Metal contamination and antioxidant responses of Mytilus galloprovincialis along the west coast of the Cape Peninsula, South Africa

Abstract

Metals are prevalent in the marine environment and come from various sources that are of both natural and anthropogenic origins. High concentrations of metals can have negative impacts on marine organisms and hence knowledge about the levels of contamination is important. However, knowledge about the levels of metal contamination in southern Africa and in particular, Cape Town, is sparse and there is a need to ascertain the levels and effects of metals in both the environment and biota. Metal bioaccumulation in Mytilus galloprovincialis sampled from 1985 to 2008 during the Mussel Watch Programme (MWP) was analysed. Analysis of MWP mussels from the west coast of the Cape Peninsula, South Africa showed that metal contamination varied significantly between sites and seasons. The mean order of combined metal concentrations for the study period was Zn>Fe>Cd>Cu>Mn>Hg. The concentrations of metals in the water, sediment and mussels (Mytilus galloprovincialis) were measured along the west coast of the Cape Peninsula from autumn 2010 to autumn 2011. Sampling took place at Scarborough, Hout Bay, Green Point, Milnerton and Bloubergstrand. The samples were digested using nitric acid and analysed using an Inductively-Coupled Plasma-Mass Spectrophotometer (ICP-MS). The results of the water and sediment analysis showed that metal contamination was lowest in Scarborough and highest in Hout Bay. Seasonally, pollution loads were highest in autumn 2011 and lowest in autumn 2010. The high contamination levels reported in the sediment suggested that localised contamination from anthropogenic sources as well as natural weathering were responsible for the high concentrations of metals reported. The Pollution Load Index of metals in the sediment showed that the order of polluted sites were Hout Bay > Bloubergstrand > Green Point > Milnerton > Scarborough. The bioaccumulation of metals in M. galloprovincialis showed that most metals were significantly lowest at Scarborough and that metals in mussels differed significantly between seasons, with winter 2010 showing the highest significant difference. The efficiency of metal accumulation was measured using the Biosediment Accumulation Factor (BSAF). The results showed that the BSAF was highest for Cd, Pb, Zn and Cu, with the lowest BSAF reported for Fe and Mn. These results indicated that metals were bioavailable for uptake and suggested that M. galloprovincialis accumulated Cd, Pb and Zn at higher rates than the other metals sampled, and could support the notion that these metals be ideally suited as tools for biomonitoring. A laboratory experiment was done to investigate correlations between the concentrations of Cu in M. galloprovincialis and antioxidant responses caused by potential stress induced by Cu exposure. The results of the experiment indicated that copper accumulated in M. galloprovincialis during a 21-day exposure period. Mussels exposed to low dosages (40 μg/L) of Cu resulted in a 4-fold increase in Cu bioaccumulation in its tissue, whereas mussels exposed to high dosages (100 μg/L) of Cu resulted in a 10-fold increase in Cu bioaccumulation in its tissue. The potential of oxidative stress as a biomarker was investigated using a battery of antioxidant biomarkers in the mussels exposed to Cu. Total antioxidant capacity was measured using FRAP and ORAC, enzyme activity was determined using CAT, SOD and GSH and lipid peroxidation was determined using TBARS and CDs. The results showed that M. galloprovincialis exposed to high dosages of Cu had significantly higher antioxidant activities, assumed as a response to the high dosages of Cu that the organisms were exposed to. For GSH, ORAC and TBARS, the antioxidant activities by day 21 were significantly higher than at the start of the experiment. There were significant differences between exposure groups and times, but these differences did not change consistently over time. The results of this chapter suggested that antioxidant responses could be considered as biomarkers of toxicity and it is recommended that future research considers mixtures of contaminants that reflect the metals in the natural environment. Antioxidant responses in M. galloprovincialis were determined from samples collected at Scarborough, Hout Bay, Green Point, Milnerton and Bloubergstrand. The results showed variable antioxidant responses at the respective sites, making interpretation of the antioxidant biomarker activities difficult. The results suggest that, based on antioxidant enzyme activity, Milnerton be considered a relatively polluted site. The antioxidant capacity biomarkers suggested that Green Point, Milnerton and Bloubergstrand be considered relatively polluted. The lipid peroxidation responses suggested that only Milnerton and Bloubergstrand be considered relatively polluted. However, it is noted that there could have been other factors that could have caused oxidative stress, including naturally induced activities such wave action, desiccation, lack of food, etc. The oxidative stress results should therefore be considered with caution since all the sites sampled are not considered to be polluted when compared to international standards. Based on these findings, it was recommended that Milnerton and Bloubergstrand could be considered relatively polluted, Hout Bay and Green point mildly relatively polluted and Scarborough, unpolluted. It was concluded that antioxidant responses was an appropriate biomarker of stress but that more environmental parameters be considered when interpreting the antioxidant responses.