Antiviral agent modulates freshwater methanogenesis at the nanotrace level.

Antiviral drugs (ATVs), such as oseltamivir carboxylate (OTC), are widely used to treat viral infections. Although environmental levels are in the ng/L to µg/L range, knowledge on risks posed by this substance class is hindered by limited assessments. Evidence on effects on microbially-driven ecosystem processes is especially scarce, which is concerning because viruses are omnipresent in the microbiome and likely to influence microbial functioning. We assessed the influence of OTC on anaerobic methane production, a greenhouse gas largely emitted from natural systems and responsible for around one third of greenhouse gas-driven warming, as a proxy for microbial ecosystem function. Oseltamivir carboxylate inhibited initial methanogenesis between 15% and 40% even at nanotrace levels (i.e., 0.006-600 µg/L). Given that in most treatments methane levels returned to those of the control, the initial inhibition is potentially attributed to a temporally limited OTC-induced shift from a neutral/temperate viral infection of prokaryotes toward a lytic viral replication. Mostly unaffected 16S rRNA metabarcoding community data indicate that this effect might be uniform throughout the community. This study points to ecosystem-level effects at the ng/L range of ATVs, suggesting a significant knowledge gap, which warrants further attention to this group of chemicals.