Advancing environmental monitoring across the water continuum combining biomarker analysis in multiple sentinel species: A case study in the Seine-Normandie Basin (France).

Nowadays, biomarkers are recognized as valuable tools to complement chemical and ecological assessments in biomonitoring programs. They provide insights into the effects of contaminant exposures on individuals and establish connections between environmental pressure and biological response at higher levels. In the last decade, strong improvements in the design of experimental protocols and the result interpretation facilitated the use of biomarker across wide geographical areas, including aquatic continua. Notably, the statistical establishment of reference values and thresholds enabled the discrimination of contamination effects in environmental conditions, allowed interspecies comparisons, and eliminated the need of a reference site. The aim of this work was to study freshwater-estuarine-coastal water continua by applying biomarker measurements in multi-species caged organisms. During two campaigns, eight sentinel species, encompassing fish, mollusks, and crustaceans, were deployed to cover 25 sites from rivers to the sea. As much as possible, a common methodology was employed for biomarker measurements (DNA damage and phagocytosis efficiency) and data interpretation based on guidelines established using reference values and induction/inhibition thresholds (establishment of three effect levels). The methodology was successfully implemented and allowed us to assess the environmental quality. Employing multiple species per site enhances confidence in observed trends. The results highlight the feasibility of integrating biomarker-based environmental monitoring programs across a continuum scale. Biomarker results align with Water Framework Directive indicators in cases of poor site quality. Additionally, when discrepancies arise between chemical and ecological statuses, biomarker findings offer a comprehensive perspective to elucidate the disparities. Presented as a pilot project, this work contributes to gain insights into current biomonitoring needs, providing new questions and perspectives.

Assessing viral freshwater hazard using a toxicokinetic model and Dreissena polymorpha.

The detection all pathogenic enteric viruses in water is expensive, time-consuming, and limited by numerous technical difficulties. Consequently, using reliable indicators such as F-specific RNA phages (FRNAPH) can be well adapted to assess the risk of viral contamination of fecal origin in surface waters. However, the variability of results inherent to the water matrix makes it difficult to use them routinely and to interpret viral risk. Spatial and temporal variability of surface waters can lead to underestimate this risk, in particular in the case of low loading. The use of bivalve mollusks as accumulating systems appears as a promising alternative, as recently highlighted with the freshwater mussel Dreissena polymorpha, but its capacity to accumulate and depurate FRNAPH needs to be better understood and described. The purpose of this study is to characterise the kinetics of accumulation and elimination of infectious FRNAPH by D. polymorpha in laboratory conditions, formalised by a toxico-kinetic (TK) mechanistic model. Accumulation and depuration experiments were performed at a laboratory scale to determine the relationship between the concentration of infectious FRNAPH in water and the concentration accumulated by D. polymorpha. The mussels accumulated infectious FRNAPH (3-5.4 × 104 PFU/g) in a fast and concentration-dependent way in only 48 h, as already recently demonstrated. The second exposure demonstrated that the kinetics of infectious FRNAPH depuration by D. polymorpha was independent to the exposure dose, with a T90 (time required to depurate 90 % of the accumulated concentration) of approximately 6 days. These results highlight the capacities of D. polymorpha to detect and reflect the viral pollution in an integrative way and over time, which is not possible with point water sampling. Different TK models were fitted based on the concentrations measured in the digestive tissues (DT) of D. polymorpha. The model has been developed to formalise the kinetics of phage accumulation in mussels tissues through the simultaneous estimation of accumulation and depuration rates. This model showed that accumulation depended on the exposure concentration, while depuration did not. Standardized D. polymorpha could be easily transplanted to the environment to predict viral concentrations using the TK model defined in the present study to predict the level of contamination of bodies of water on the basis of the level of phages accumulated by the organisms. It will be also provide a better understanding of the dynamics of the virus in continental waters at different time and spatial scales, and thereby contribute to the protection of freshwater resources.

