Generalist parasites persist in degraded environments: a lesson learned from microsporidian diversity in amphipods

Histopathological screening of Pontogammarus robustoides (Amphipoda), an invader on route to the United Kingdom

Biological invasions may act as conduits for pathogen introduction. To determine which invasive non-native species pose the biggest threat, we must first determine the symbionts (pathogens, parasites, commensals, mutualists) they carry, via pathological surveys that can be conducted in multiple ways (i.e., molecular, pathological, and histological). Whole animal histopathology allows for the observation of pathogenic agents (virus to Metazoa), based on their pathological effect upon host tissue. Where the technique cannot accurately predict pathogen taxonomy, it does highlight pathogen groups of importance. This study provides a histopathological survey of Pontogammarus robustoides (invasive amphipod in Europe) as a baseline for symbiont groups that may translocate to other areas/hosts in future invasions. Pontogammarus robustoides (n = 1,141) collected throughout Poland (seven sites), were noted to include a total of 13 symbiotic groups: a putative gut epithelia virus (overall prevalence = 0.6%), a putative hepatopancreatic cytoplasmic virus (1.4%), a hepatopancreatic bacilliform virus (15.7%), systemic bacteria (0.7%), fouling ciliates (62.0%), gut gregarines (39.5%), hepatopancreatic gregarines (0.4%), haplosporidians (0.4%), muscle infecting microsporidians (6.4%), digeneans (3.5%), external rotifers (3.0%), an endoparasitic arthropod (putatively: Isopoda) (0.1%), and Gregarines with putative microsporidian infections (1.4%). Parasite assemblages partially differed across collection sites. Co-infection patterns revealed strong positive and negative associations between five parasites. Microsporidians were common across sites and could easily spread to other areas following the invasion of P. robustoides. By providing this initial histopathological survey, we hope to provide a concise list of symbiont groups for risk-assessment in the case of a novel invasion by this highly invasive amphipod.

Widespread infection, diversification and old host associations of Nosema Microsporidia in European freshwater gammarids (Amphipoda)

The microsporidian genus Nosema is primarily known to infect insects of economic importance stimulating high research interest, while other hosts remain understudied. Nosema granulosis is one of the formally described Nosema species infecting amphipod crustaceans, being known to infect only two host species. Our first aim was to characterize Nosema spp. infections in different amphipod species from various European localities using the small subunit ribosomal DNA (SSU) marker. Second, we aimed to assess the phylogenetic diversity, host specificity and to explore the evolutionary history that may explain the diversity of gammarid-infecting Nosema lineages by performing a phylogenetic reconstruction based on RNA polymerase II subunit B1 (RPB1) gene sequences. For the host species Gammarus balcanicus, we also analyzed whether parasites were in excess in females to test for sex ratio distortion in relation with Nosema infection. We identified Nosema spp. in 316 individuals from nine amphipod species being widespread in Europe. The RPB1-based phylogenetic reconstruction using newly reported sequences and available data from other invertebrates identified 39 haplogroups being associated with amphipods. These haplogroups clustered into five clades (A-E) that did not form a single amphipod-infecting monophyletic group. Closely related sister clades C and D correspond to Nosema granulosis. Clades A, B and E might represent unknown Nosema species infecting amphipods. Host specificity seemed to be variable with some clades being restricted to single hosts, and some that could be found in several host species. We show that Nosema parasite richness in gammarid hosts is much higher than expected, illustrating the advantage of the use of RPB1 marker over SSU. Finally, we found no hint of sex ratio distortion in Nosema clade A infecting G. balcanicus. This study shows that Nosema spp. are abundant, widespread and diverse in European gammarids. Thus, Nosema is as diverse in aquatic as in terrestrial hosts.

