Escalating Antibiotic Inputs Alter Aquatic Plant Detritus Decomposition and Organic Carbon Mineralization: A Case Study of Gehu Lake, China

Human activities have elevated antibiotic concentrations, such as ciprofloxacin (CIP), in freshwater lakes. While lakes are pivotal to carbon cycling, the effects of increasing antibiotic levels on them remain unclear. Here, we used microcosms with samples from a eutrophic lake, Gehu Lake in China, using six experimental groups (A–F) with initial CIP concentrations of 0, 10, 50, 100, 500, and 1000 μg/L, to assess how CIP influences dissolved organic matter (DOM) transformation during aquatic plant decomposition. Our results showed that increasing CIP concentrations altered plant residue decomposition pathways and microbial-mediated DOM processing by facilitating residue breakdown, transforming DOM into more chemically recalcitrant forms, and suppressing microbial mineralization of DOM. Under high CIP exposure, continuous DOM release was associated with enhanced structural disintegration of plant residues. In the 1000 μg/L treatment, lignin content in residues decreased from 15.52% to 1.58% compared with the no-CIP treatment. Under CIP stress, the molecular composition of DOM shifted toward regions with lower hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, which corresponded with a systematic decrease in component 1 (C1) components and an increase in component 3 (C3) components, which are characterized by highly aromatic, condensed structures with greater environmental persistence. This indicates an overall increase in DOM aromaticity and condensation, reflecting a transition toward more stable and microbially resistant organic carbon pools. In addition, high CIP concentrations caused substantial losses of microbial diversity, favoring the persistence of antibiotic-resistant but functionally limited taxa. In the 1000 μg/L CIP treatment, the Chao1 and Shannon indices were only 129.8 and 2.16, respectively. The reduction in microbial diversity further weakened the metabolic capacity to degrade DOM, particularly its structurally complex fractions, thereby retaining more organic carbon being retained within the lake system. Our study highlights the influence of antibiotic pollution on organic carbon processing during aquatic plant residue decomposition and suggests potential implications for carbon management in freshwater environments.