Biochemical basis of resistance to multiple contaminations in the native and invasive populations of Dreissena polymorpha.

The zebra mussel Dreissena polymorpha (Pallas, 1771) is an invasive species and a valuable bioindicator in the inland waters. Nevertheless, the biochemical reasons for the unique competitiveness of zebra mussels are not clear. This study aimed to compare the native and invasive populations of D. polymorpha in their ability to withstand the same multiple environmental challenges (i.e. chemical: microplastics and caffeine; physical: temperatures). The specimens from the invasive population in west Ukraine (Tn) and native population at lower streams of river Dnipro (south Ukraine, Kh) were sampled in the August of 2021 y. Molluscs from both populations were treated simultaneously with microplastics (MP, 1 mg L-1, pore size 2 µm); caffeine (Caf, 20.0 µg L-1) at 18 °C, elevated temperature (25 °C) and MP and Caf combinations at 18 °C and 25 °C for 14 days. Untreated molluscs exposed at 18 °C represented control groups. A set of the 20 markers of oxidative stress, biotransformation, detoxification and apoptosis were assayed in the total soft tissues. From the two controls, Kh-group indicated lower stressful impact. However, both populations increased caspase-3 and GST activities and lysosomal instability in most exposures, and cholinesterase and phenoloxidase activities under the heating and combine exposures, indicating the remarkable properties to respond to new challenges and synergistic effect of mixtures. Inter-population differences were related to the metallothionein, cathepsin D, cytochrome P540 and oxidative stress responses that reflect population-dependent adverse outcome pathways. The discriminant analysis separated two populations with a substantially lesser magnitude of responses in the invasive population as a sign of higher resistance.

Organ-oriented proteogenomics functional atlas of three aquatic invertebrate sentinel species.

Proteogenomic methodologies have enabled the identification of protein sequences in wild species without annotated genomes, shedding light on molecular mechanisms affected by pollution. However, proteomic resources for sentinel species are limited, and organ-level investigations are necessary to expand our understanding of their molecular biology. This study presents proteomic resources obtained from proteogenomic analyses of key organs (hepatopancreas, gills, hemolymph) from three established aquatic sentinel invertebrate species of interest in ecotoxicological/ecological research and environmental monitoring: Gammarus fossarum, Dreissena polymorpha, and Palaemon serratus. Proteogenomic analyses identified thousands of proteins for each species, with over 90% of them being annotated to putative function. Functional analysis validated the relevance of the proteomic atlases by revealing similarities in functional annotation of catalogues of proteins across analogous organs in the three species, while deep contrasts between functional profiles are delimited across different organs in the same organism. These organ-level proteomic atlases are crucial for future research on these sentinel animals, aiding in the evaluation of aquatic environmental risks and providing a valuable resource for ecotoxicological studies.

Modulation of haemocyte motility by chemical and biological stresses in Mytilus edulis and Dreissena polymorpha.

Mussels are constantly exposed to various pollutants in the environment, which can impair their immune defences against microbes and thus threaten their survival. In this study, we expand the insight into a key parameter of immune response in two mussel species by exploring the impact of exposure to pollutants or bacteria or simultaneous chemical and biological exposure on haemocyte motility. Basal haemocyte velocity in primary culture was high and increasing over time in Mytilus edulis (mean cell speed of 2.32 µm/min ± 1.57) whereas Dreissena polymorpha showed a constant and rather low cell motility with time (mean cell speed of 0.59 µm/min ± 0.1). In the presence of bacteria, the motility of haemocytes was instantly enhanced and slowed down after 90 min for M. edulis. In contrast, in vitro exposure of haemocytes to chemicals, either Bisphenol A, oestradiol, copper, or caffeine, induced an inhibition of cell motility in both mussel species. Finally, the cellular activation observed during bacterial challenges was inhibited by simultaneous exposure to bacteria and pollutants. Overall, our results indicate that chemical contaminants can alter haemocyte migration in mussels which can weaken their response to pathogens and therefore increase their susceptibility to infectious diseases.