Possible seasonal and diurnal modulation of Gammarus pulex (Crustacea, Amphipoda) drift by microsporidian parasites

In lotic freshwater ecosystems, the drift or downstream movement of animals (e.g., macroinvertebrates) constitutes a key dispersal pathway, thus shaping ecological and evolutionary patterns. There is evidence that macroinvertebrate drift may be modulated by parasites. However, most studies on parasite modulation of host drifting behavior have focused on acanthocephalans, whereas other parasites, such as microsporidians, have been largely neglected. This study provides new insight into possible seasonal and diurnal modulation of amphipod (Crustacea: Gammaridae) drift by microsporidian parasites. Three 72 h drift experiments were deployed in a German lowland stream in October 2021, April, and July 2022. The prevalence and composition of ten microsporidian parasites in Gammarus pulex clade E varied seasonally, diurnally, and between drifting and stationary specimens of G. pulex. Prevalence was generally higher in drifting amphipods than in stationary ones, mainly due to differences in host size. However, for two parasites, the prevalence in drift samples was highest during daytime suggesting changes in host phototaxis likely related to the parasite's mode of transmission and site of infection. Alterations in drifting behavior may have important implications for G. pulex population dynamics and microsporidians' dispersal. The underlying mechanisms are more complex than previously thought.

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.

Temporal disturbance of a model stream ecosystem by high microbial diversity from treated wastewater

Microbial communities in freshwater streams play an essential role in ecosystem functioning via biogeochemical cycling. Yet, the impacts of treated wastewater influx into stream ecosystems on microbial strain diversity remain mostly unexplored. Here, we coupled full-length 16S ribosomal RNA gene Nanopore sequencing and strain-resolved metagenomics to investigate the impact of treated wastewater on a mesocosm system (AquaFlow) run with restored river water. Over 10 days, community Bray-Curtis dissimilarities between treated and control mesocosm decreased (0.57 ± 0.058 to 0.26 ± 0.046) based on ribosomal protein S3 gene clustering, finally converging to nearly identical communities. Similarly, strain-resolved metagenomics revealed a high diversity of bacteria and viruses after the introduction of treated wastewater; these microbes also decreased over time resulting in the same strain clusters in control and treatment at the end of the experiment. Specifically, 39.2% of viral strains detected in all samples were present after the introduction of treated wastewater only. Although bacteria present at low abundance in the treated wastewater introduced additional antibiotic resistance genes, signals of naturally occurring ARG-encoding organisms resembled the resistome at the endpoint. Our results suggest that the previously stressed freshwater stream and its microbial community are resilient to a substantial introduction of treated wastewater.

Microsporidian diversity in the aquatic isopod Asellus aquaticus

We conducted a molecular survey on microsporidian diversity in different lineages (operational taxonomic units = OTUs) of Asellus aquaticus from 30 sites throughout Europe. Host body length was determined, and DNA was extracted from host tissue excluding the intestine and amplified by microsporidian-specific primers. In total, 247 A. aquaticus specimens were analysed from which 26.7% were PCR-positive for microsporidians, with significantly more infections in larger individuals. Prevalence ranged between 10 and 90%. At 9 sites, no microsporidians were detected. A significant relationship was found between the frequency of infected individuals and habitat type, as well as host OTU. The lowest proportion of infected individuals was detected in spring-habitats (8.7%, n = 46) and the highest in ponds (37.7%, n = 53). Proportion of infected individuals among host OTUs A, D and J was 31.7, 21.7 and 32.1%, respectively. No infections were detected in OTU F. Our results are, however, accompanied by a partially low sample size, as only a minimum of 5 individuals was available at a few locations. Overall, 17 different microsporidian molecular taxonomic units (MICMOTUs) were distinguished with 5 abundant isolates (found in 4–17 host individuals) while the remaining 12 MICMOTUs were “rare” and found only in 1–3 host individuals. No obvious spatio-genetic pattern could be observed. The MICMOTUs predominantly belonged to Nosematida and Enterocytozoonida. The present study shows that microsporidians in A. aquaticus are abundant and diverse but do not show obvious patterns related to host genetic lineages or geography.