BIOCHEMICAL AND PHYSIOLOGICAL RESPONSES OF THE GREEN MUSSEL, PERNA VIRIDIS L. TO SOME HEAVY METALS

Abstract

Biochemical and physiological responses of the green mussel, Perna viridis L. to heavy metals (cadmium, copper, lead, mercury and zinc), especially lead, were investigated in the present study. The green mussels were collected from Lim Chu Kang and Changi Village of Singapore. Glutathione (GSH) levels in gills, kidney and digestive glands of the green mussels exposed to mercury (0.25 µg/1) and lead (1.0 mg/I) were determined. The results indicated that GSl-1 levels in the mussel's tissues were associated with the mercury concentrations in the tissues. On the other hand, lead accumulated in the mussel's kidney did not exist in conjugation with the -SH group. The results suggested that there might be different mechanisms between lead and mercury in terms of GSH metabolism in mussels. Influence of heavy metals on the a-amylase activity in digestive glands of the green mussels was investigated. All heavy metals tested in this experiment (Cd, Hg, Pb and Zn) showed an in vitro inhibition of the amylase activity in the homogenate from mussel's digestive glands. Mercury was the most effective inhibitor of the amylase activity with a LC50 value of 0.9 mM, followed by zinc (11.6 mM), cadmium (15.3 mM) and lead (17.1 mM) in that order. All of the four metals caused a significant reduction of the apparent K," and Vm." values of the amylase. The data also showed that both lead exposure and starvation affected the amylase activity. Starvation had a greater effect than lead exposure on the amylase activity. Lead taken up from the food and the seawater had almost an equal inhibitory effect on the amylase activity of the digestive glands. Vil Some enzymological properties of the acetylcholinesterase in gills of the green mussel were investigated and the effects of lead- and mercury-exposure on its activity were also examined. AchE from mussel's gills was different from that present in mammalian erythrocytes or the electTic organs of fish in their enzymological properties. Mercury showed both in vitro and in viva inhibition of the AchE activity in the mussel's gills. Lead showed an in vitro activation of the AchE activity, but in viva inhibition of the AchE activity. This discrepancy implied that the AchE might not be a specific target acted in viva by lead. Rates of feeding, respiration and ammonia excretion of the green mussels exposed to lead ranging from 0.01 ppm to 2.0 ppm were also investigated. Some physiologically integrated parameters such as O:N ratio, scope for growth and growth efficiency were calculated from the determinations of individual physiological components. A decline in the feeding rates of the exposed mussels indicated a toxic effect of lead on the neural control of gill cilia. Oxygen consumption was not altered by lead exposure. Increased ammonia excretion in the lead-treated mussels suggested an elevation in lhe protein catabolism. The calculated O:N ratio showed a similar pattern as the ammonia excretion. The lower O:N ratio of the exposed mussels probably resulted from the lower digestive ability of the mussels. The decline in lhe scope for growth and the growth efficiency of the mussels exposed to high concentration of lead or longer period (7 days) of lead exposure, might be due to the significant decreases in feeding rates.