Integrative biomarker response - Threshold (IBR-T): Refinement of IBRv2 to consider the reference and threshold values of biomarkers.

The Integrated Biomarker Response (IBR) is one of the most used index in biomonitoring, especially the IBRv2 integrating a reference condition. However, some limitations remain for its routine and large-scale use. The IBRv2 is proportional to the total number of biomarkers, is dependent on the nature of biomarkers and considers all biomarkers modulations, even small and biologically non-significant. In addition, IBRv2 relies on reference values but the references are often different between each study, making it difficult to compare results between studies and/or campaigns. To overcome these limitations, the present work proposed a new index called IBR-T ("Integrated Biomarker Response - Threshold") which considers the threshold values of biomarkers by limiting the calculation of the IBR value to biomarkers with significant modulations. The IBRv2 and the IBR-T were calculated and compared on four datasets from active biomonitoring campaigns using Dreissena polymorpha, a bivalve widely used in freshwater biomonitoring studies. The comparison between indices has demonstrated that the IBR-T presents a better correlation (0.907 < r2 < 0.998) with the percentage of biomarkers significantly modulated than the IBRv2 (0.002 < r2 < 0.759). The IBRv2 could not be equal to 0 (0.915 < intercept <1.694) because the value was dependent on the total number of biomarkers, whereas the IBR-T reached 0 when no biomarker was significantly modulated, which appears more biologically relevant. The final ranking of sites was different between the two index and the IBR-T ranking tends to be more ecologically relevant that the IBRv2 ranking. This IBR-T have shown an undeniable interest for biomonitoring and could be used by environmental managers to simplify the interpretation of large datasets, directly interpret the contamination status of the site, use it to decision-making, and finally to easily communicate the results of biomonitoring studies to the general public.

Comparative immune responses of blue mussel and zebra mussel haemocytes to simultaneous chemical and bacterial exposure.

Biomonitoring at the scale of the aquatic continuum and based on biomarkers, requires various representative species and a knowledge of their sensitivity to contaminants. Mussel immunomarkers are established tools for evaluating immunotoxic stress, but little is known about the consequences of an immune activation by local microorganisms on their response to pollution. This study aims to compare the sensitivity of cellular immunomarkers in two mussel species from different environments, the marine mussel Mytilus edulis (blue mussel) and the freshwater mussel Dreissena polymorpha (zebra mussel), to chemical stressors combined with bacterial challenge. Haemocytes were exposed ex vivo to the contaminants (bisphenol A, caffeine, copper chloride, oestradiol, ionomycin) for 4 h. The chemical exposures were coupled with simultaneous bacterial challenges (Vibrio splendidus and Pseudomonas fluorescens) to trigger activation of the immune response. Cellular mortality, phagocytosis efficiency and phagocytosis avidity were then measured by flow cytometry. The two mussel species had different basal levels since D. polymorpha showed higher cell mortality than M. edulis (23.9 ± 11% and 5.5 ± 3% dead cells respectively), and lower phagocytosis efficiency (52.6 ± 12% and 62.2 ± 9%), but similar phagocytosis avidity (17.4 ± 5 and 13.4 ± 4 internalised beads). Both bacterial strains led to an increase in cellular mortality (+8.4% dead cells in D. polymorpha, +4.9% in M. edulis), as well an activation of phagocytosis (+9.2% of efficient cells in D. polymorpha, +6.2% efficient cells and +3 internalised beads per cell in M. edulis). All chemicals triggered an increase in haemocyte mortality and/or phagocytotic modulations, except for bisphenol A. The two species differed in the amplitude of their response. The addition of a bacterial challenge significantly altered cell responses to chemicals with synergetic and antagonistic variations compared to a single exposure, depending on the compound used and the mussel species. This work highlights the species-specific sensitivity of mussel immunomarkers to contaminants, with or without bacterial challenge, and the necessity of considering the presence of in natura non-pathogenic microorganisms for future in situ applications of immunomarkers.

