Acanthocephalans as pollutant sinks? Higher pollutant accumulation in parasites may relieve their crustacean host

Increasing chemical pollution calls for a closer look at ecologically highly relevant host-parasite interactions to understand the persistence of organisms and populations in a polluted environment. The impact of chemical exposure within the host-parasite interactions - particularly the distinctive bioaccumulation behavior of organic micropollutants - can substantially influence the persistence of a species. This significance has been emphasized by previous research showing a higher tolerance of Gammarus roeselii (Amphipoda, Crustacea) infected with acanthocephalans during acute exposure to a pyrethroid. This suggests the presence of infection-related benefits within polluted environments. The present study addressed this complex relationship by investigating the chemical body burden and internal pollutant concentrations of both G. roeselii and its acanthocephalan parasites across a pollution gradient. Specifically, we analyzed 405 organic micropollutants and identified 123 of these in gammarids and their acanthocephalan parasites. Among the detected compounds, 22 are either banned or are no longer permitted for use in Germany. Remarkably, we discovered that the concentrations of pollutants were up to 35 times higher in the acanthocephalan parasites than in their crustacean intermediate hosts. The log K_OW, the most frequently used measure of chemical hydrophobicity, could not explain the accumulation. Instead, the accumulation is likely explained by the unique physiology and high absorption capacity of acanthocephalans, combined with potentially limited biotransformation and excretion ability. This results in a redistribution of micropollutants within the host-parasite system, reducing the burden on the host up to 13.9 % and potentially explaining the observed helpful effects of parasitized G. roeselii in polluted environments. Our study underscores the often overlooked but significant role of host-parasite interactions in human-altered ecosystems, revealing how these relationships can mediate and amplify the impacts of micropollutants within aquatic communities. These insights stress the need to consider the pervasive influence of metazoan parasites in environmental assessments and pollution management strategies.