This ability was shown to be mediated by interactions between carbohydrates associated with the cell surface of microalgae and C-type lectins present in mucus covering the feeding organs ( Pales Espinosa et al., 2010 Pales Espinosa and Allam, 2018). To optimize energy uptake from their surroundings, these organisms preferentially ingest nutrient-rich particles while rejecting poor quality ones in pseudofeces ( Loosanoff and Engle, 1947 Morton, 1960). The strikingly-efficient ability of suspension feeding bivalves to select their food particles have been recognized for decades ( Newell and Jordan, 1983 Pastoureaud et al., 1996). The role of carbohydrate residues in food capture is not unique to unicellular organisms and has also been demonstrated in suspension feeding bivalves, including mussels ( Pales Espinosa et al., 2008 Pales Espinosa and Allam, 2013). Finally, several heterotrophic dinoflagellates are known to use a similar mechanism involving the same carbohydrate residues (i.e., mannose and/or glucose) to catch their prey ( Ucko et al., 1999 Wootton et al., 2007 Martel, 2009). Similarly, a 40-kDa glycoprotein (gp40) presents on the cell surface of the heliozoon Actinophrys sol and displaying mannose- and/or glucose-related residues was found to be involved in the immobilization and ingestion of prey flagellates ( Sakaguchi et al., 2001). For example, the endosymbiotic dinoflagellate ( Symbiodinium C1f) cell surface displays glycan ligands, including mannose/glucose and galactose, that are involved in recognition during initial contact at the onset of symbiosis with their coral host larvae, Fungia scutaria ( Wood-Charlson et al., 2006). In marine organisms, carbohydrates are also involved in many recognition processes including reproduction, chemical communication and host-symbiont interactions ( Caldwell and Pagett, 2010). A comprehensive overview of about fifty biological functions of glycans, including intercellular signaling, intercellular adhesion, cell–matrix interactions, recognition of pathogen-associated molecular patterns or immune modulation of host by symbionts (including parasites), is provided by Varki (2017). In fact, almost all cell surface molecules and secreted proteins undergo glycosylation resulting in their modification by covalently-linked carbohydrate moieties. Beyond these basic functions, carbohydrates are also recognized as key elements in various molecular recognition and interaction processes ( Varki, 1993 Sears and Wong, 1996 Wang and Boons, 2011 Varki, 2017). The elementary functions of carbohydrates are to provide energy to the body and to offer structural support to cell walls. These findings support that selection of food particles by mussels rely on the strength of the bond between suspended particle and the mucosal layer that mediate sorting, and that these bonds depend on the quantity of compatible ligands on each particle. Results showed that glucose-BSA, but not fucose-BSA, has an effect on particle sorting in mussels, and 1.08 x 10 9 molecules of glucose per microspheres, corresponding to a density of 6.99 x 10 6 molecules of glucose per µm 2, triggers particle selection. Further, mussels were fed with microspheres coated with neoglycoproteins. The ability of commercial lectins to inhibit sorting was not linear and a threshold was noted between 30 and 45 ug lectins per million algae cells. Mussels fed with Chlamydomonas which MCSC were blocked with ConA or PEA lectins (affinity to mannose and glucose) led to a significant decrease of the sorting efficiencies, not observed when the lectin UEA (affinity to fucose) was used. The nature and quantities of surface carbohydrates required to trigger sorting in mussels was evaluated and the relationship between ligand quantities and sorting efficiency (SE) was determined. In this study, sorting efficiencies in blue mussels Mytilus edulis fed with microalgae having modified MCSC and engineered microspheres coated with target carbohydrates was measured. Microalga cell surface carbohydrates (MCSC) represent unique patterns that can be considered as species-specific fingerprints. Interactions between carbohydrate-binding proteins (lectins) expressed on the feeding organs and carbohydrates present on microbial cell surface can provide this specificity. The adhesion between food particles and mucus is a fundamental process in particle sorting in suspension-feeding bivalves that requires specific recognition. School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, United States.Emmanuelle Pales Espinosa * Margot Eckstein Bassem Allam
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