Interest of a multispecies approach in active biomonitoring: Application in the Meuse watershed.

A biomonitoring approach based on a single model species cannot be representative of the contaminations impacts on the ecosystem overall. As part of the Interreg DIADeM program ("Development of an integrated approach for the diagnosis of the water quality of the River Meuse"), a study was conducted to establish the proof of concept that the use of a multispecies active biomonitoring approach improves diagnostic of aquatic systems. The complementarity of the biomarker responses was tested in four model species belonging to various ecological compartments: the bryophyte Fontinalis antipyretica, the bivalve Dreissena polymorpha, the amphipod Gammarus fossarum and the fish Gasterosteus aculeatus. The species have been caged upstream and downstream from five wastewater treatment plants (WWTPs) in the Meuse watershed. After the exposure, a battery of biomarkers was measured and results were compiled in an Integrated Biomarker Response (IBR) for each species. A multispecies IBR value was then proposed to assess the quality of the receiving environment upstream the WWTPs. The effluent toxicity was variable according to the caged species and the WWTP. However, the calculated IBR were high for all species and upstream sites, suggesting that the water quality was already downgraded upstream the WWTP. This contamination of the receiving environment was confirmed by the multispecies IBR which has allowed to rank the rivers from the less to the most contaminated. This study has demonstrated the interest of the IBR in the assessment of biological impacts of a point-source contamination (WWTP effluent) but also of the receiving environment, thanks to the use of independent references. Moreover, this study has highlighted the complementarity between the different species and has emphasized the interest of this multispecies approach to consider the variability of the species exposition pathway and sensibility as well as the mechanism of contaminants toxicity in the final diagnosis.

First evidence of SARS-CoV-2 genome detection in zebra mussel (Dreissena polymorpha).

The uses of bivalve molluscs in environmental biomonitoring have recently gained momentum due to their ability to indicate and concentrate human pathogenic microorganisms. In the context of the health crisis caused by the COVID-19 epidemic, the objective of this study was to determine if the SARS-CoV-2 ribonucleic acid genome can be detected in zebra mussels (Dreissena polymorpha) exposed to raw and treated urban wastewaters from two separate plants to support its interest as bioindicator of the SARS-CoV-2 genome contamination in water. The zebra mussels were exposed to treated wastewater through caging at the outlet of two plants located in France, as well as to raw wastewater in controlled conditions. Within their digestive tissues, our results showed that SARS-CoV-2 genome was detected in zebra mussels, whether in raw and treated wastewaters. Moreover, the detection of the SARS-CoV-2 genome in such bivalve molluscans appeared even with low concentrations in raw wastewaters. This is the first detection of the SARS-CoV-2 genome in the tissues of a sentinel species exposed to raw and treated urban wastewaters. Despite the need for development for quantitative approaches, these results support the importance of such invertebrate organisms, especially zebra mussel, for the active surveillance of pathogenic microorganisms and their indicators in environmental waters.

Blue mussel (Mytilus edulis)-A bioindicator of marine water contamination by protozoa: Laboratory and in situ approaches.

AIMS: The protozoan parasites Cryptosporidium spp., Giardia duodenalis and Toxoplasma gondii are identified as public health priorities and are present in a wide variety of environments including the marine ecosystem. The objective of this study was to demonstrate that the marine bivalve blue mussel (Mytilus edulis) can be used as a tool to monitor the contamination of marine waters by the three protozoa over time. METHODS AND RESULTS: In order to achieve a proof of concept, mussels were exposed to three concentrations of G. duodenalis cysts and Cryptosporidium parvum/T. gondii oocysts for 21 days, followed by 21 days of depuration in clear water. Then, natural contamination by these protozoa was sought for in wild marine blue mussels along the northwest coast of France to validate their relevance as bioindicators in the field. Our results highlighted that: (a) blue mussels bioaccumulated the parasites for 21 days, according to the conditions of exposure, and parasites could still be detected during the depuration period (until 21 days); (b) the percentage of protozoa-positive M. edulis varied under the degree of protozoan contamination in water; (c) mussel samples from eight out of nine in situ sites were positive for at least one of the protozoa. CONCLUSIONS: The blue mussel M. edulis can bioaccumulate protozoan parasites over long time periods, according to the degree of contamination of waters they are inhabiting, and can highlight recent but also past contaminations (at least 21 days). SIGNIFICANCE AND IMPACT OF THE STUDY: Mytilus edulis is a relevant bioaccumulators of protozoan (oo)cysts in laboratory and field conditions, hence its potential use for monitoring parasite contamination in marine waters.

