Autophagic and lysosomal reactions to stress in the hepatopancreas of blue mussels.

The aim of this investigation was to test the reactions of the hepatopancreatic digestive cells of blue mussels (Mytilus edulis and Mytilus galloprovincialis) to a variety of environmental stressors. These stressors included anoxia, hyperthermia, polycyclic aromatic hydrocarbons, copper and a combination of copper+nutritional deprivation. Paraquat was used as an experimental generator of reactive oxygen species (ROS). All of these stressors induced adverse reactions in the lysosomal system of the digestive cells and many also induced autophagy. Changes induced by anoxia and hyperthermia were reversible, whereas autophagic reactions caused by PAHs were incomplete resulting in swelling and accumulation of lipid and phospholipid in the autolysosomes. The lysosomotropic drug chloroquine, an inducer of incomplete autophagy, enhanced the toxicity of phenanthrene but was not itself toxic at the experimental concentration used. Nutritional deprivation-induced autophagy had a protective effect on lysosomal stability in mussels exposed to copper. These findings complement previous findings and support a mechanistic model for lysosomal responses to free radicals and reactive oxygen (ROS, reactive oxygen species) which are generated by normal metabolism and often enhanced by stress and toxic xenobiotics and metals. The protective role of autophagy induced by nutritional deprivation against oxidative stress can be explained by this model, where autophagy boosts "cellular housekeeping" through enhanced removal of ROS-damaged proteins and organelles minimising formation of harmful stress/age pigment (lipofuscin). Finally, we discuss the possibility of low level triggering of autophagy in mussels by fluctuating environmental regimes providing a mechanism for tolerance to environmental stress.

Effects of pyrene on mussels in different experimental conditions.

The toxicity of the polyaromatic hydrocarbon pyrene to Mytilus edulis and Mytilus galloprovincialis was investigated in experiments conducted in the United Kingdom for M. edulis and in Turkey for M. galloprovincialis. Experimental conditions of temperature and salinity were chosen to be appropriate to the ambient conditions in which the mussels typically live. The effect of different feeding regimes on pyrene bioaccumulation and toxicity was also investigated. Feeding rate and neutral red retention biomarker techniques were used for toxicity assessment. An experiment with M. edulis demonstrated that mussels exposed to pyrene accumulated increasing amounts of this compound throughout a 15-day exposure period and that accumulation increased in relation to exposure concentration and with increasing concentration of unicellular algal food material. However, in these experiments, which were protected from UV light, there was no clear relationship between pyrene concentration in tissues and feeding rate. A clear concentration-response relationship was observed between tissue concentration and neutral red retention for days 1 and 7 of the experiment, but this relationship was lost by day 15, with evidence of the cells recovering. A similar experiment was conducted with M. galloprovincialis at a single (high) pyrene concentration in darkness. The feeding rate of the exposed mussels was always lower than the feeding rate of the control mussels, although the difference was insignificant at 7-day exposure. A decreasing trend in health status of the mussels was indicated by the neutral red retention assay results after 7 and 15 days of exposure to pyrene. In a third experiment (with M. galloprovincialis), an illuminated algal chemostat system containing pyrene was inserted in the exposure system as the food source for the mussels. Both biomarker results showed conclusively that toxic effects correlated with pyrene bioaccumulation, though there was no clear evidence for light-enhanced toxicity. These experiments showed that experimental conditions such as salinity, temperature and feeding regime and perhaps the species of mussel used may influence whether pyrene is perceived as being a toxic molecule.

Toxic effects of unresolved complex mixtures of aromatic hydrocarbons accumulated by mussels, Mytilus edulis, from contaminated field sites.

Exposure of marine mussels (Mytilus edulis) to an unresolved complex mixture (UCM) of aromatic hydrocarbons isolated from a crude oil has been shown to reduce their feeding rate by 40%. The present study was undertaken to determine whether UCMs bioaccumulated by mussels in the field are also toxic. The feeding rate of mussels derived from polluted sites increased when they were placed in clean water, pointing to a loss of toxic agents from the tissues. At the end of the depuration period, water in which mussels from an oil-polluted site had been held contained a UCM. Steam-distillation extracts of the tissues of mussels taken from several polluted sites were shown to be highly toxic to the feeding activity of juvenile mussels. The tissues of mussels from these sites contained UCMs. Nontoxic steam distillates from clean mussels did not. Steam-distillation extracts of mussels from an oil-polluted site were fractionated by normal-phase high-performance liquid chromatography. A fraction, largely comprising a "monoaromatic" UCM, reduced the feeding rate of juvenile mussels by 70%. Two later-eluting fractions containing aromatic UCMs also produced smaller depressions in feeding rate. These results support our contention that some aromatic UCM hydrocarbons constitute a forgotten pollutant burden in the marine environment.