Resource allocation and extracellular acid–base status in the sea urchin Strongylocentrotus droebachiensis in response to CO2 induced seawater acidification

► Animals tolerated a pCO2 up to 145Pa, their health status deteriorated at 284Pa. ► Exposure to elevated pCO2 reduced somatic and reproductive growth. ► N excretion increased, feeding rate decreased leading to a shift in energy budgets. ► First report of extracellular pH compensation by HCO3-accumulation in echinoderms. ► At high pCO2 71% of animals ceased feeding and revealed a strong metabolic acidosis. Anthropogenic CO2 emission will lead to an increase in seawater pCO2 of up to 80–100Pa (800–1000μatm) within this century and to an acidification of the oceans. Green sea urchins (Strongylocentrotus droebachiensis) occurring in Kattegat experience seasonal hypercapnic and hypoxic conditions already today. Thus, anthropogenic CO2 emissions will add up to existing values and will lead to even higher pCO2 values >200Pa (>2000μatm). To estimate the green sea urchins’ potential to acclimate to acidified seawater, we calculated an energy budget and determined the extracellular acid base status of adult S. droebachiensis exposed to moderately (102–145Pa, 1007–1431μatm) and highly (284–385Pa, 2800–3800μatm) elevated seawater pCO2 for 10 and 45 days. A 45-day exposure to elevated pCO2 resulted in a shift in energy budgets, leading to reduced somatic and reproductive growth. Metabolic rates were not significantly affected, but ammonium excretion increased in response to elevated pCO2. This led to decreased O:N ratios. These findings suggest that protein metabolism is possibly enhanced under elevated pCO2 in order to support ion homeostasis by increasing net acid extrusion. The perivisceral coelomic fluid acid–base status revealed that S. droebachiensis is able to fully (intermediate pCO2) or partially (high pCO2) compensate extracellular pH (pHe) changes by accumulation of bicarbonate (maximum increases 2.5mM), albeit at a slower rate than typically observed in other taxa (10-day duration for full pHe compensation). At intermediate pCO2, sea urchins were able to maintain fully compensated pHe for 45 days. Sea urchins from the higher pCO2 treatment could be divided into two groups following medium-term acclimation: one group of experimental animals (29%) contained remnants of food in their digestive system and maintained partially compensated pHe (+2.3mM HCO3−), while the other group (71%) exhibited an empty digestive system and a severe metabolic acidosis (−0.5 pH units, −2.4mM HCO3−). There was no difference in mortality between the three pCO2 treatments. The res