Lysosomal membrane stability and respiration rate in zebra mussel (Dreissena polymorpha Pallas, 1771) as biomarkers for ex situ heavy metal exposure Abstract Background and purpose: In the current study we aimed to investigate the lysosomal membrane stability in haemocytes of the invasive mollusk zebra mussel (Dreissena polymorpha Pallas, 1771) by applying the neutral red retention assay (NRRA), as well as changes in the respiration rate and survival under acute heavy metal exposure. (Pb)
Materials and methods: The mussels were treated with different decreasing concentrations of nickel (Ni) and lead
INTRODUCTION
In recent years, heavy metal pollution in the environment and food has been considered as a global issue (1,2). Large quantities of heavy metals have been released into aquatic systems, both fresh and marine worldwide due to a global rapid population growth and intensive domestic activities, as well as expanding industrial and agricultural production (3,4). Thus, heavy metals have severely deteriorated the aquatic ecosystems because of their toxicity, abundance, persistence, and subsequent bioaccumulation and biomagnification (5).To monitor the health of aquatic systems, sentinel organisms such as mussels (bivalves) have been proposed to be suitable ''biomonitors'' of pollution as they accumulate heavy metals in their tissues and shells (6,7). Their filtering habits, low metabolism and ability to bioaccumulate pollutants make them an excellent choice to assess their bioavailability and effects (8-10). Mussels are also sessile, sedentary, have a reasonable size, they are worldwide distributed and often found in large amounts (11-13).Marine mussels such as Mytilus sp., have been most often addressed as sensitive indicators for monitoring toxic substances in water (14-17). However, other bivalve species belonging to different taxa, such as Dreissena polymorpha have also been successfully used in ecotoxicological studies and monitoring programs (18)(19)(20)(21)(22)(23)(24)(25)(26)(27).The immune defence of mussels is comprised of cellmediated and humoral mechanisms, in which the haemocytes or blood cells play a key role (28). They are also involved in the detoxification process through the accumulation of metallic and organic xenobiotics in their well-developed endolysosomal system (29). Lysosomes are highly conserved multi-functional cellular organelles present in almost all cells of eukaryotic organisms, from yeast to humans. Their function in the cellular economy includes the degradation of redundant or damaged organelles (e.g., mitochondria and endoplasmic reticulum) and longer lived proteins as part of the autophagic cellular turnover (30). Lysosomes are also involved in normal physiological responses such as the digestion of materials ingested by endocytosis and phagocytosis (i.e., intracellular digestion) (30,31). Moreover, the lysosomal system, which is remarkably well-developed in mussel haemocytes is well-known as a target site for toxic metals and organic chemicals, due to its ability to accum...