Parasitism reduces oxidative stress of fish host experimentally exposed to PAHs

Cigarette butts enable toxigenic cyanobacteria growth by inhibiting their lethal fungal infections

Cigarette butts (CBs), of which around 4.5 trillion are discarded annually, are one of the most common types of litter worldwide. CBs contain various chemicals, including metals, nicotine, and polycyclic aromatic hydrocarbons, which can leach into water and pose a threat to aquatic organisms such as cyanobacteria and chytrid fungi. Chytrids, zoosporic fungi that parasitize cyanobacteria lethally, play a crucial role in regulating cyanobacteria blooms by delaying or suppressing bloom formation. Despite the prevalence of CBs in the environment, the impact of their leachates on cyanobacteria-chytrid interactions is not well understood. We assessed the effects of CB leachate on the interaction between the toxigenic cyanobacterium Planktothrix agardhii and its chytrid parasite Rhizophydium megarrhizum. CB leachate inhibited cyanobacterial growth in uninfected cultures. Infection prevalence decreased at 0.2, 2, and 10 CB L-1, with the two highest concentrations completely suppressing infection. Interestingly, at the highest CB concentration, cyanobacterial biomass in infected cultures was comparable to that of uninfected cultures not exposed to CB leachate, suggesting that the presence of chytrids mitigates the impact of the leachate. This study demonstrates that CB leachates are a potential environmental hazard that can enable cyanobacterial growth by inhibiting chytrid infections during epidemics. In addition, our research highlights the importance of assessing the effects of chemical mixtures, such as those leached from CBs, on multi-species interactions, such as host-parasite dynamics. These assessments are crucial to better understand the impact of pollutants on aquatic ecosystems.

A class of their own? Water-soluble polymer pollution impacting a freshwater host-pathogen system

While the inclusion of synthetic polymers such as primary microplastics within personal care products have been widely restricted under EU/UK Law, water-soluble polymers (WSPs) have so far slipped the net of global chemical regulation despite evidence that these could be polluting wastewater effluents at concentrations greatly exceeding those of microplastics. Polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP) represent WSPs with common industry and household uses, down-the-drain disposal and a direct route to wastewater treatment plants, conveying high risk of environmental leaching into freshwater ecosystems. The current study is the first investigating the impacts of predicted environmental concentrations of these WSPs on life-history traits of two freshwater species also constituting a disease model (fish - Poecilia reticulata and parasite - Gyrodactylus turnbulli). Single effects of WSPs on fish as well as their interactive effects with infection of the ectoparasite were determined over a 45-day exposure. Generally, WSPs reduced fish growth and increased routine metabolic rate of fish implying a depleted energetic budget, however these effects were dose, exposure time and polymer dependent. Parasitic infection alone caused a significant reduction in fish growth and enhanced fish routine metabolic rate. In contrast, a non-additive effect on metabolic rate was evident in fish experiencing simultaneous infection and WSP exposure, suggesting a protective effect of the two WSPs for fish also exposed to a metazoan ectoparasite. Off-host parasite survival was significantly lowered by both WSPs; however, parasite counts of infected fish also exposed to WSP were not significantly different from the control, implying more complex mechanisms may underpin this stressor interaction. Distinct detrimental impacts were inflicted on both organisms implying environmental leaching of WSPs may be causing significant disruption to interspecies interactions within freshwater ecosystems. Additionally, these results could contribute to sustainable development in industry, as we conclude PVA represents a less harmful alternative to PVP.

Parasites and Pollutants: Effects of Multiple Stressors on Aquatic Organisms

Parasites can affect their hosts in various ways, and this implies that parasites may act as additional biotic stressors in a multiple-stressor scenario, resembling conditions often found in the field if, for example, pollutants and parasites occur simultaneously. Therefore, parasites represent important modulators of host reactions in ecotoxicological studies when measuring the response of organisms to stressors such as pollutants. In the present study, we introduce the most important groups of parasites occurring in organisms commonly used in ecotoxicological studies ranging from laboratory to field investigations. After briefly explaining their life cycles, we focus on parasite stages affecting selected ecotoxicologically relevant target species belonging to crustaceans, molluscs, and fish. We included ecotoxicological studies that consider the combination of effects of parasites and pollutants on the respective model organism with respect to aquatic host-parasite systems. We show that parasites from different taxonomic groups (e.g., Microsporidia, Monogenea, Trematoda, Cestoda, Acanthocephala, and Nematoda) clearly modulate the response to stressors in their hosts. The combined effects of environmental stressors and parasites can range from additive, antagonistic to synergistic. Our study points to potential drawbacks of ecotoxicological tests if parasite infections of test organisms, especially from the field, remain undetected and unaddressed. If these parasites are not detected and quantified, their physiological effects on the host cannot be separated from the ecotoxicological effects. This may render this type of ecotoxicological test erroneous. In laboratory tests, for example to determine effect or lethal concentrations, the presence of a parasite can also have a direct effect on the concentrations to be determined and thus on the subsequently determined security levels, such as predicted no-effect concentrations. Environ Toxicol Chem 2023;42:1946-1959. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Combined threats of climate change and contaminant exposure through the lens of bioenergetics

