Free-living stage of the unicellular algae Coccomyxa sp. parasite of the blue mussel (Mytilus edulis): Low-light adaptation, capacity for growth at a very wide salinity range and tolerance to low pH

Study of the Impact of the Parasitic Microalgae Coccomyxa parasitica on the Health of Bivalve Modiolus kurilensis

The invasion of bivalves by parasitic microalgae Coccomyxa is widespread and causes pathologies and dysfunctions of the organs, especially in the most valuable products: the mantle and the muscle. The pathogenesis of the disease remains completely unknown. In this study, based on a macroscopic examination of Modiolus kurilensis and microalgae count in each infected individual, four stages of disease development with characteristic pathognomonic symptoms were described. During the progression of the disease, the concentration of alkaline phosphatase, glucose, calcium, hemolytic and agglutinating activities, number of basophils, eosinophils, phagocytes, and cells with reactive oxygen species increased in the hemolymph, while number of agranulocytes, cells with lysosomes, dead hemocytes, total protein concentration, as well as the weight of mollusks decreased. In the nephridia and digestive gland, necrosis, invasion of Nematopsis sp., hemocyte infiltration, and fibrosis increased. The ratio of changed tubules and occurrence of granulocytomas increased in the digestive gland, while the base membrane, nephrocytes and concretions changed in the nephridia. This study helps establish the variability of these parameters under normal conditions and their alteration during the disease. Moreover, these findings can be used for veterinary monitoring of the state of bivalves in natural and aquaculture populations.

Pre-exposure to Cu2+ and CuO NPs leads to infection of caged blue mussels, Mytilus edulis L., by pathogenic microalga: Pilot study in the Lower St. Lawrence Estuary (Québec, Canada)

As evidenced from literature, exposure to non-lethal concentrations of dissolved copper (Cu²⁺) and copper nanoparticles (CuO NPs) promotes blue mussels susceptibility to various bacterial infections. We study whether pre-exposure (3.5 h) with CuSO₄ (100 μg Cu L^-1) and CuO NPs (1000 μg Cu L^-1) will result in infection of M. edulis L. with pathogenic microalga Coccomyxa sp. under field conditions. In May - September 2019, cages were installed in the site Metis-sur-Mer, St. Lawrence Estuary (QC, Canada) where the native mussel population is known to be infected with the pathogen. Untreated and pre-exposed mussels were grown for up to 130 days. Only the mussels pre-exposed to copper were infected by Coccomyxa. This finding allows proposing that occurrences of Coccomyxa-infected mussels worldwide might have an association with water pollution with xenobiotics. Pre-exposure of caged mussels to copper, as a protocol monitoring for other infectious agents, can be recommended to test.

Cultivation of Chlorella sp. with livestock waste compost for lipid production

Cultivation of microalgae Chlorella sp. with livestock waste compost as an alternative nutrient source was investigated in this present study. Five culture media with different nutrient concentrations were prepared. The characteristics of algal growth and lipid production were examined. The results showed that the specific growth rate together with biomass and lipid productivities was different among all the cultures. As the initial nutrient concentration decreased, the lipid content of Chlorella sp. increased. The variations in lipid productivity of Chlorella sp. among all the cultures were mainly due to the deviations in biomass productivity. The livestock waste compost medium with 2000mgL^-1COD provided an optimal nutrient concentration for Chlorella sp. cultivation, where the highest productivities of biomass (288.84mgL^-1day^-1) and lipid (104.89mgL^-1day^-1) were presented.

Permanent culture and parasitic impact of the microalga Coccomyxa parasitica, isolated from horse mussel Modiolus kurilensis

Animals with deformed shells and microalgal invasion have been identified in the natural population of the horse mussel species Modiolus kurilensis of Peter the Great Bay in the Sea of Japan. The haemolymph is initially infested with algae, followed by the rectum, siphons, mantles and gonads located in the posterior body areas. Mantles, which are primarily exposed to light, are major depots for algae. The microscopic analysis of algal cells has revealed the absence of flagella and pyrenoids, the presence of single chloroplast, and reproduction by autosporulation, with dispores prevailing over tetraspores. These results, together with the nearly complete sequence analysis of small subunit (SSU) 18S rDNA (1728bp), have confirmed that these cells are Coccomyxa parasitica. A newly developed method of isolating microalgae from mollusk tissues has facilitated the continuous pure - probably axenic - culture of C. parasitica, thereby providing a description of the time course of each life stage. Histological analyses have revealed significant haemocyte infiltration into the mantles, gonads, kidneys and digestive gland tissues infested with microalgae and the gill tissues, in which the intruder was not identified. Algal encapsulation with major focal areas of fibrosis and amorphic necrosis has been revealed in these infested organs. The spaces between the gonad follicles and digestive gland tubules were significantly widened as these areas were filled with a mass of algae and phagocytic haemocytes, showing acini with a thickened basement membrane. The mantles and kidneys of Modiolus displayed significant morphological deviations of different cells in epithelial, connective and muscle tissues, resulting in the dysfunction of the infested organs. Therefore, C. parasitica, which reproduces in the culture, regardless of the host, is a facultative parasite, causing major pathological alterations, such as anomalous histomorphological patterns and infested organ dysfunctions.