Biodegradation masks the overall chemical impact of seasonal pandemics on riverine systems: The case of quaternary ammonium disinfectants during COVID-19

The environmental impact of pandemics is often evaluated by measuring disinfectant or drug concentrations in surface waters. This approach assumes that increased used of those chemicals leads directly to higher environmental concentrations. However, this assumption does not consider microbial degradation. Quaternary ammonium compounds (QACs), a group of emerging pollutants which were widely used during the COVID-19 pandemic, offer a clear example of this limitation in riverine systems.

In this study, we investigated the relationship between QAC concentrations and microbial biodegradation capacity in five major Turkish rivers sampled seasonally over one year. Summer samples were used as reference conditions, characterized by low COVID-19 case numbers obtained during the wastewater surveillance program. Winter and spring showed with statistically higher case numbers. Surface waters were analyzed for six QACs using LC-MS/MS. Total QAC concentrations ranged from <2 nM in low-impact rivers to >200 nM in urban-influenced systems. Despite relatively high COVID-19 cases, QAC concentrations during winter and spring often remained low (typically 1-10 nM) in several rivers.

Microbial measurements revealed that low QAC concentrations during peak pandemic periods were not due to reduced inputs but to enhanced biodegradation. Culture-based assays showed strong seasonal enrichment of QAC-degrading bacteria during winter and spring. Quantitative PCR targeting the QAC biodegradation gene qxyA showed copy numbers 2-5 fold higher in winter and spring samples compared to summer reference conditions. In multiple rivers, high qxyA copy numbers coincided with low or non-detectable QAC concentrations. In addition, microbial community resistance to fluoroquinolone antibiotics, which is also related to QAC exposure, substantially increased during winter and spring.

These results demonstrate a feedback mechanism in which increased QAC use selects for specialized degraders that rapidly remove QACs from the water column. As a result, chemical measurements alone underestimate the ecological impact of disinfectant use. Functional markers such as qxyA provide a more reliable indicator of anthropogenic pressure and microbial adaptation in riverine ecosystems under global change.