Organisms face energetic challenges of climate change in combination with suites of natural and anthropogenic stressors. In particular, chemical contaminant exposure has neurotoxic, endocrine-disrupting, and behavioral effects which may additively or interactively combine with challenges associated with climate change. We used a literature review across animal taxa and contaminant classes, but focused on Arctic endotherms and contaminants important in Arctic ecosystems, to demonstrate potential for interactive effects across five bioenergetic domains: (1) energy supply, (2) energy demand, (3) energy storage, (4) energy allocation tradeoffs, and (5) energy management strategies; and involving four climate change-sensitive environmental stressors: changes in resource availability, temperature, predation risk, and parasitism. Identified examples included relatively equal numbers of synergistic and antagonistic interactions. Synergies are often suggested to be particularly problematic, since they magnify biological effects. However, we emphasize that antagonistic effects on bioenergetic traits can be equally problematic, since they can reflect dampening of beneficial responses and result in negative synergistic effects on fitness. Our review also highlights that empirical demonstrations remain limited, especially in endotherms. Elucidating the nature of climate change-by-contaminant interactive effects on bioenergetic traits will build toward determining overall outcomes for energy balance and fitness. Progressing to determine critical species, life stages, and target areas in which transformative effects arise will aid in forecasting broad-scale bioenergetic outcomes under global change scenarios.

The "parasite detoxification hypothesis": lead exposure potentially changes the ecological interaction from parasitism to mutualism

In urban areas, organisms are exposed to high pollutant levels, especially element trace metals that may impact host-parasite interactions. Indeed, parasites have been reported to reduce the negative effects of pollutants on their hosts. The fitness of parasitized organisms in polluted environments may therefore be greater than that of unparasitized organisms. In our study, we used an experimental approach to test this hypothesis on feral pigeons (Columba livia), which are endemically parasitized by nematodes and exposed to high levels of lead in urban areas. We tested the combined effects of lead exposure and helminth parasitism on different pigeon fitness components: preening, immunocompetence, abundance of lice (Columbicola columbae) and haemosporidian parasites (Heamoproteus spp., Plasmodium spp.), reproduction investment, and oxidative stress. Our results show that among pigeons exposed to lead treatment, individuals harboring nematode parasites exhibit more preening activity and have fewer ectoparasites lice than nematode-free individuals. Benefits for nematode-parasitized individuals exposed to lead were not detected for other fitness parameters. Further studies are required to confirm the "parasite detoxification hypothesis" in pigeons and to identify the mechanisms by which this detoxification occurs.

Environmental parasitology: stressor effects on aquatic parasites

Anthropogenic stressors are causing fundamental changes in aquatic habitats and to the organisms inhabiting these ecosystems. Yet, we are still far from understanding the diverse responses of parasites and their hosts to these environmental stressors and predicting how these stressors will affect host-parasite communities. Here, we provide an overview of the impacts of major stressors affecting aquatic ecosystems in the Anthropocene (habitat alteration, global warming, and pollution) and highlight their consequences for aquatic parasites at multiple levels of organisation, from the individual to the community level. We provide directions and ideas for future research to better understand responses to stressors in aquatic host-parasite systems.

Narrower isotopic niche size in fish infected by the intestinal parasite Pomphorhynchus sp. compared to uninfected ones

Examples of parasite-related effects on intermediate crustacean hosts are numerous but their ecological consequences on their vertebrate hosts are scarce. Here, we address the role of macroparasite infections on the trophic niche structure of definitive hosts and its potential physiological consequences using wild fish populations infected with an acantochephalan parasite Pomphorhynchus sp., a trophically transmitted intestinal worm. Infected and uninfected fish were sampled from six populations on the Marne River, France and the prevalence of intestinal parasites in the host populations ranged from 50% to 90%. Although the isotopic ratios (δ¹³ C and δ¹⁵ N) did not differ between infected and uninfected fish, we found a consistent pattern of isotopic niche size being considerably smaller in infected hosts when compared with noninfected ones. This was not explained by interindividual differences in intrinsic factors such as length/age or body condition between infected and uninfected fish. These results suggest a potential niche specialization of infected fish, which did not impair their energetic status.