Cellular and molecular complementary immune stress markers for the model species Dreissena polymorpha.

This study aimed to combine cellular and molecular analyses for better detail the effects of various stresses on a sentinel species of freshwater invertebrate. For this purpose, the hemocytes of the zebra mussel, Dreissena polymorpha, were exposed to different stresses at two different intensities, high or low: chemical (cadmium and ionomycin), physical (ultraviolet B), or biological ones (Cryptosporidium parvum and Toxoplasma gondii). After exposure, flow cytometry and droplet digital PCR analyses were performed on the same pools of hemocytes. Several responses related to necrosis, apoptosis, phagocytosis, production of nitric oxide and expression level of several genes related to the antioxidant, detoxification and immune systems were evaluated. Results showed that hemocyte integrity was compromised by both chemical and physical stress, and cellular markers of phagocytosis reacted to ionomycin and protozoa. While cadmium induced oxidative stress and necrosis, ionomycin tends to modulate the immune response of hemocytes. Although both biological stresses led to a similar immune response, C. parvum oocysts induced more effects than T. gondii, notably through the expression of effector caspases gene and an increase in hemocyte necrosis. This suggests different management of the two protozoa by the cell. This work provides new knowledge of biomarkers in the zebra mussel, at both cellular and molecular levels, and contributes to elucidate the mechanisms of action of different kinds of stress in this species.

From shotgun to targeted proteomics: rapid Scout-MRM assay development for monitoring potential immunomarkers in Dreissena polymorpha.

A highly multiplexed liquid chromatography mass spectrometry-multiple reaction monitoring (MRM)-based assay has been developed for evaluating 107 candidate immune biomarkers in both hemocytes and plasma of the zebra mussel Dreissena polymorpha. The Scout-MRM strategy was employed for the first time, shortening the implementation of a targeted MRM bottom-up proteomics assay using selected immune protein-related peptides identified by shotgun discovery proteogenomics. This strategy relies on spiking scout peptides during the discovery phase and using them to build and deploy the MRM targeted proteomics method. It proved to be highly relevant, since about 90% of the targeted peptides and proteins were monitored and rapidly measured in both hemocyte and plasma samples. The sample preparation protocol was optimized by evaluating the digestion efficiency of tryptic peptides over time. The accuracy and precision of 50 stable isotope-labeled peptides were evaluated for use as internal standards. Finally, the specificity of the transitions was thoroughly assessed to ensure the reliable measurement of protein biomarkers. Several analytical and biological validation criteria were evaluated across hemocytes and plasma samples exposed ex vivo to biological contaminants, resulting in the validation of two Scout-MRM assays for the relative quantitation of 85 and 89 proteins in hemocytes and plasma, respectively. Graphical abstract.

Identification of immune-related proteins of Dreissena polymorpha hemocytes and plasma involved in host-microbe interactions by differential proteomics.

