Uptake without inactivation of human adenovirus type 2 by Tetrahymena pyriformis ciliates

Abstract Human adenoviruses are ubiquitous contaminants of surface water. Indigenous protists may interact with adenoviruses and contribute to their removal from the water column, though the associated kinetics and mechanisms differ between protist species. In this work, we investigated the interaction of human adenovirus type 2 (HAdV2) with the ciliate Tetrahymena pyriformis . In co-incubation experiments in a freshwater matrix, T. pyriformis was found to efficiently remove HAdV2, with ≥ 4 log 10 removal over 72 hours. Neither sorption onto the ciliate nor secreted compounds contributed to the observed loss of infectious HAdV2. Instead, internalization was shown to be the dominant removal mechanism, resulting in the presence of viral particles inside food vacuoles of T. pyriformis, as visualized by transmission electron microscopy. The fate of HAdV2 once ingested was scrutinized and no evidence of virus digestion was found over the course of 48 hours. This work shows that T. pyriformis can exert a dual role on microbial water quality: while they remove infectious adenovirus from the water column, they can also accumulate and potentially protect infectious viruses from exposure to environmental or engineered stressors. Environmental significance Human viruses are ubiquitous contaminants of surface water. The fate of human viruses, once discharged into the environment, is modulated by indigenous microorganisms in the surrounding water body. Among microorganisms, the role of protists in controlling virus persistence has been overlooked. Here, we investigate the interactions of the ciliate Tetrahymena pyriformis with human adenovirus type 2. We demonstrate that T. pyriformis can serve as a sink of adenovirus from the water column, but also act as a reservoir and potential protective barrier for infectious viruses against stressors. T. pyriformis ciliates are among the most abundant protists in surface waters. Understanding this dual role of protists is important for assessing and maintaining microbial water quality and infection risks.