DNA strand breaks in marine invertebrates

Abstract

In their natural environment, marine organisms are exposed to a wide range of chemical and physical factors that can damage their genetic material. Polycyclic aromatic hydrocarbons (PAHs) and trace metals (e.g. copper) are chemicals that are known to have such effects on marine organisms, either directly or indirectly. Arguably the most important mechanisms for toxicity are chemical interactions of contaminants, their metabolites or reactive intermediates with nucleic acids, as this can potentially cause genotoxicity. Damage to DNA can ultimately cause cell death, mutations and carcinogenesis, which may not only affect the individual, but can be transferred to offspring, with potential transgenerational consequences. A major reason for the genotoxicity of some contaminants is the generation of radical oxygen species (ROS), either directly or as a consequence of cellular processing or detoxification. This thesis aimed to elucidate how environmental stressors in the form of oxidative stress, lowmolecular- weight PAHs and metals, affect DNA strand breaks and recovery from such damage in marine invertebrates. The alkaline single cell gel electrophoresis or comet assay was the method used to quantify DNA strand breaks. Baseline levels of DNA strand breaks were determined in hemocytes and coelomocytes of blue mussel (Mytilus edulis), shore crab (Carcinus maenas), vase tunicate (Ciona intestinalis) and common starfish (Asterias rubens). The sensitivity to oxidative stress in the same cells and species was assessed following exposure to hydrogen peroxide (H2O2). Lymphocytes from Atlantic cod (Gadus morhua) were used as a reference. Then, the genotoxic potential of the low-molecular-weight PAHs phenanthrene and dibenzothiophene was examined in vitro in hemocytes from M. edulis and C. maenas and coelomocytes from A. rubens. Finally, in vivo copper genotoxicity and mortality was explored in the calanoid copepods Acartia tonsa, Temora longicornis and the harpacticoid copepod Tigriopus brevicornis. The results demonstrated species-dependent differences in the sensitivity of circulating cells to oxidative stress. Invertebrate circulating cells were more susceptible to oxidative stress than cod lymphocytes (paper I). Oxidative stress responses in paper II indicate that coelomocytes from Asterias rubens were more susceptible than hemocytes from Mytilus edulis and Carcinus maenas (paper I). Low-molecular-weight PAHs exposure resulted in a weak genotoxic effect of phenanthrene to Asterias rubens coelomocytes and Mytilus edulis hemocytes, but not to Carcinus maenas hemocytes. Dibenzothiophene was not genotoxic to cells from any of the examined species (paper II). The three copepods Acartia tonsa, Temora longicornis and Tigriopus brevicornis differed significantly in their apparent genotoxic sensitivity to copper exposure, with T. brevicornis being more robust than the other two species, but there were no clear links between Cu exposure and DNA strand breaks in any of the species (paper III). This thesis exhibited the sensitivity of invertebrate hemocytes/coelomocytes as model systems in genotoxicity studies, including challenges and advantages in their applicability. The thesis highlights the potential of oxidative stress, low molecular weight PAHs and copper to cause DNA strand breaks in circulating cells and differences in sensitivity between species.