Mechanismen der CO2 Toleranz bei Seeigeln des Genus Strongylocentrotus = Mechanisms of CO2 tolerance in sea urchins of the genus Strongylocentrotus

Increasing atmospheric pCO2 due to anthropogenic CO2 emissions are altering the carbonate chemistry of the oceans, inducing a drop in surface seawater pH (pHSW) and [CO32-] and an increase in seawater pCO2 and [HCO3-]. This phenomenon has been termed “ocean acidification” and has lately received considerable public and scientific attention. Atmospheric pCO2 of 1000 ppm and a concomitant decrease in surface ocean pH of 0.4 units can be expected by the year 2100-2300.The organisms examined in this study – echinoids – are keystone species in several ecosystems as well as economically important. Echinoids are characterized by a calcified skeleton in adult as well as larval stages. Calcifying invertebrates have been shown to be relatively vulnerable to CO2 induced changes in seawater carbonate chemistry. In most studies, echinoid adults and larvae responded with reduced growth and developmental rates to elevated seawater pCO2, but the underlying mechanisms are unknown. In order to fill some of the gaps in knowledge, the present work was aimed at characterizing pCO2 induced changes in acid-base regulatory capacity and energy budgets in two sea urchin species, Strongylocentrotus droebachiensis and S. purpuratus. Furthermore, this study investigated the adults’ physiological acclimation potential and studied ‘carry-over’ effects between different life cycle stages in response to environmental hypercapnia. Using feeding rates, aerobic metabolic rates and egestion/excretion rates measured in larval and adult sea urchins exposed to current (approx. 40 Pa, 390 µatm) and elevated pCO2 conditions (100 - 385 Pa, 990 - 3800 µatm), the present study demonstrated that the energy available for growth and development – so called ‘scope for growth (SfG)’ – was reduced in response to hypercapnic conditions and that SfG correlated with observed decreases in growth and development. In S. purpuratus larvae, the reduction in SfG was due to elevated energy demands for maintenance processes as indicated by highly increased metabolic rates which rose by up to 100%, while food ingestion was slightly but not significantly reduced. In adult S. droebachiensis, SfG was reduced due to a significant decrease in feeding rates and an increase in N excretion, while aerobic metabolic rates were not altered in response to elevated seawater pCO2. This drop in SfG significantly reduced gonad growth and fertilization success in sea urchins exposed to hypercapnic conditions for up to 4 months. In order to identify the processes that were supplied with increased energy in response to elevated pCO2, extracellular pH (pHe) was determined in larval and adult S. droebachiensis. Planktotrophic sea urchin larvae basically have two fluid compartments that could be impacted by changes in seawater carbonate chemistry: 1. the digestive system consisting of mouth, stomach and intestine and 2. the primary body cavity (PBC), filled with an extracellular matrix similar in structural appearance to the scyphozoan mesoglea, in which the calcifying primary mesenchyme cells (PMCs) construct the larval spicules. Acute and chronic declines in pHSW both led to linear decreases in PBC pHe indicating that PBC pHe is pH conform to the surrounding seawater and that no pHe compensation occurs in this cell free gelatinous space. Furthermore, environmental hypercapnia (250 Pa, 2470 µatm) led to a 0.4 units decrease in the highly alkaline stomach pH (pH 9.5) and may thus negatively impact energy acquisition by potentially reducing larval digestion potential. While several genes important for ion regulation and calcification were down regulated in S. purpuratus pluteus larvae, gene transcript abundance of several metabolic genes and Na+/K+-ATPase were increased in response to elevated pCO2 conditions. This is in line with the observed increased metabolic rates which may be due to increased energetic demands for ion regulation, particularly for additional alkalization of the stomach lumen. Furthermore, albeit larval survival was not impacted by larval exposure to elevated pCO2 (1175 µatm), a 5 times higher mortality of S. dreobachiensis juveniles after metamorphosis of exposed larvae indicated a clear bottleneck at the transition from the larva to the juvenile. Adult S. droebachiensis fully compensated hypercapnia induced acid-base disturbances by significantly accumulating [HCO3-]e after 10 days of exposure to 145 Pa CO2 (1430 µatm). Furthermore, they could sustain full pHe compensation and high [HCO3-]e for 45 days. Adult S. droebachiensis exposed to 123 Pa pCO2 for 16 month exhibited a fertility rate not different from control animals. Both experiments suggest that there is acclimation/adaptation potential in S. droebachiensis which is possibly due to its current distribution in a habitat with seasonal hypercapnic conditions. In contrast, sea urchins exposed to >284 Pa CO2 (2800 µatm) only partially compensated pHe changes with [HCO3-]e accumulation of maximal 2.5 mM above control values similar to the [HCO3-]e values of animals exposed to the intermediate pCO2. 71% of all animals could not sustain this compensation for 45 days exhibiting a strong metabolic acidosis. Possibly, pCO2 conditions above 280 Pa (2760 µatm) exceed the acclimation potential of S. droebachiensis. Accordingly, the present thesis demonstrated that environmental hypercapnia primarily impacts the energy balance and energy partitioning of different ontogenetic stages of Strongylocentrotid sea urchins leading to the observed reductions in growth and development. Shifts in energy partitioning likely disturb transitions between life cycle stages and may thus decrease recruitment success of sea urchins.