Microbial processes dominate DOM degradation in alpine karst lakes over photochemical effects

Karst lakes, known as major inorganic carbon sinks, have recently been recognized as stable reservoirs of organic carbon. However, the mechanisms governing organic carbon stability in these systems remain poorly understood. In particular, the role of dissolved organic matter (DOM) degradation in shaping DOM composition and stability through photochemical and microbial pathways has not been well characterized. By combining fluorescence spectroscopy and FT-ICR-MS with microbial high-throughput sequencing, we conducted a controlled experiment on water- and sediment-derived DOM from a Jiuzhaigou karst lake, employing light-only, microbe-only, and combined treatments. While photochemical processes contribute to changes in DOM properties, microbial activity primarily dominates DOM degradation under photo-biological conditions. Specifically, photochemical processes primarily degraded aromatic compounds into aliphatic forms, resulting in reduced O/C ratios and increased H/C ratios. In contrast, microorganisms preferentially degraded compounds with low O/C and high H/C ratios. Notably, DOM containing nitrogen and sulfur exhibited higher biological reactivity, whereas CHO compounds were more likely to contribute to recalcitrant DOM pools. Furthermore, high-molecular-weight DOM restricted microbial diversity, whereas DOM with high O/C ratios facilitated more complex microbial networks. This study provides insights into the stability of water and sediment DOM and the mechanisms driving its degradation, offering a deeper understanding of C cycling in karst ecosystems.