Pollutants bioavailability and toxicological risk from microplastics to marine mussels.

Lysozymes (EC 3.2.1.17) are hydrolytic enzymes, characterized by their ability to cleave the β-(1,4)-glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in peptidoglycan, the major bacterial cell wall polymer. In the animal kingdom, three major distinct lysozyme types have been identified — the c-type (chicken or conventional type), the g-type (goose-type) and the i-type (invertebrate type) lysozyme. Examination of the phylogenetic distribution of these lysozymes reveals that c-type lysozymes are predominantly present in the phylum of the Chordata and in different classes of the Arthropoda. Moreover, g-type lysozymes (or at least their corresponding genes) are found in members of the Chordata, as well as in some bivalve mollusks belonging to the invertebrates. In general, the latter animals are known to produce i-type lysozymes. Although the homology in primary structure for representatives of these three lysozyme types is limited, their three-dimensional structures show striking similarities. Nevertheless, some variation exists in their catalytic mechanisms and the genomic organization of their genes. Regarding their biological role, the widely recognized function of lysozymes is their contribution to antibacterial defence but, additionally, some lysozymes (belonging to different types) are known to function as digestive enzymes.

Lysozymes in the animal kingdom

Marine bivalves (such as mussels, oysters, and clams) are widespread mollusks in coastal waters at different latitudes; due to their filter-feeding habits, they accumulate large numbers of bacteria from the harvesting waters and may act as passive carriers of human pathogens. To cope with this challenge, bivalves possess both humoral and cellular defense mechanisms with remarkably effective capabilities. The circulating cells, or hemocytes, are primarily responsible for defense against parasites and pathogens; microbial killing results from the combined action of the phagocytic process with humoral defense factors such as agglutinins (e.g., lectins), lysosomal enzymes (e.g., acid phosphatase, lysozyme), toxic oxygen intermediates, and various antimicrobial peptides. In this work, current knowledge of the mechanisms underlying the interactions between bacteria and the hemolymph components of marine bivalves is summarized. Bacterial susceptibility to hemolymph killing in different bivalve species may be a consequence of the different ability of bacterial products to attract phagocytes, the presence or absence of specific opsonizing molecules, the hemocyte capability to bind and engulf different bacteria, and the different bacterial sensitivity to intracellular killing. The role of soluble (e.g., agglutinins and opsonins) and surface-bound factors in bacterial phagocytosis by hemocytes of the most common marine bivalve species is described and the possibility that environmental temperatures and other seasonal factors may influence this process is considered. Moreover, the potential strategies used by bacteria to evade phagocytic killing by hemocytes are discussed. From the available data it is clear that several questions need further investigation; the elucidation of the factors influencing phagocytosis in bivalves and the fundamental strategies used by bacteria to escape hemolymph killing are important not only to understand bivalve immune defenses but also to explain the persistence of pathogenic bacteria in bivalve tissues and to predict the consequent impact on human health.

Bacteria- hemocyte Interactions and Phagocytosis in Marine Bivalves

Defense mechanisms and disease prevention in farmed marine invertebrates

Concern is growing regarding the impact of chemicals suspected of altering the function of the immune system in humans and wildlife. There are numerous examples of links between pollution and increased susceptibility to disease in wildlife species, including immunosuppression in harbour seals feeding on fish from contaminated sites, altered immune function in riverine fish and decreased host resistance in birds exposed to pollutants. Laboratory tests have identified potential immunological hazards posed by a range of anthropogenic chemicals in mammals and higher vertebrates. However, few reports have considered the ecological relevance of pollution-induced immunosuppression in invertebrate phyla, which constitute around 95% of all animal species and occupy key structural and functional roles in ecosystems. In this paper effects of chemicals on immune function in invertebrates are briefly reviewed and biomarkers of immunotoxicity are identified. Examples of new approaches for the measurement of immunological inflammatory reactions and stress in molluscan haemocytes are detailed. The relevance of defining the immune system as a target organ of toxicity in invertebrates is discussed and an integrated approach for the use of immunological biomarkers in environment management is proposed, combining measures of immune function and organismal viability at the biochemical, cellular and population level.

Immunotoxicity in invertebrates: measurement and ecotoxicological relevance.

Enzymes Involved in Defense Functions of Hemocytes of Mussel Mytilus galloprovincialis

Mediterranean mussels Mytilus galloprovincialis were experi~nentally cultured from 5 rafts located in 4 Galician Rias, following the established industrial procedure. Cultures were sampled monthly until mussels exceeded market slze. Observation of histological sections of sampled mussels by light microscopy demonstrated symbionts which could be classified into 3 groups according to their pathogenicity. The first group consisted of symbionts with unnoticeable pathogenic effects including: prokaryotic inclusion bodies (PlB) in digestive gland and gills, an unidentified protistan in digestive primary ducts, a kidney coccidian, intracytoplasmic ciliates in digestive tubules, gill ciliates and a turbellarian in the ~ntestinal lumen. The second group comprised syn~bionts that could damage the host, although unlikely to be lethal, including: the microsporidian Steinhausia mytilovum, the flatworm Urastoma cyprinae, and the copepod Mytilicola intestinahs. The third group included the protistan Marteilia refringens and the trematode Proctoeces rnaculatus, potentially lethal pathogens. In addition, mussels with haemocytic infiltration of tissues and granulocytomas and a few cases of disseminated neoplasia were detected. The qualitative composition of mussel symbiont community was similar at the 5 study sites, except for 3 symbionts which were not detected at some sites Quantitatively, symbiont loads were higher and histological signs of stress more abundant in Moana and Vilagarcia (the most inner sites in the Rias), intermediate in Illa de Arousa and Muros, and lower in Lorbe. Symbionts increased in prevalence as mussels grew. Some of the symbionts were detected in mussel seed at the beginning of the experimental cultures.

https://www.int-res.com/articles/dao/31/d031p127.pdf

Carmen López

Papers

Enzymes Involved in Defense Functions of Hemocytes of Mussel Mytilus galloprovincialis

Symbionts and diseases of farmed mussels Mytilus galloprovincialis throughout the culture process in the Rías of Galicia (NW Spain)

In vitrostudy of phagocytic ability ofMytilus galloprovincialisLmk. haemocytes

Seasonal variation and effects of age, food availability, size, gonadal development, and parasitism on the hemogram of mytilus galloprovincialis

Morphological Characterization of the Hemocytes of the Clam,Ruditapes decussatus(Mollusca: Bivalvia)