Biological responses of zebra mussel Dreissena polymorpha are investigated to assess the impact of contaminants on aquatic organisms and ecosystems. In addition to concentrate chemical contaminants in their tissues, zebra mussels accumulate several microorganisms such as viruses, protozoa and bacteria. In order to understand the molecular mechanisms involved in the defence against microorganisms this study aims at identifying immune proteins from D. polymorpha hemolymph involved in defence against protozoa and viruses. For this purpose, hemolymph were exposed ex vivo to Cryptosporidium parvum and RNA poly I:C. Differential proteomics on both hemocytes and plasma revealed immune proteins modulated under exposures. Different patterns of response were observed after C. parvum and RNA poly I:C exposures. The number of modulated proteins per hemolymphatic compartments suggest that C. parvum is managed in cells while RNA poly I:C is managed in plasma after 4 h exposure. BLAST annotation and GO terms enrichment analysis revealed further characteristics of immune mechanisms. Results showed that many proteins involved in the recognition and destruction of microorganisms were modulated in both exposure conditions, while proteins related to phagocytosis and apoptosis were exclusively modulated by C. parvum. This differential proteomic analysis highlights in zebra mussels modulated proteins involved in the response to microorganisms, which reflect a broad range of immune mechanisms such as recognition, internalization and destruction of microorganisms. This study paves the way for the identification of new markers of immune processes that can be used to assess the impact of both chemical and biological contaminations on the health status of aquatic organisms.

A new protocol for the simultaneous flow cytometric analysis of cytotoxicity and immunotoxicity on zebra mussel (Dreissena polymorpha) hemocytes.

Immunotoxicity analysis receives a strong interest in environmental a priori and a posteriori risk assessment procedures considering the direct involvement of the immune system in the health status of organisms, populations and thus ecosystems. The freshwater mussel Dreissena polymorpha is an invasive species widely used in ecotoxicology studies and biomonitoring surveys to evaluate the impacts of contaminants on aquatic fauna. Bivalve hemocytes are the immunocompetent cells circulating in the open circulatory system of the organism. However, there is nowadays no consensus on a protocol to evaluate the immunocompetent state of this particular cell type using flow cytometry. Wild species such as D. polymorpha present several technical barriers complicating their analyze including (i) the quality and the purity of the hemolymph sample, (ii) the controversial characterization of hemocyte subpopulations and their diversity, (iii) the quantity of biological material, and (iv) the high inter-individual variability of hemocyte responses. The present work proposes several technical and analytical improvements to control the above-mentioned issues. The inclusion of sedimentation and cell detachment steps in the pre-analytical phase of the protocol substantially ameliorate the quality of the hemolymph sample as well as the accuracy of the cytometric measurements, by selecting the analyzed cells on their adhesion ability and by increasing the concentration of the analyzed events. The development of an effective triple-labeling procedure including the cellular probe Hoechst® 33342, the membrane impermeant dye propidium iodide and yellow-green fluorescent microspheres allowed the simultaneous analysis of cytotoxicity and phagocytosis activity in hemocytes. It also significantly enhanced the accuracy of hemocyte endpoint measurements by eliminating non-target events from the analysis and allowing relevant gating strategies. Finally, the use of pooled samples of hemolymph noticeably reduced inter-sample variability while providing more plasticity in the experimental design and improving the discriminating potency between treatments. The developed protocol is suitable for ex vivo exposure of hemocyte in a chemical/environmental toxicity assessment as well as for in vivo exposure in the laboratory or in situ biomonitoring surveys with few adaptations.

Comparative Proteomics of Ostreid Herpesvirus 1 and Pacific Oyster Interactions With Two Families Exhibiting Contrasted Susceptibility to Viral Infection.

