Abstract
Ocean acidification refers to the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide from the atmosphere. This ocean acidification is considered to be a threat to marine organisms and is suspected to act as a physiological stressor especially in Antarctic marine environments where the effects of ocean acidification will be first observed. There have been few studies that have investigated the effects of ocean acidification on key physiological processes. To address this gap in the knowledge base of climate change related physiological responses this thesis aimed to investigate the effects of ocean acidification on oxidative stress responses in the embryos of 2 polar echinoderm species, Sterechinus neumayeri and Odontaster validus and a temperate species Patiriella regularis. Embryos and early larval stages are investigated because these early life history stages are particularly vulnerable and are a potential bottleneck for wider population ecology.
Oxidative stress has been selected as a biomarker for stress because it is a highly conserved physiological process that is common in all life. Oxidative stress is the production and accumulation of reactive oxygen species beyond the capacity of an organism to quench those reactive species using antioxidants. Failure to quench these reactive oxygen species results in the damage of lipids, proteins and DNA. In this study I measured the concentration of lipid hydroperoxides, protein carbonyls and 8-OHdG in DNA as biomarkers for oxidative damage. In response to decreased seawater pH the antioxidant molecules superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), glutathione peroxidase (GPOX), glutathione S-transferase (GST), total glutathione and the % of reduced glutathione in embryos were measured.
To test the hypotheses that decreased seawater pH will cause oxidative damage and secondly that this damage will elicit an antioxidant response in S. neumayeri, O. validus and P. regularis embryos, freshly fertilised eggs were exposed to 3 seawater pH treatments, pH 8.1 (ambient seawater), pH 7.8 (IPCC 100 year scenario) and pH 7.6 (IPCC 200 year scenario) and left them to develop until the late blastula phase. It was found that development within seawater with a decreased pH significantly increased levels of protein carbonyls, lipid hydroperoxides and 8-OHdG, increased abnormality and the cross sectional area of S. neumayeri blastula. In response to our second hypothesis, S. neumayeri blastula had significant increases in SOD, CAT, GR, GPOX, GST and had a significant decrease in the total amount of glutathione. O. validus showed a significant increase in the amount of protein carbonyls and had a significant increase in the amount of GR. The baseline levels of antioxidants were higher than those expressed by S. neumayeri. For P. regularis there was a significant increase in the proportion of abnormal blastula. The size of the blastula that developed in seawater with a decreased pH was also significantly smaller. P. regularis blastula showed significant increases in lipid hydroperoxides and protein carbonyls. As well as O. validus, P. regularis had high baseline levels of antioxidants. S. neumayeri embryos appear to be vulnerable to oxidative stress caused by decreased seawater pH whereas the high levels of antioxidants within O. validus and P. regularis appear to buffer these species to the effects of oxidative stress caused by decreased seawater pH.
Marine stressors seldom act on marine organisms in isolation and this thesis also investigates the interaction of ocean acidification and ultraviolet radiation (UV-R) on oxidative stress in S. neumayeri embryos. In response to our hypothesis that decreased seawater pH would will be additive with exposure to a secondary stressor of UV-R freshly fertilised eggs were exposed to 3 pH treatments (pH 8.1, 7.8 and 7.6) and after 7 days of development the blastula were exposed to a 1 hour light treatment that either blocked UV-A and UV-B light or exposed blastula to PAR, UV-A and UV-B that was similar to 1 hour on the surface of the sea ice near Ross Island, Antarctica. These were preserved to measure oxidative damage and antioxidant defence. It was observed that there was a significant increase in the size of embryos that were exposed to UV-R. Lipid hydroperoxides and 8-OHdG increased with exposure to both seawater with a decreased pH and UV-R. Protein carbonyls increased with exposure to decreased seawater pH, UV-R and there was a significant interaction where the increase in protein carbonyls was greater than expected with exposure to pH 7.6 and UV-R. The antioxidant response included significant increases in SOD, GR and GPOX with a significant interaction caused by a decrease in antioxidants in the pH 7.6/UV-R treatment. CAT significantly increased with exposure to UV-R with a significant interaction again in the pH 7.6/UV-R treatment. GST and total glutathione only increased with exposure to decreased seawater pH. The percentage of reduced glutathione increased with exposure to UV-R. Seawater with a decreased pH appears to increase the amount of oxidative stress inflicted when there is a short exposure to UV-R than either stressor causes individually.
In conclusion seawater with a decreased pH caused oxidative stress in S. neumayeri and the addition of UV-R stress was additive. There was some oxidative damage in O. validus and P. regularis however high baseline antioxidants may increase the resilience of these species.