Telomeres and anthropogenic disturbances in wildlife: A systematic review and meta-analysis

Human-driven environmental changes are affecting wildlife across the globe. These challenges do not influence species or populations to the same extent and therefore a comprehensive evaluation of organismal health is needed to determine their ultimate impact. Evidence suggests that telomeres (the terminal chromosomal regions) are sensitive to environmental conditions and have been posited as a surrogate for animal health and fitness. Evaluation of their use in an applied ecological context is still scarce. Here, using information from molecular and occupational biomedical studies, we aim to provide ecologists and evolutionary biologists with an accessible synthesis of the links between human disturbances and telomere length. In addition, we perform a systematic review and meta-analysis on studies measuring telomere length in wild/wild-derived animals facing anthropogenic disturbances. Despite the relatively small number of studies to date, our meta-analysis revealed a significant small negative association between disturbances and telomere length (-0.092 [-0.153, -0.031]; n = 28; k = 159). Yet, our systematic review suggests that the use of telomeres as a biomarker to understand the anthropogenic impact on wildlife is limited. We propose some research avenues that will help to broadly evaluate their suitability: (i) further causal studies on the link between human disturbances and telomeres; (ii) investigating the organismal implications, in terms of fitness and performance, of a given telomere length in anthropogenically disturbed scenarios; and (iii) better understanding of the underlying mechanisms of telomere dynamics. Future studies in these facets will help to ultimately determine their role as markers of health and fitness in wildlife facing anthropogenic disturbances.

Dysbiosis of fish gut microbiota is associated with helminths parasitism rather than exposure to PAHs at environmentally relevant concentrations

Although parasite infection and pollution are common threats facing wild populations, the response of the gut microbiota to the joint impact of these stressors remains largely understudied. Here, we experimentally investigated the effects of exposure to Polycyclic Aromatic Hydrocarbons (PAHs) and infection by a common acanthocephalan intestinal parasite (Pomphorhynchus sp.) on the gut microbial flora of a freshwater fish, the European chub (Squalius cephalus). Naturally infected or uninfected individuals were exposed to PAHs at environmentally realistic concentrations over a five-week period. Characterization of the gut bacterial community through 16S rRNA gene amplicon sequencing revealed that parasitic infection was a more structuring factor of bacterial diversity and composition than PAH exposure. Specifically, chub infected by Pomphorhynchus sp. harbored significantly less evenly represented gut bacterial communities than the uninfected ones. In addition, substantial changes in sequence abundance were observed within the main bacterial phyla, including the Firmicutes, Fusobacteriota, Actinobacteriota, and Proteobacteria. Again, these compositional changes correlated with host infection with Pomphorhynchus sp., confirming its pivotal role in gut microbial assemblage. Overall, these results highlight the importance of defining the parasitic status of individuals when conducting microbial ecotoxicological analyses at the digestive tract level, as this should lead to better understanding of microbiota modulations and help to identify microbial markers specifically associated with chemicals.

Benefits of Parasitism in Polluted Environments: A Review and Perspectives

The frequency and strength of biotic interactions are thought to be shaped by environmental conditions. In this study, we reviewed and discussed the potential effects of toxic chemicals in driving shifts along the parasite-mutualist continuum. Some parasites have the astonishing capacity to accumulate trace metals and organic pollutants from various taxa within freshwater, marine, and terrestrial ecosystems. Recent studies have provided evidence of clear benefits for the host: when exposed to contaminants, infected organisms exhibited reduced contamination levels, less severe oxidative stress, and histological alterations, as well as higher body condition and survival rate compared with their uninfected conspecifics. Such effects might arise when the costs of parasitism are lower than their benefits in specific environmental conditions. Assessing the potential outcomes for parasites exploiting contaminated hosts is a crucial but neglected issue, since ecotoxicological effects on parasites may alter interspecific relationships. We identified possible avenues for future research using innovative tools and long-term experimental manipulations of both parasitism and pollution to better understand how toxic chemicals can modulate the strength and direction of host-parasite interactions.

Bridging the gap: aquatic parasites in the One Health concept

The One Health framework emphasizes the interconnectedness of humans, animals, and the environment but often remains focused on human health. Here we highlight how the evolutionary and ecological dynamics of aquatic parasites are crucial to our understanding of these connected health aspects, especially in the light of environmental changes.