• We determined the sensitivity thresholds for survival development and growth in sea urchin larvae exposed to acidified conditions.
• Determination of physiological parameters including midgut pH homeostasis, metabolic rates and expression of midgut acid-base transporters demonstrates a physiological tipping point at pH 7.2.
• This work demonstrates substantial resilience of an important environmental engineer to the ongoing phenomenon of ocean acidification.
Sea urchin larvae reduce developmental rates accompanied by changes in their energy budget when exposed to acidified conditions. The necessity to maintain highly alkaline conditions in their digestive systems led to the hypothesis that gastric pH homeostasis is a key trait affecting larval energy budgets leading to distinct tipping points for growth and survival.
To test this hypothesis, sea urchin larvae were reared for 10 days in different pH conditions ranging from pH 7.0 to pH 8.2. Survival, development and growth rates were determined demonstrating severe impacts < pH 7.2. To test the effects of pH on midgut alkalization we measured midgut pH and monitored the expression of acid-base transporters. While larvae were able to maintain their midgut pH at 8.9–9.1 up to an acidification level of pH 7.2, midgut pH was decreased in the lower pH treatments. The maintenance of midgut pH under low pH conditions was accompanied by dynamic changes in the expression level of midgut acid-base transporters. Metabolic rates of the larvae increased with decreasing pH and reached a threshold between pH 7.0 and pH 7.3 where metabolic rates decreased again. Methylation analyses on promoter CpG islands were performed for midgut acid-base transporter genes to test for possible epigenetic modifications after 10-day exposure to different pH conditions. This analysis demonstrated no correlation between methylation level and pH treatments suggesting low potential for epigenetic modification of acid-base transporters upon short-term exposure. Since a clear tipping point was identified at pH 7.2, which is much lower than near-future OA scenarios, this study suggests that the early development of the purple sea urchin larva has a comparatively high tolerance to seawater acidification with substantial acclimation capacity and plasticity in a key physiological trait under near-future OA conditions.
Lee H.-G., Stumpp M., Yan J.-J., Tseng Y.-C., Heinzel S. & Hu M. Y., in press. Tipping points of gastric pH regulation and energetics in the sea urchin larva exposed to CO2 induced seawater acidification. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology. Article (subscription required)