Vertical dynamics of DOM-specialized bacteria and fungi drive stability in stratified reservoirs: Mechanisms revealed by machine learning

Thermal stratification in reservoirs exacerbates the vertical differentiation of dissolved organic matter (DOM). However, the mechanisms underlying microbial community stability and carbon cycle regulation remain unclear. In this study, the vertical patterns of DOM-related microorganisms: obligate labile and obligate refractory (OLB and ORB) DOM-associated bacteria and fungi (OLF/ORF) were investigated in a thermally stratified drinking water reservoir. Labile DOM was predominantly enriched in the euphotic layer (79.22 % ± 8.91 %), and refractory DOM accumulated in the hypolimnion. Species replacement resulted in significant vertical differences. OLB and OLF diversity peaked in the thermocline, whereas ORB and ORF diversity was highest in the hypolimnion. The BISSE model analysis revealed higher speciation than extinction rates for the OLB across all layers, in contrast to the ORB. Both OLB speciation and ORB extinction rates decreased with increasing depth. OLF and ORF exhibited higher overall speciation rates. Stochastic processes dominated community assembly, with stochasticity increasing in the case of OLB/ORB and deterministic processes strengthening in OLF/ORF with depth. The OLB networks were found to be most complex in the epilimnion, ORB/OLF in the euphotic layer, and ORF in the thermocline. Microbial community stability was highest in the hypolimnion for OLB/ORF and in the epilimnion for ORB/OLF. OLB-OLF interactions were consistently more complex than ORB-ORF interactions across the layers, with peak complexity in the euphotic zone and thermocline. OLB and ORB were positively regulated by changes in temperature and dissolved oxygen, whereas OLF and ORF were dominated by TN and NO₃^--N, respectively. Furthermore, this study reveals the vertical dynamics of DOM-specialized bacteria and fungi and environmental factors in thermally stratified reservoirs.