The role of oxidative stress in the reproductive fitness of marine invertebrates exposed to environmental stressors

Abstract

Coastal marine environments have become increasingly impacted by anthropogenic contamination, particularly from industrialisation, agricultural run-off and marine traffic. It is essential that we better understand the underlying mechanisms of toxicity that these pollutants pose to natural populations of marine species. Oxidative stress (OS) in organisms occurs when the rate of reactive oxygen species (ROS) generation exceeds the scavenging capacity of an organism’s antioxidant (AO) system and is an important unifying feature underlying the toxicity of many chemical contaminants in aquatic organisms. Oxidative stress is also thought to be a possible mediator of trade-offs between current and future reproduction. The studies included in this thesis utilized AO enzyme activities, levels of the molecular antioxidant glutathione, protein carbonylation, lipid peroxidation and levels of 8-OHdG in DNA as OS biomarkers in a range of marine invertebrates. The objectives were to evaluate the general effectiveness of these biomarkers for detecting contaminant stress and, more specifically, to assess the reproductive impacts of contaminant-induced OS using sea urchins as a model system.

Antioxidants and oxidative damage were shown to be significantly elevated in Austrovenus stutchburyi (Bivalvia: Veneridae) and Micrelenchus tenebrosus (Gastropoda: Trochidae) from a contaminated site in Otago Harbour, New Zealand and in Laternula elliptica (Bivalvia: Laternulidae) and Sterechinus neumayeri (Echinoidea: Echinidae) from a contaminated site in McMurdo Sound, Antarctica when compared with levels in individuals collected from clean reference sites in both locations. Contaminants at these sites predominantly included heavy metals and polycyclic aromatic hydrocarbons (PAHs). The same trends were observed in laboratory experiments in which the New Zealand sea urchin Evechinus chloroticus (Echinoidea: Echinometridae) was exposed to dietary PAHs, specifically phenanthrene, fluoranthene, pyrene and benzo[a]pyrene, indicating that these chemical contaminants had the capacity to induce OS in this species. All biomarkers that were tested proved to be useful indicators of contamination exposure although, in general, oxidative damage showed the greatest sensitivity as a diagnostic measure.

To explore the physiological and ecological costs of such contaminants, later experiments assessed correlations between OS status and core components of fitness including reproductive effort, gamete quality and the carry-over potential from parent-to-offspring in E. chloroticus and S. neumayeri. Contaminant exposure resulted in oxidative damage in the reproductively mature gonad tissues of these sea urchins despite a significant upregulation of AO defences, however oxidative damage to eggs for both species was generally negligible, demonstrating that damage measured in the gonad was predominantly occurring in immature ova and/or somatic tissues. Sperm had negligible concentrations of the majority of antioxidants, and experienced greater levels of DNA damage than eggs. An additional key finding was that early stage offspring reflected maternal antioxidant status with populations derived from contaminant-experienced mothers demonstrating significantly higher baseline AO levels compared to those derived from contaminant-naïve mothers. This maternally inherited protection enhanced the capacity of embryos to minimise oxidative damage to lipids, proteins and DNA during early development when embryos were exposed to additional hydrocarbon contaminants. In contrast, when early embryos were exposed to UVR, a different stressor to that which parents were conditioned for, while the greater levels of AO afforded protection from lipid and protein damage, no additional protection was provided against oxidative DNA damage. In contrast, paternal contaminant history had limited influence on whether embryos were more or less capable of protecting themselves from oxidative damage in response to additional stressors. Interestingly, abnormal embryonic development in response to either pollutants or UVR was largely independent of oxidative damage, implying that an inherited resilience against OS may not necessarily translate to a fitness or survival gain in these species, at least in the early life stages.

In conclusion, these OS biomarkers provide a useful tool, not only to evaluate the biological effects of exposure to chemical pollutants, but also to understand the mode of action of the toxicants. As such they are of increasing interest in biomonitoring programs as well as in ecotoxicological studies. Furthermore, results suggest that antioxidants are strong candidates for a ‘currency’ underlying resource trade-offs in sea urchins with reduced fecundity acting as at least one apparent cost.