Massive mortality outbreaks affecting Pacific oysters (Crassostrea gigas) spat/juveniles are often associated with the detection of a herpesvirus called ostreid herpesvirus type 1 (OsHV-1). In this work, experimental infection trials of C. gigas spat with OsHV-1 were conducted using two contrasted Pacific oyster families for their susceptibility to viral infection. Live oysters were sampled at 12, 26, and 144 h post infection (hpi) to analyze host-pathogen interactions using comparative proteomics. Shotgun proteomics allowed the detection of seven viral proteins in infected oysters, some of them with potential immunomodulatoy functions. Viral proteins were mainly detected in susceptible oysters sampled at 26 hpi, which correlates with the mortality and viral load observed in this oyster family. Concerning the Pacific oyster proteome, more than 3,000 proteins were identified and contrasted proteomic responses were observed between infected A- and P-oysters, sampled at different post-injection times. Gene ontology (GO) and KEGG pathway enrichment analysis performed on significantly modulated proteins uncover the main immune processes (such as RNA interference, interferon-like pathway, antioxidant defense) which contribute to the defense and resistance of Pacific oysters to viral infection. In the more susceptible Pacific oysters, results suggest that OsHV-1 manipulate the molecular machinery of host immune response, in particular the autophagy system. This immunomodulation may lead to weakening and consecutively triggering death of Pacific oysters. The identification of several highly modulated and defense-related Pacific oyster proteins from the most resistant oysters supports the crucial role played by the innate immune system against OsHV-1 and the viral infection. Our results confirm the implication of proteins involved in an interferon-like pathway for efficient antiviral defenses and suggest that proteins involved in RNA interference process prevent viral replication in C. gigas. Overall, this study shows the interest of multi-omic approaches applied on groups of animals with differing sensitivities and provides novel insight into the interaction between Pacific oyster and OsHV-1 with key proteins involved in viral infection resistance.

Comparison of viability and phagocytic responses of hemocytes withdrawn from the bivalves Mytilus edulis and Dreissena polymorpha, and exposed to human parasitic protozoa.

Bivalve molluscs are now considered indicator species of aquatic contamination by human parasitic protozoa. Nonetheless, the possible effects of these protozoa on the immune system of their paratenic hosts are poorly documented. The aim of this study was to evaluate the effects of two protozoa on hemocyte viability and phagocytosis from two mussels, the zebra mussel (freshwater habitat) and the blue mussel (seawater habitat). For these purposes, viability and phagocytic markers have been analysed on hemocytes from mussels without biological stress (control hemocytes), and on hemocytes exposed to a biological stress (Toxoplasma gondii and Cryptosporidium parvum oocysts). We report, for the first known time, the interactions between protozoa and hemocytes of mussels from different aquatic environments. Zebra mussel hemocytes showed a decrease in phagocytosis of fluorescent microbeads after exposure to both protozoa, while blue mussel hemocytes reacted only to T. gondii oocysts. These decreases in the ingestion of microbeads can be caused by competition between beads and oocysts and can be influenced by the size of the oocysts. New characterisations of their immune capacities, including aggregation, remain to be developed to understand the specificities of both mussels.

Use of the bivalve Dreissena polymorpha as a biomonitoring tool to reflect the protozoan load in freshwater bodies.

Cryptosporidium parvum, Toxoplasma gondii and Giardia duodenalis are worldwide pathogenic protozoa recognized as major causal agents of waterborne disease outbreaks. To overcome the normative process (ISO 15553/2006) limitations of protozoa detection in aquatic systems, we propose to use the zebra mussel (Dreissena polymorpha), a freshwater bivalve mollusc, as a tool for biomonitoring protozoan contamination. Mussels were exposed to three concentrations of C. parvum oocysts, G. duodenalis cysts or T. gondii oocysts for 21 days followed by 21 days of depuration in clear water. D. polymorpha accumulated protozoa in its tissues and haemolymph. Concerning T. gondii and G. duodenalis, the percentage of protozoa positive mussels reflected the contamination level in water bodies. As for C. parvum detection, oocysts did accumulate in mussel tissues and haemolymph, but in small quantities, and the limit of detection was high (between 50 and 100 oocysts). Low levels of T. gondii (1-5 oocysts/mussel) and G. duodenalis (less than 1 cyst/mussel) were quantified in D. polymorpha tissues. The ability of zebra mussels to reflect contamination by the three protozoa for weeks after the contamination event makes them a good integrative matrix for the biomonitoring of aquatic ecosystems.

