa after the schmutzdecke maturity. The closed water cellar lined with ceramic tiles could reduce the deterioration of epikarst spring water during storage. Via 16S rRNA sequencing, it was found that the high abundance of TM6_Dependentiae in purified epikarst spring water (PESW) suggested that the M-SSF system relies on the formation of a closed food chain to achieve effective water purification. The decrease of Pseudarcicella abundance in PESW indicated that M-SSF could effectively prevent the water quality from external influences represented by leeches. Besides, the 16S function prediction was used to qualitatively characterize microbial nitrogen metabolism, as well as organic matter degradation in water purification. HIGHLIGHTS The quality of epikarst spring water was significantly improved using the M-SSF.; TM6_Dependentiae indicated that the purification of M-SSF relies on the establishment of a closed food chain.; The M-SSF could prevent the purified epikarst spring water from being influenced by external pollution.

Additional file 1. Supplementary experimental section. Fig. S1. Mycelial morphology of E. cristatum NWAFU-1. Fig. S2. Molecular network of the metabolic products from E. cristatum NWAFU-1. Fig. S3. Proposed biosynthetic gene clusters of echinulin and function analysis of each gene in the BGC. Fig. S4. Sequence similarity network analysis of CriC. Fig. S5. 1H-NMR spectrum of Cyclo-TA (CD3OD-d4, 500 MHz). Fig. S6. 13C-NMR spectrum of Cyclo-TA (CD3OD-d4, 125 MHz). Fig. S7. HSQC spectrum of Cyclo-TA (CD3OD-d4). Fig. S8. HMBC spectrum of Cyclo-TA (CD3OD-d4). Fig. S9. 1H-1H COSY spectrum of Cyclo-TA (CD3OD-d4). Fig. S10. NOESY spectrum of Cyclo-TA (CD3OD-d4). Fig. S11. HPLC traces of time-course biochemical assays for microsome containing CriC. Fig. S12. Substrate promiscuity analysis of CriC. Fig. S13. HPLC traces of AO-criC product under non-linear increasing concentration gradient substrate feeding. Fig. S14. Domain analysis and speculative reaction mechanism for CriC. Fig. S15. Genome mining-based CriC uncovered serval BGCs responsible for Cyclo-TA containing compounds. Table S1. Primers used for construction of expression plasmids. Table S2. NMR Data of Cyclo-TA in CD3OD-d4 (500 MHz for 1H NMR, 125 MHz for 13C NMR). Table S3. Percent identity matrix of CriC and its homologies in CriC group branch on the evolutionary tree.