The immune system of the freshwater zebra mussel, Dreissena polymorpha, decrypted by proteogenomics of hemocytes and plasma compartments.

The immune system of bivalves is of great interest since it reflects the health status of these organisms during stressful conditions. While immune molecular responses are well documented for marine bivalves, few information is available for continental bivalves such as the zebra mussel, Dreissena polymorpha. A proteogenomic approach was conducted on both hemocytes and plasma to identified immune proteins of this non-model species. Combining transcriptomic sequences with mass spectrometry data acquired on proteins is a relevant strategy since 3020 proteins were identified, representing the largest protein inventory for this sentinel organism. Functional annotation and gene ontology (GO) analysis performed on the identified proteins described the main molecular players of hemocytes and plasma in immunity. GO analysis highlights the complementary immune functions of these two compartments in the management of micro-organisms. Functional annotation revealed new mechanisms in the immune defence of the zebra mussel. Proteins rarely observed in the hemolymph of bivalves were pinpointed such as natterin-like and thaumatin-like proteins. Furthermore, the high abundance of complement-related proteins observed in plasma suggested a strong implication of the complement system in the immune defence of D. polymorpha. This work brings a better understanding of the molecular mechanisms involved in zebra mussel immunity. SIGNIFICANCE: Although the molecular mechanisms of marine bivalves are widely investigated, little information is known for continental bivalves. Moreover, few proteomic studies described the complementarity of both hemolymphatic compartments (cellular and plasmatic) in the immune defence of invertebrates. The recent proteogenomics concept made it possible to discover proteins in non-model organisms. Here, we propose a proteogenomic strategy with the zebra mussel, a key sentinel species for biomonitoring of freshwater, to identify and describe the molecular actors involved in the immune system in both hemocytes and plasma compartments. More widely, this study provided new insight into bivalve immunity.

First evidence of cytotoxic effects of human protozoan parasites on zebra mussel (Dreissena polymorpha) haemocytes.

The interaction between human protozoan parasites and the immune cells of bivalves, that can accumulate them, is poorly described. The purpose of this study is to consider the mechanisms of action of some of these protozoa on zebra mussel haemocytes, by evaluating their cytotoxic potential. Haemocytes were exposed to Toxoplasma gondii, Giardia duodenalis or Cryptosporidium parvum (oo)cysts. The results showed a cytotoxic potency of the two largest protozoa on haemocytes and suggested the formation of haemocyte aggregates. Thus, this study reveals the first signs of a haemocyte:protozoan interaction.

Genotoxic and Cytotoxic Effects on the Immune Cells of the Freshwater Bivalve Dreissena polymorpha Exposed to the Environmental Neurotoxin BMAA.

The environmental neurotoxin ß-N-Methylamino-l-alanine (BMAA) has been pointed out to be involved in human neurodegenerative diseases. This molecule is known to be bioaccumulated by bivalves. However, little data about its toxic effects on freshwater mussels is available, particularly on the hemolymphatic compartment and its hemocyte cells involved in various physiological processes such as immune defenses, digestion and excretion, tissue repair, and shell production. Here we exposed Dreissena polymorpha to dissolved BMAA, at the environmental concentration of 7.5 µg of /mussel/3 days, during 21 days followed by 14 days of depuration in clear water, with the objective of assessing the BMAA presence in the hemolymphatic compartment, as well as the impact of the hemocyte cells in terms of potential cytotoxicity, immunotoxicity, and genotoxiciy. Data showed that hemocytes were in contact with BMAA. The presence of BMAA in hemolymph did not induce significant effect on hemocytes phagocytosis activity. However, significant DNA damage on hemocytes occurred during the first week (days 3 and 8) of BMAA exposure, followed by an increase of hemocyte mortality after 2 weeks of exposure. Those effects might be an indirect consequence of the BMAA-induced oxidative stress in cells. However, DNA strand breaks and mortality did not persist during the entire exposure, despite the BMAA persistence in the hemolymph, suggesting potential induction of some DNA-repair mechanisms.